By thus considering the descent and relationship of scientific conceptions, we can alone understand what we really owe to Black's discovery of latent heat. In regard to the method of the discovery, little need be said, since every student of the Baconian philosophy must see, that the discovery was of a kind for which none of the maxims of that system had provided. As latent heat escapes the senses, it could not obey the rules of a philosophy, which grounds all truth on observation and experiment. The subject of the inquiry being supersensual, there was no scope for what Bacon called crucial experiments and separations of nature. The truth was in the idea; experiments, therefore, might illustrate it, might bring it up to the surface, and so enable men to grasp it, but could not prove it. And this, which appears on the very face of the discovery, is confirmed by the express testimony of Dr. Thomson, who knew Black, and was, indeed, one of the most eminent of his pupils. We are assured by this unimpeachable witness, that Black, about the year 1759 began to speculate concerning heat; that the result of those speculations was the theory of latent heat; that he publicly taught that theory in the year 1761; but that the experiments which were necessary to convince the world of it were not made till 1764,[761] though, as I need hardly add, according to the inductive method, it was a breach of all the rules of philosophy to be satisfied with the theory three years before the experiments were made, and it was a still greater breach, not only to be satisfied with it, but to have openly promulgated it as an original and unquestionable truth, which explained, in a new manner, the economy of the material world.

The intellect of Black belonged to a class, which, in the eighteenth century, was almost universal in Scotland, but was hardly to be found in England, and which, for want of a better word, we are compelled to call deductive, though fully admitting that even the most deductive minds have in them a large amount of induction, since, indeed, without induction, the common business of life could not be carried on. But for the purposes of scientific classification, we may say, that a man or an age is deductive, when the favourite process is reasoning from principles instead of reasoning to them, and when there is a tendency to underrate the value of specific experience. That this was the case with the illustrious discoverer of latent heat, we have seen, both from the nature of the discovery, and from the decisive testimony of his friend and pupil. And a further confirmation may be found in the circumstance, that, having once propounded his great idea, he, instead of instituting a long series of laborious experiments, by which it might be verified in its different branches, preferred reasoning from it according to the general maxims of dialectic; pushing it to its logical consequences, rather than tracking it into regions where the senses might either confirm or refute it.[762] By following this process of thought, he was led to some beautiful speculations, which are so remote from experience, that even now, with all the additional resources of our knowledge, we cannot tell whether they are true or false. Of this kind were his views respecting the causes of the preservation of man, whose existence would, he thought, be endangered, except for the power which heat possesses of lying latent and unobserved. Thus, for example, when a long and severe winter was followed by sudden warmth, it appeared natural that the ice and snow should melt with corresponding suddenness; and if this were to happen, the result would be such terrible inundations, that it would be hardly possible for man to escape from their ravages. Even if he escaped, his works, that is, the material products of his civilization, would perish. From this catastrophe, nothing saves him but the latent power of heat. Owing to this power, directly the ice and snow begin to melt at their surface, the heat enters their structure, where a large part of it remains in abeyance, and thus losing much of its power, the process of liquefaction is arrested. This dreadful agent is lulled, and becomes dormant. It is weakened at the outset of its career, and is laid up, as in a storehouse, from which it can afterwards emerge, gradually, and with safety to the human species.[763]

In this way, as summer advances, a vast magazine of heat is accumulated, and is preserved in the midst of water, where it can do man no injury, since, indeed, his senses are unable to feel it. There the heat remains buried, until, in the rotation of the seasons, winter returns, and the waters are congealed into ice. In the process of congelation, that treasury of heat, which had been hidden all the summer, reappears; it ceases to be latent; and now, for the first time, striking the senses of man, it tempers, on his behalf, the severity of winter. The faster the water freezes, the faster the heat is disengaged; so that, by virtue of this great law of nature, cold actually generates warmth, and the inclemency of every season, though it cannot be hindered, is softened in proportion as the inclemency is more threatening.[764]

Thus, again, inasmuch as heat becomes latent, and flies from the senses, not only when ice is passing into water, but also when water is passing into steam, we find in this latter circumstance, one of the reasons why man and other animals can live in the tropics, which, but for this, would be deserted. They are constantly suffering from the heat which is collected in their bodies, and which, considered by itself, is enough to destroy them. But this heat causes thirst, and they consequently swallow great quantities of fluid, much of which exudes through the pores of the skin in the form of vapour. And as, according to the theory of latent heat, vapour cannot be produced without a vast amount of heat being buried within it, such vapour absorbs and carries off from the body, that which, if left in the system, would prove fatal. To this we must add, that, in the tropics, the evaporation of water is necessarily rapid, and the vapour which is thus produced, becomes another storehouse of heat, and a vehicle by which it is removed from the earth, and prevented from unduly interfering with the economy of life.[765]

From these and many other arguments, all of which were so essentially speculative, and dealt with such hidden processes of nature, that even now we are not justified either in confidently admitting them or in positively denying them, Black was led to that great doctrine of the indestructibility of heat,[766] which, as I have pointed out, has, in its connexion with the indestructibility of force, a moral and social importance even superior to its scientific value. Though the evidence of which he was possessed was far more scanty than what we now have, he, by the reach of his commanding intellect, rather than by the number and accuracy of his facts, became so penetrated with a conviction of the stability of physical affairs, that he not only applied that idea to the subtle phenomena of heat, but, what was much harder to do, he applied it to cases in which heat so entirely escapes the senses, that man has no cognizance of it, except through the medium of the imagination. According to his view, heat passes through an immense variety of changes, during which it appears to be lost; changes which no eye can ever see, which no touch can ever experience, and which no instrument can ever measure. Still, and in the midst of all these changes, it remains intact. From it nothing can be taken, and to it nothing can be added. In one of those fine passages of his Lectures, which, badly reported as they are,[767] bear the impress of his elevated genius, Black, after stating what would probably happen, if the total amount of heat existing in the world were to be diminished, proceeds to speculate on the consequences of its being increased. Were it possible for any power to add to it ever so little, it would at once overstep its bounds; the equilibrium would be disturbed; the framework of affairs would be disjointed. The evil rapidly increasing, and acting with accumulated force, nothing would be able to stop its ravages. It must continue to gain ground, till all other principles are absorbed and conquered. Sweeping on, unhindered, and irresistible, before it, every animal must perish, the whole vegetable world must disappear, the waters must pass into vapour, and the solid parts of the globe be merged and melted, until, at length, the glorious fabric, loosened and dissolved, would fall away, and return to that original chaos out of which it had been evolved.[768]

These, like many other of the speculations of this great thinker, will find small favour with those purely inductive philosophers, who not only suppose, perhaps rightly, that all our knowledge is in its beginning built upon facts, but who countenance, what seems to me, the very dangerous opinion, that every increase of knowledge must be preceded by an increase of facts. To such men it will appear, that Black had far better have occupied himself in making new observations, or devising new experiments, than in thus indulging his imagination in wild and unprofitable dreams. They will think, that these flights of fancy are suitable, indeed, to the poet, but unworthy of that severe accuracy, and of that close attention to facts, which ought to characterize a philosopher. In England, especially, there is, among physical inquirers, an avowed determination to separate philosophy from poetry, and to look upon them, not only as different, but as hostile. Among that class of thinkers, whose zeal and ability are beyond all praise, and to whom we owe almost unbounded obligations, there does undoubtedly exist a very strong opinion, that, in their own pursuit, the imagination is extremely dangerous, as leading to speculations, of which the basis is not yet assured, and generating a desire to catch too eagerly at distant glimpses before the intermediate ground has been traversed. That the imagination has this tendency is undeniable. But they who object to it on this account, and who would, therefore, divorce poetry from philosophy, have, I apprehend, taken a too limited view of the functions of the human mind, and of the manner in which truth is obtained. There is, in poetry, a divine and prophetic power, and an insight into the turn and aspect of things, which, if properly used, would make it the ally of science instead of the enemy. By the poet, nature is contemplated on the side of the emotions; by the man of science, on the side of the understanding. But the emotions are as much a part of us as the understanding: they are as truthful; they are as likely to be right. Though their view is different, it is not capricious. They obey fixed laws; they follow an orderly and uniform course; they run in sequences; they have their logic and method of inference. Poetry, therefore, is a part of philosophy, simply because the emotions are a part of the mind. If the man of science despises their teaching, so much the worse for him. He has only half his weapons; his arsenal is unfilled. Conquests, indeed, he may make, because his native strength may compensate the defects of his equipment. But his success would be more complete and more rapid, if he were properly furnished and made ready for the battle. And I cannot but regard as the worst intellectual symptom of this great country, what I must venture to call the imperfect education of physical philosophers, as exhibited both in their writings and in their trains of thought. This is the more serious, because they, as a body, form the most important class in England, whether we look at their ability, or at the benefits we have received from them, or at the influence they are exercising, and are likely to exercise, over the progress of society. It cannot, however, be concealed, that they display an inordinate respect for experiments, an undue love of minute detail, and a disposition to overrate the inventors of new instruments, and the discoverers of new, but often insignificant, facts. Their predecessors of the seventeenth century, by using hypotheses more boldly, and by indulging their imagination more frequently, did certainly effect greater things, in comparison with the then state of knowledge, than our contemporaries, with much superior resources, have been able to achieve. The magnificent generalizations of Newton and Harvey could never have been completed in an age absorbed in one unvarying round of experiments and observations. We are in that predicament, that our facts have outstripped our knowledge, and are now encumbering its march. The publications of our scientific institutions, and of our scientific authors, overflow with minute and countless details, which perplex the judgment, and which no memory can retain. In vain do we demand that they should be generalized, and reduced into order. Instead of that, the heap continues to swell. We want ideas, and we get more facts. We hear constantly of what nature is doing, but we rarely hear of what man is thinking. Owing to the indefatigable industry of this and the preceding century, we are in possession of a huge and incoherent mass of observations, which have been stored up with great care, but which, until they are connected by some presiding idea, will be utterly useless. The most effective way of turning them to account, would be to give more scope to the imagination, and incorporate the spirit of poetry with the spirit of science. By this means, our philosophers would double their resources, instead of working, as now, maimed, and with only half their nature. They fear the imagination, on account of its tendency to form hasty theories. But, surely, all our faculties are needed in the pursuit of truth, and we cannot be justified in discrediting any part of the human mind. And I can hardly doubt, that one of the reasons why we, in England, made such wonderful discoveries during the seventeenth century, was because that century was also the great age of English poetry. The two mightiest intellects our country has produced are Shakspeare and Newton; and that Shakspeare should have preceded Newton, was, I believe, no casual or unmeaning event. Shakspeare and the poets sowed the seed, which Newton and the philosophers reaped. Discarding the old scholastic and theological pursuits, they drew attention to nature, and thus became the real founders of all natural science. They did even more than this. They first impregnated the mind of England with bold and lofty conceptions. They taught the men of their generation to crave after the unseen. They taught them to pine for the ideal, and to rise above the visible world of sense. In this way, by cultivating the emotions, they opened one of the paths which lead to truth. The impetus which they communicated, survived their own day, and, like all great movements, was felt in every department of thought. But now it is gone; and, unless I am greatly mistaken, physical science is at present suffering from its absence. Since the seventeenth century, we have had no poet of the highest order, though Shelley, had he lived, would perhaps have become one. He had something of that burning passion, that sacred fire, which kindles the soul, as though it came fresh from the altar of the gods. But he was cut off in his early prime, when his splendid genius was still in its dawn. If we except his immature, though marvellous, efforts, we may assuredly say, that, for nearly two hundred years, England has produced no poetry which bears those unmistakable marks of inspiration which we find in Spenser, in Shakspeare, and in Milton. The result is, that we, separated by so long an interval from those great feeders of the imagination, who nurtured our ancestors, and being unable to enter fully into the feelings of poets, who wrote when nearly all opinions, and, therefore, nearly all forms of emotion, were very different to what they now are, cannot possibly sympathize with those immortal productions so closely as their contemporaries did. The noble English poetry of the sixteenth and seventeenth centuries is read more than ever, but it does not colour our thoughts; it does not shape our understandings, as it shaped the understandings of our forefathers. Between us and them is a chasm, which we cannot entirely bridge. We are so far removed from the associations amid which those poems were composed, that they do not flash upon us with that reality and distinctness of aim, which they would have done, had we lived when they were written. Their garb is strange, and belongs to another time. Not merely their dialect and their dress, but their very complexion and their inmost sentiments, tell of bygone days, of which we have no firm hold. There is, no doubt, a certain ornamental culture, which the most highly educated persons receive from the literature of the past, and by which they sometimes refine their taste, and sometimes enlarge their ideas. But the real culture of a great people, that which supplies each generation with its principal strength, consists of what is learnt from the generation immediately preceding. Though we are often unconscious of the process, we build nearly all our conceptions on the basis recognized by those who went just before us. Our closest contact is, not with our forefathers, but with our fathers. To them we are linked by a genuine affinity, which, being spontaneous, costs us no effort, and from which, indeed, we cannot escape. We inherit their notions, and modify them, just as they modified the notions of their predecessors. At each successive modification, something is lost and something is gained, until, at length, the original type almost disappears. Therefore it is, that ideas entertained several generations ago, bear about the same relation to us, as ideas preserved in a foreign literature. In both cases, the ideas may adorn our knowledge, but they are never so thoroughly incorporated with our minds, as to be the knowledge itself. The assimilation is incomplete, because the sympathy is incomplete. We have now no great poets; and our poverty in this respect is not compensated by the fact, that we once had them, and that we may, and do, read their works. The movement has gone by; the charm is broken; the bond of union, though not cancelled, is seriously weakened. Hence, our age, great as it is, and, in nearly all respects, greater than any the world has yet seen, has, notwithstanding its large and generous sentiments, its unexampled toleration, its love of liberty, and its profuse, and almost reckless, charity, a certain material, unimaginative, and unheroic character, which has made several observers tremble for the future. So far as I can understand our present condition, I do not participate in these fears, because I believe that the good we have already gained, is beyond all comparison greater than what we have lost. But that something has been lost, is unquestionable. We have lost much of that imagination, which, though, in practical life, it often misleads, is, in speculative life, one of the highest of all qualities, being suggestive as well as creative. Even practically, we should cherish it, because the commerce of the affections mainly depends on it. It is, however, declining; while, at the same time, the increasing refinement of society accustoms us more and more to suppress our emotions, lest they should be disagreeable to others. And as the play of the emotions is the chief study of the poet, we see, in this circumstance, another reason which makes it difficult to rival that great body of poetry which our ancestors possessed. Therefore, it is doubly incumbent on physical philosophers to cultivate the imagination. It is a duty they owe to their own pursuits, which would be enriched and invigorated by such an enlargement of their resources. It is also a duty which they owe to society in general; since they, whose intellectual influence is already greater than that of any other class, and whose authority is perceptibly on the increase, might have power enough to correct the most serious deficiency of the present age, and to make us some amends for our inability to produce such a splendid imaginative literature as that which our forefathers created, and in which the choicest spirits of the seventeenth century did, if I may so say, dwell and have their being.

If, therefore, Black had done nothing more than set the example of a great physical philosopher giving free scope to the imagination, he would have conferred upon us a boon, the magnitude of which it is not easy to overrate. And it is very remarkable, that, before he died, that department of inorganic physics, which he cultivated with such success, was taken up by another eminent Scotchman, who pursued exactly the same plan, though with somewhat inferior genius. I allude, of course, to Leslie, whose researches on heat are well known to those who are occupied with this subject; while, for our present purpose, they are chiefly interesting as illustrating that peculiar method which, in the eighteenth century, seemed essential to the Scotch mind.

About thirty years after Black propounded his famous theory of heat, Leslie began to investigate the same topic, and, in 1804, published a special dissertation upon it.[769] In that work, and in some papers in his Treatises on Philosophy, are contained his views, several of which are now known to be inaccurate,[770] though some are of sufficient value to mark an epoch in the history of science. Such was his generalization respecting the connexion between the radiation of heat, and its reflection; bodies which reflect it most, radiating it least, and those which radiate it most, reflecting it least. Such, too, was another wide conclusion, which the best inquirers have since confirmed, namely, that, while heat is radiating from a body, the intensity of each ray is as the sine of the angle which it makes with the surface of that body.

These were important steps, and they were the result of experiments, preceded by large and judicious hypotheses. In relation, however, to the economy of nature, considered as a whole, they are of small account in comparison with what Leslie effected towards consolidating the great idea of light and heat being identical, and thus preparing his contemporaries for that theory of the interchange of forces, which is the capital intellectual achievement of the nineteenth century. But it is interesting to observe, that, with all his ardour, he could not go beyond a certain length. He was so hampered by the material tendencies of his time, that he could not bring himself to conceive heat as a purely supersensual force, of which temperature was the external manifestation.[771] For this, the age was barely ripe. We accordingly find him asserting, that heat is an elastic fluid, extremely subtle, but still a fluid.[772] His real merit was, that, notwithstanding the difficulties which beset his path, he firmly seized the great truth, that there is no fundamental difference between light and heat. As he puts it, each is merely a metamorphosis of the other. Heat is light in complete repose. Light is heat in rapid motion. Directly light is combined with a body, it becomes heat; but when it is thrown off from that body, it again becomes light.[773]

Whether this is true or false, we cannot tell; and many years, perhaps many generations, will have to elapse before we shall be able to tell. But the service rendered by Leslie is quite independent of the accuracy of his opinion, as to the manner in which light and heat are interchanged. That they are interchanged, is the essential and paramount idea. And we must remember, that he made this idea the basis of his researches, at a period when some very important facts, or, I should rather say, some very conspicuous facts, were opposed to it; while the main facts which favoured it were still unknown. When he composed his work, the analogies between light and heat, with which we are now acquainted, had not been discovered; no one being aware, that double refraction, polarization, and other curious properties, are common to both. To grasp so wide a truth in the face of such obstacles, was a rare stroke of sagacity. But, on account of the obstacles, the inductive mind of England refused to receive the truth, as it was not generalized from a survey of all the facts. And Leslie, unfortunately for himself, died too soon to enjoy the exquisite pleasure of witnessing the empirical corroboration of his doctrine by direct experiment, although he clearly perceived that the march of discovery, in reference to polarization, was leading the scientific world to a point, of which his keen eye had discerned the nature, when, to others, it was an almost invisible speck, dim in the distant offing.[774]

In regard to the method adopted by Leslie, he assures us, that, in assuming the principles from which he reasoned, he derived great aid from poetry; for he knew that the poets are, after their own manner, consummate observers, and that their united observations form a treasury of truths, which are nowise inferior to the truths of science, and of which science must either avail herself, or else suffer from neglecting them.[775] To apply these truths rightly, and to fit them to the exigencies of physical inquiry, is, no doubt, a most difficult task, since it involves nothing less than holding the balance between the conflicting claims of the emotions and the understanding. Like all great enterprises, it is full of danger, and, if undertaken by an ordinary mind, would certainly fail. But there are two circumstances which make it less dangerous in our time, than in any earlier period. The first circumstance is, that the supremacy of the human understanding, and its right to judge all theories for itself, is now more generally admitted than ever; so that there can be little fear of our leaning to the opposite side, and allowing poetry to encroach on science. The other circumstance is, that our knowledge of the laws of nature is much greater than that possessed by any previous age; and there is, consequently, less risk of the imagination leading us into error, inasmuch as we have a large number of well-ascertained truths, which we can confront with every speculation, no matter how plausible or ingenious it may appear.

On both these grounds, Leslie was, I apprehend, justified in taking the course which he did. At all events, it is certain, that, by following it, he came nearer than would otherwise have been possible, to the conceptions of the most advanced scientific thinkers of our day. He distinctly recognized that, in the material world, there is neither break nor pause; so that what we call the divisions of nature have no existence, except in our minds.[776] He was even almost prepared to do away with that imaginary difference between the organic and inorganic world, which still troubles many of our physicists, and prevents them from comprehending the unity and uninterrupted march of affairs. They, with their old notions of inanimate matter, are unable to see that all matter is living, and that what we term death is a mere expression by which we signify a fresh form of life. Towards this conclusion, all our knowledge is now converging; and it is certainly no small merit in Leslie, that he, sixty years ago, when really comprehensive views, embracing the whole creation, were scarcely known among scientific men, should have strongly insisted that all forces are of the same kind, and that we have no right to distinguish between them, as if some were living, and others were dead.[777]

We owe much to him, by whom such views were advocated. But they were then, and in a certain, though far smaller degree, they are now, so out of the domain of physical experience, that Leslie never could have obtained them by generalizing in the way which the inductive philosophy enjoins. His great work on heat was executed, as well as conceived, on the opposite plan;[778] and his prejudices on this point were so strong, that we are assured by his biographer, that he would allow no merit to Bacon, who organized the inductive method into a system, and to whose authority we in England pay a willing, and I had almost said a servile, homage.[779]

Another curious illustration of the skill with which the Scotch mind, when once possessed of a principle, worked from it deductively, appears in the geological speculations of Hutton, late in the eighteenth century. It is well known, that the two great powers which have altered the condition of our planet, and made it what it is, are fire and water. Each has played so considerable a part, that we can hardly measure their relative importance. Judging, however, from the present appearance of the crust of the earth, there is reason to believe, that the older rocks are chiefly the result of fusion, and that the younger are aqueous deposits. It is, therefore, not unlikely, that, in the order in which the energies of nature have unfolded themselves, fire preceded water, and was its necessary precursor.[780] But, all that we are as yet justified in asserting is, that these two causes, the igneous and the aqueous, were in full operation long before man existed, and are still busily working. Perhaps they are preparing another change in our habitation, suitable to new forms of life, as superior to man, as man is superior to the beings who occupied the earth before his time. Be this as it may, fire and water are the two most important and most general principles with which geologists are concerned; and though, on a superficial view, each is extremely destructive, it is certain that they can really destroy nothing, but can only decompose and recompose; shifting the arrangements of nature, but leaving nature herself intact. Whether one of these elements will ever again get the upper hand of its opponent, is a speculation of extreme interest. For, there is reason to suspect, that, at one period, fire was more active than water, and that, at another period, water was more active than fire. That they are engaged in incessant warfare, is a fact with which geologists are perfectly familiar, though, in this, as in many other cases, the poets were the first to discern the truth. To the eye of the geologist, water is constantly labouring to reduce all the inequalities of the earth to a single level; while fire, with its volcanic action, is equally busy in restoring those inequalities, by throwing up matter to the surface, and in various ways disturbing the crust of the globe.[781] And as the beauty of the material world mainly depends on that irregularity of aspect, without which scenery would have presented no variety of form, and but little variety of colour, we shall, I think, not be guilty of too refined a subtlety, if we say that fire, by saving us from the monotony to which water would have condemned us, has been the remote cause of that development of the imagination which has given us our poetry, our painting, and our sculpture, and has thereby not only wonderfully increased the pleasures of life, but has imparted to the human mind a completeness of function, to which, in the absence of such a stimulus, it could not have attained.

When geologists began to study the laws according to which fire and water had altered the structure of the earth, two different courses were open to them, namely, the inductive and the deductive. The deductive plan was to compute the probable consequences of fire and water, by reasoning from the sciences of thermotics and hydrodynamics; tracking each element by an independent line of argument, and afterwards coördinating into a single scheme the results which had been separately obtained. It would then only remain to inquire, how far this imaginary scheme harmonized with the actual state of things; and if the discrepancy between the ideal and the actual were not greater than might fairly be expected from the perturbations produced by other causes, the ratiocination would be complete, and geology would, in its inorganic department, become a deductive science. That our knowledge is ripe for such a process, I am far, indeed, from supposing; but this is the path which a deductive mind would take, so far as it was able. On the other hand, an inductive mind, instead of beginning with fire and water, would begin with the effects which fire and water had produced, and would first study these two agents, not in their own separate sciences, but in their united action as exhibited on the crust of the earth. An inquirer of this sort would assume, that the best way of arriving at truth would be to proceed from effects to causes, observing what had actually happened, and rising from the complex results up to a knowledge of the simple agents, by whose power the results have been brought about.

If the reader has followed the train of thought which I have endeavoured to establish in this chapter, and in the first volume, he will be prepared to expect that when, in the latter half of the eighteenth century, geology was first seriously studied, the inductive plan of proceeding from effects to causes became the favourite one in England; while the deductive plan of proceeding from causes to effects, was adopted in Scotland and in Germany. And such was really the case. It is generally admitted, that, in England, scientific geology owes its origin to William Smith, whose mind was singularly averse to system, and who, believing that the best way of understanding former causes was to study present effects, occupied himself, between the years 1790 and 1815, in a laborious examination of different strata.[782] In 1815, he, after traversing the whole of England on foot, published the first complete geological map which ever appeared, and thus took the first great step towards accumulating the materials for an inductive generalization.[783] In 1807, and, therefore, before he had brought his arduous task to an end, there was formed in London the Geological Society, the express object of which, we are assured, was, to observe the condition of the earth, but by no means to generalize the causes which had produced that condition.[784] The resolution was, perhaps, a wise one. At all events, it was highly characteristic of the sober and patient spirit of the English intellect. With what energy and unsparing toil it has been executed, and how the most eminent members of the Geological Society have, in the pursuit of truth, not only explored every part of Europe, but examined the shell of the earth in America and in Northern Asia, is well known to all who are interested in these matters; nor can it be denied, that the great works of Lyell and Murchison prove that the men who are capable of such laborious enterprises, are also capable of the still more difficult achievement of generalizing their facts and refining them into ideas. They did not go as mere observers, but they went with the noble object of making their observations subservient to a discovery of the laws of nature. That was their aim; and all honour be to them for it. Still, it is evident, that their process is essentially inductive; it is a procedure from the observation of complex phenomena, up to the elements to which those phenomena are owing; it is, in other words, a study of natural effects, in order to learn the operation of natural causes.

Very different was the process in Germany and Scotland. In 1787, that is, only three years before William Smith began his labours, Werner, by his work on the classification of mountains, laid the foundation of the German school of geology.[785] His influence was immense; and among his pupils we find the names of Mohs, Raumer, and Von Buch, and even that of Alexander Humboldt.[786] But the geological theory which he propounded, depended entirely on a chain of argument from cause to effect. He assumed, that all the great changes through which the earth had passed, were due to the action of water. Taking this for granted, he reasoned deductively from premisses with which his knowledge of water supplied him. Without entering into details respecting his system, it is enough to say, that, according to it, there was originally one vast and primeval sea, which, in the course of time, deposited the primitive rocks. The base of all was granite; then gneiss; and others followed in their order. In the bosom of the water, which at first was tranquil, agitations gradually arose, which, destroying part of the earliest deposits, gave birth to new rocks, formed out of their ruins. The stratified thus succeeded to the unstratified, and something like variety was established. Then came another period, in which the face of the waters, instead of being merely agitated, was convulsed by tempests, and, amid their play and collision, life was generated, and plants and animals sprung into existence. The vast solitude was slowly peopled, the sea gradually retired; and a foundation was laid for that epoch, during which man entered the scene, bringing with him the rudiments of order and of social improvement.[787]

These were the leading views of a system which, we must remember, exercised great sway in the scientific world, and won over to its side minds of considerable power. Erroneous and far-fetched though it was, it had the merit of calling attention to one of the two chief principles which have determined the present condition of our planet. It had the further merit of provoking a controversy, which was eminently serviceable to the interests of truth. For, the great enemy of knowledge is not error, but inertness. All that we want is discussion, and then we are sure to do well, no matter what our blunders may be. One error conflicts with another; each destroys its opponent, and truth is evolved. This is the course of the human mind, and it is from this point of view that the authors of new ideas, the proposers of new contrivances, and the originators of new heresies, are benefactors of their species. Whether they are right or wrong, is the least part of the question. They tend to excite the mind; they open up the faculties; they stimulate us to fresh inquiry; they place old subjects under new aspects; they disturb the public sloth; and they interrupt, rudely, but with most salutary effect, that love of routine, which, by inducing men to go grovelling on in the ways of their ancestors, stands in the path of every improvement, as a constant, an outlying, and, too often, a fatal obstacle.

The method adopted by Werner was evidently deductive, since he argued from a supposed cause, and reasoned from it to the effects. In that cause, he found his major premiss, and thence he worked downwards to his conclusion, until he reached the world of sense and of reality. He trusted in his one great idea, and he handled that idea with consummate skill. On that very account, did he pay less attention to existing facts. Had he chosen, he, like other men, could have collected them, and subjected them to an inductive generalization. But he preferred the opposite path. To reproach him with this is irrational; for, in his journey after truth, he chose one of the only two roads which are open to the human mind. In England, indeed, we are apt to take for granted that one road is infinitely preferable to the other. It may be so; but on this, as on many other subjects, assertions are current which have never been proved. At all events, Werner was so satisfied with his method, that he would not be at the pains of examining the position of rocks and their strata, as they are variously exhibited in different countries; he did not even explore his own country, but, confining himself to a corner of Germany, he began and completed his celebrated system, without investigating the facts on which, according to the inductive method, that system should have been built.[788]

Exactly the same process, on the same subject, and at the same time, was going on in Scotland. Hutton, who was the founder of Scotch geology, and who, in 1788, published his Theory of the Earth, conducted the inquiry just as Werner did; though, when he began his speculations, he had no knowledge of what Werner was doing.[789] The only difference between them was, that while Werner reasoned from the agency of water, Hutton reasoned from the agency of fire. The cause of this may, I think, be explained. Hutton lived in a country where some of the most important laws of heat had, for the first time, been generalized, and where consequently, that department of inorganic physics had acquired great reputation. It was natural for a Scotchman to take more than ordinary interest in a subject in which Scotland had been so successful, and had obtained so much fame. We need not, therefore, wonder that Hutton, who, like all men, felt the intellectual bent of the time in which he lived, should have yielded to an influence of which he was, perhaps, unconscious. In obedience to the general mental habits of his country he adopted the deductive method. In further obedience to the more special circumstances connected with his own immediate pursuits, he gathered the principles from which he reasoned from a study of fire, instead of gathering them, as Werner did, from a study of water.

Hence it is, that, in the history of geology, the followers of Werner are known as Neptunists, and those of Hutton as Plutonists.[790] And these terms represent the only difference between the two great masters. In the most important points, namely their method, they were entirely agreed. Both were essentially one-sided; both paid a too exclusive attention to one of the two principal agents which have altered, and are still altering, the crust of the earth; both reasoned from those agents, instead of reasoning to them; and both constructed their system without sufficiently studying the actual and existing facts; committing, in this respect, an error which the English geologists were the first to rectify.

As I am writing a history, not of science, but of scientific method, I can only briefly glance at the nature of those services which Hutton rendered to geology, and which are so considerable, that his system has been called its present basis.[791] This, however, is too strongly expressed; for, though Hutton was far from denying the influence of water,[792] he did not concede enough to it, and there is a tendency among several geologists to admit that the system of Werner considered as an aqueous theory, contains a larger amount of truth than the advocates of the igneous theory are willing to allow. Still, what Hutton did was most remarkable, especially in reference to what are now termed metamorphic rocks, the theory of whose formation he was the first to conceive.[793] Into this, and into their connexion, on the one hand, with the sedimentary rocks, and, on the other hand, with those rocks whose origin is perhaps purely igneous, I could not enter without treading on debatable ground. But, putting aside what is yet uncertain, I will mention two circumstances respecting Hutton which are undisputed, and which will give some idea of his method, and of the turn of his mind. The first circumstance is, that, although he ascribed to subterranean heat, as exhibited in volcanic action, a greater and more constant energy than any previous inquirers had ventured to do,[794] he preferred speculating on the probable consequences of that action, rather than drawing inferences from the facts which the action presented; he being on this point so indifferent, that he arrived at his conclusions without inspecting even a single region of active volcanoes, where he might have watched the workings of nature, and seen what she was really about.[795] The other circumstance is equally characteristic. Hutton, in his speculations concerning the geological effects of heat, naturally availed himself of the laws which Black had unfolded. One of those laws was, that certain earths owe their fusibility to the presence of fixed air in them before heat has expelled it; so that if it were possible to force them to retain their fixed air, or carbonic acid gas, as we now call it, no amount of heat could deprive them of the capability of being fused. The fertile mind of Hutton saw, in this discovery, a principle from which he could construct a geological argument. It occurred to him, that great pressure would prevent the escape of fixed air from heated rocks, and would thus enable them to be fused, notwithstanding their elevated temperature. He then supposed that, at a period anterior to the existence of man, such a process had taken place under the surface of the sea, and that the weight of so great a column of water had prevented the rocks from being decomposed while they were subjected to the action of fire. In this way, their volatile parts were held together, and they themselves might be melted, which could not have happened except for this enormous pressure. By following this line of argument, he accounted for the consolidation of strata by heat; since, according to the premisses from which he started, the oily, or bituminous parts, would remain, in spite of the efforts of heat to disperse them.[796] This striking speculation led to the inference, that the volatile components of a substance, and its fixed components, may be made to cohere, in the very teeth of that apparently irresistible agent whose business it is to effect their separation. Such an inference was contrary to all experience; or, to say the least, no man had ever seen an instance of it.[797] Indeed, the event was only supposed to happen in consequence of circumstances which were never met with on the surface of the globe, and which, therefore, were out of the range of all human observation.[798] The utmost that could be expected was, that, by means of our instruments, we might, perhaps, on a small scale, imitate the process which Hutton had imagined. It was possible, that a direct experiment might artificially combine great pressure with great heat, and that the result might be, that the senses would realize what the intellect had conceived.[799] But the experiment had never been tried, and Hutton, who delighted in reasoning from ideas rather than from facts, was not likely to undertake it.[800] He cast his speculation on the world, and left it to its fate.[801] Fortunately, however, for the reception of his system, a very ingenious and skilful experimenter of that day, Sir James Hall, determined to test the speculation by an appeal to facts; and as nature did not supply the facts which he wanted, he created them for himself. He applied heat to powdered chalk, while, at the same time, with great delicacy of manipulation, he subjected the chalk to a pressure about equal to the weight of a column of water half a mile high. The result was, that, under that pressure, the volatile parts of the chalk were held together; the carbonic acid gas was unable to escape; the generation of quicklime was stopped; the ordinary operations of nature were baffled, and the whole composition, being preserved in its integrity, was fused, and, on subsequently cooling, actually crystallized into solid marble.[802] Never was triumph more complete. Never did a fact more fully confirm an idea.[803] But, in the mind of Hutton, the idea preceded the fact by a long interval; since, before the fact was known, the theory had been raised, and the system which was built upon it had, indeed, been published several years. It, therefore, appears that one of the chief parts of the Huttonian Theory, and certainly its most successful part, was conceived in opposition to all preceding experience; that it pre-supposed a combination of events which no one had ever observed, and the mere possibility of which nothing but artificial experiment could prove; and, finally, that Hutton was so confident of the validity of his own method of inquiry, that he disdained to make the experiment himself, but left to another mind that empirical branch of the investigation which he deemed of little moment, but which we, in England, are taught to believe is the only safe foundation of physical research.[804]

I have now given an account of all the most important discoveries made by Scotland, in the eighteenth century, respecting the laws of the inorganic world. I have said nothing of Watt, because, although the steam-engine, which we owe to him, is of incalculable importance, it is not a discovery, but an invention. An invention it may justly be termed, rather than an improvement.[805] Notwithstanding what had been effected in the seventeenth century, by De Caus, Worcester, Papin, and Savery, and notwithstanding the later additions of Newcomen and others, the real originality of Watt is unimpeachable. His engine was, essentially, a new invention; but, under its scientific aspect, it was merely a skilful adaptation of laws previously known; and one of its most important points, namely, the economy of heat, was a practical application of ideas promulgated by Black.[806] The only discovery made by Watt, was that of the composition of water. Though his claims are disputed by the friends of Cavendish, it would appear that he was the first who ascertained that water, instead of being an element, is a compound of two gases.[807] This discovery was a considerable step in the history of chemical analysis, but it neither involved nor suggested any new law of nature, and has, therefore, no claim to mark an epoch in the history of the human mind.[808] There is, however, one circumstance connected with it which is too characteristic to be passed over in silence. The discovery was made in 1783, by Watt, the Scotchman, and by Cavendish, the Englishman, neither of whom seems to have been aware of what the other was doing.[809] But between the two there was this difference. Watt, for several years previously, had been speculating on the subject of water in connexion with air, and having, by Black's law of latent heat, associated them together, he was prepared to believe that one is convertible into the other.[810] The idea of an intimate analogy between the two bodies having once entered his mind, gradually ripened; and when he, at last, completed the discovery, it was merely by reasoning from data which others possessed besides himself. Instead of bringing to light new facts, he drew new conclusions from former ideas.[811] Cavendish, on the other hand, obtained his result by the method natural to an Englishman. He did not venture to draw a fresh inference, until he had first ascertained some fresh facts. Indeed, his discovery was so completely an induction from his own experiments, that he omitted to take into consideration the theory of latent heat, from which Watt had reasoned, and where that eminent Scotchman had found the premisses of his argument.[812] Both of these great inquirers arrived at truth, but each accomplished his journey by a different path. And this antithesis is accurately expressed by one of the most celebrated of living chemists, who, in his remarks on the composition of water, truly says, that while Cavendish established the facts, Watt established the idea.[813]

Thus much, as to what was effected by the Scotch in the department of inorganic science. If we now turn to organic science, we shall find that, there also, their labours were very remarkable. To those who are capable of a certain elevation and compass of thought, it will appear, in the highest degree, probable, that, between the organic and inorganic world, there is no real difference. That they are separated, as is commonly asserted, by a sharp line of demarcation, which indicates where one abruptly ends, and the other abruptly begins, seems to be a supposition altogether untenable. Nature does not pause, and break off in this fitful and irregular manner. In her works there is neither gap nor chasm. To a really scientific mind, the material world presents one vast and uninterrupted series, gradually rising from the lowest to the highest forms, but never stopping. In one part of that series, we find a particular structure, which, so far as our observations have yet extended, we, in another part, cannot find. We also observe particular functions, which correspond to the structure, and, as we believe, result from it. This is all we know. Yet, from these scanty facts, we, who, at present, are still in the infancy of knowledge, and have but skimmed the surface of things, are expected to infer, that there must be a point, in the chain of existence, where both structure and function suddenly cease, and, after which, we may vainly search for signs of life. It would be difficult to conceive a conclusion more repugnant to the whole march and analogy of modern thought. In every department, the speculations of the greatest thinkers are constantly tending to coördinate all phenomena, and to regard them as different, indeed, in degree, but by no means as different in kind. Formerly, men were content to ground their conviction of this difference in kind, on the evidence of the eye, which, on a cursory inspection, saw an organization in some bodies, and not in others. From the organization, they inferred the life, and supposed that plants, for instance, had life, but that minerals had none. This sort of argument was long deemed satisfactory; but, in the course of time, it broke down; more evidence was required, and, since the middle of the seventeenth century, it has been universally admitted, that the eye, by itself, is an untrustworthy witness, and that we must employ the microscope, instead of relying on the unaided testimony of our own puny and precarious senses. But the microscope is steadily improving, and we cannot tell what limits there are to its capacity for improvement. Consequently, we cannot tell what fresh secrets it may disclose. Neither can we say, that it may not be altogether superseded by some new artificial resource, which shall furnish us with evidence, as superior to any yet supplied, as our present evidence is superior to that of the naked eye. Even already, and notwithstanding the shortness of time during which the microscope has been a really effective instrument, it has revealed to us organizations, the existence of which no one had previously suspected. It has proved, that what, for thousands of years, had been deemed mere specks of inert matter, are, in truth, animals possessing most of the functions which we possess, reproducing their species in regular and orderly succession, and endowed with a nervous system, which shows that they must be susceptible of pain and enjoyment. It has detected life hidden in the glaciers of Switzerland; it has found it embedded in the polar ice, and, if it can flourish there, it is hard to say from what quarter it can be shut out. So unwilling, however, are most men to relinquish old notions, that the resources of chemistry have been called in, to ascertain the supposed difference between organic and inorganic matter: it being asserted, that, in the organic world, there is a greater complexity of molecular combination, than in the inorganic.[814] Chemists further assert, that, in organic nature, there is a predominance of carbon, and, in inorganic, a predominance of silicon.[815] But chemical analysis, like microscopic observation, is making such rapid strides, that each generation, I had almost said each year, is unsettling some of the conclusions previously established; so that, now, and for a long time hence, we must regard those conclusions as empirical, and, indeed, as merely tentative. Surely a permanent and universal inference cannot be drawn from shifting and precarious facts, which are admitted to-day, and may be overthrown to-morrow. It would, therefore, appear that, in favour of the opinion, that some bodies are living, and that others are dead, we have nothing, except the circumstance, that our researches, so far as they have yet gone, have shown that cellular structure, growth, and reproduction, are not the invariable properties of matter, but are excluded from a large part of the visible world, which, on that account, we call inanimate. This is the whole of the argument on that side of the question. On the other side, we have the fact, that our sight, and the artificial instruments, by whose aid we have arrived at this conclusion, are confessedly imperfect; and we have the further fact, that, imperfect as they are, they have proved, that the organic kingdom is infinitely more extensive than the boldest dreamer had ever imagined, while they have not been able to enlarge the boundaries of the inorganic kingdom to any thing like the same amount. This shows, that, so far as our opinions are concerned, the balance is steadily inclining in one given direction; in other words, as our knowledge advances, a belief in the organic is encroaching upon a belief in the inorganic.[816] When we, moreover, add, that all science is manifestly converging towards one simple and general theory, which shall cover the whole range of material phenomena, and that, at each successive step, some irregularities are explained away, and some inequalities are reduced, it can hardly be doubted, that such a movement tends to weaken those old distinctions, the reality of which has been too hastily assumed; and that, in their place, we must, sooner or later, substitute the more comprehensive view, that life is a property of all matter, and that the classification of bodies into animate and inanimate, or into organic and inorganic, is merely a provisional arrangement, convenient, perhaps, for our present purposes, but which, like all similar divisions, will eventually be merged in a higher and wider scheme.

Until, however, that step is taken, we must be content to reason according to the evidence supplied by our imperfect instruments, or by our still more imperfect senses. We, therefore, recognize the difference between organic and inorganic nature, not as a scientific truth, but as a scientific artifice, by which we separate in idea, what is inseparable in fact; hoping, in this way, to pursue our course with the greater ease, and ultimately to obtain results, which will make the artifice needless. Assuming, then, this division, we may refer all investigations of organic bodies to one of two objects. The first object is, to ascertain the law of those bodies, in their usual, healthy, or, as we somewhat erroneously phrase it, normal course. The other object is, to ascertain their law, in their unusual, unhealthy, or abnormal course. When we attempt to do the first of these things, we are physiologists. When we attempt to do the second, we are pathologists.[817]

Physiology and pathology are thus the two fundamental divisions of all organic science.[818] Each is intimately connected with the other; and eventually, no doubt, both will be fused into a single study, by discovering laws which will prove that here, as elsewhere, nothing is really abnormal, or irregular. Hitherto, however, the physiologists have immeasurably outstripped the pathologists in the comprehensiveness of their views, and, therefore, in the value of their results. For, the best physiologists distinctly recognize that the basis of their science must include, not only the animals below man, but also the entire vegetable kingdom, and that, without this commanding survey of the whole realm of organic nature, we cannot possibly understand even human physiology, still less general physiology. The pathologists, on the other hand, are so much in arrear, that the diseases of the lower animals rarely form part of their plan; while the diseases of plants are almost entirely neglected, although it is certain that, until all these have been studied, and some steps taken to generalize them, every pathological conclusion will be eminently empirical, on account of the narrowness of the field from which it is collected.

The science of pathology being still so backward in the conception as well as in the execution, that even men of real ability believe that it can be raised from a mere study of the human frame, it will hardly be expected that the Scotch, notwithstanding the marvellous boldness of their speculations, should have been able, in the eighteenth century, to anticipate a method which the nineteenth century has yet to employ. But they produced two pathologists of great ability, and to whom we owe considerable obligations. These were, Cullen and John Hunter.[819] Cullen was eminent only as a pathologist; but Hunter, whose fine and discursive genius took a much wider range, was great both in physiology and in pathology. A short account of their generalizations respecting organic science, will be a fitting sequel to the notices I have already given of what was done by their countrymen for inorganic science, during the same period. It will complete our survey of the Scotch intellect, and will enable the reader to form some idea of the brilliant achievements of that most remarkable people, who, contrary to the course of affairs in all other modern nations, have shown that scientific discoveries do not necessarily weaken superstition, and that it is possible for two hostile principles to flourish side by side, without ever coming into actual collision, or without sensibly impairing each other's vigour.

In 1751, Cullen was appointed professor of medicine in the University of Glasgow;[820] from which, however, in 1756, he was removed to the University of Edinburgh,[821] where he delivered those celebrated lectures, on which his fame now depends. During the early part of his career, he paid great attention to inorganic physics, and propounded some remarkable speculations, which are supposed to have suggested the theory of latent heat to Black, who was his pupil.[822] But, to follow out those views, would have required a number of minute experiments, which it did not suit the habit of his mind to make. Having, therefore, put forth his ideas, he left them to germinate, and passed on to his arduous attempt to generalize the laws of disease as they are exhibited in the human frame. In the study of disease, the phenomena being more obscure and less amenable to experiment, there was greater latitude for speculation; hence, he could more easily indulge in that love of theory, which was his ruling passion, and with an extreme devotion to which he has been reproached.[823] That the reproach is not altogether unjust, must, I think, be admitted, since we find him laying down the doctrine, that, inasmuch as, in the treatment of disease, theory could not be separated from practice, it was unimportant which came first.[824] This was tantamount to saying, that a medical practitioner might allow his theories to control his observations; for it is certain that, in an immense majority of cases, men are so tenacious of the opinions they imbibe, that whatever, in any pursuit, first occupies their understanding, is likely to mould all that comes afterwards. In ordinary minds, associations of ideas, if firmly established, become indissoluble; and the power of separating them, and of arranging them in new combinations, is one of the rarest of our endowments. An average intellect, when once possessed by a theory, can hardly ever escape from it. Hence, in practical matters, theory should be feared, just as, in scientific matters, it should be cherished; because practical pursuits are chiefly engrossed by the lower class of minds, where associations and the force of prejudice are extremely strong, while scientific pursuits concern the higher class, where such prepossessions are comparatively weak, and where close associations are more easily severed. The most powerful intellects are most accustomed to new arrangements of thought, and are, therefore, most able to break up old ones. On them, belief sits lightly, because they well know how little evidence we have for many of even our oldest beliefs. But the average, or, as we must say, without meaning offence, the inferior, minds, are not disturbed by these refinements. Theories, which they have once heartily embraced, they can hardly ever get rid of, and they often dignify them with the name of essential truths, and resent every attack upon them as a personal injury. Having inherited such theories from their fathers, they regard them with a sort of filial piety, and cling to them as if they were some rich acquisition, which no one has a right to touch.

To this latter class, nearly all men belong, who are more engaged in practical pursuits than in speculative ones. Among them, are the ordinary practitioners, whether in medicine or in any other department, extremely few of whom are willing to break up trains of thought to which they are inured.[825] Though they profess to despise theory, they are, in reality, enslaved by it. All that they can do, is to conceal their subjection, by terming their theory a necessary belief. It must, therefore, be deemed a remarkable proof of Cullen's love of deductive reasoning, that he, sagacious and clear-sighted as he was, should have supposed that, in so practical an art as medicine, theory could, with impunity, precede practice. For, it is most assuredly true, that, taking men in the average, their minds are so constructed, that it cannot precede it without controlling it. It is equally true, that such control must be hurtful. Even now, and notwithstanding the great steps which have been taken in morbid anatomy, in animal chemistry, and in the microscopic investigation both of the fluids and solids of the human frame, the treatment of disease is a question of art, far more than a question of science. What chiefly characterizes the most eminent physicians, and gives them their real superiority, is not so much the extent of their theoretical knowledge,—though that, too, is often considerable,—but it is that fine and delicate perception which they owe, partly to experience, and partly to a natural quickness in detecting analogies and differences which escape ordinary observers. The process which they follow, is one of rapid, and, in some degree, unconscious, induction. And this is the reason why the greatest physiologists and chemists, which the medical profession possesses, are not, as a matter of course, the best curers of disease. If medicine were a science, they would always be the best. But medicine, being still essentially an art, depends mainly upon qualities which each practitioner has to acquire for himself, and which no scientific theory can teach. The time for a general theory has not yet come, and probably many generations will have to elapse before it does come. To suppose, therefore, that a theory of disease should, as a matter of education, precede the treatment of disease, is not only practically dangerous, but logically false. With its practical danger I am not now concerned; but its logical aspect is a curious illustration of that passion for systematic and dialectic reasoning which characterized Scotland. It shows that Cullen, in his eagerness to argue from principles to facts, instead of from facts to principles, could, in the most important of all arts, recommend a method of procedure, for which even our knowledge is not ripe, but which, in his time, was so singularly rash and immature, that nothing can explain its adoption by a man of such vigorous understanding, except the circumstance of his living in a country in which that peculiar method reigned supreme.