At the time when Professor Tyndall was studying at Marburg University, the principal figure there was Bunsen, who had been appointed Professor of Chemistry in 1838. He was a profound chemist, and his fame as a lecturer was so eminent as to attract many foreign students. A prolific discoverer, and peculiarly happy in his manner of demonstrating his scientific teaching, he soon fascinated the ardent minds of the two students from Queenswood. For two years Tyndall attended his chemical lectures. Indeed he learned German chiefly by listening to Bunsen. He has himself stated that Bunsen treated him like a brother, giving his time, space, and appliances, for the benefit of his studies. The subject which most attracted Tyndall’s attention was electro-chemistry, upon which Bunsen delivered an admirable course of lectures in 1848. The whole principle of the voltaic pile was thus explained to him in a masterful manner. He also made himself acquainted with chemical analyses, both quantitative and qualitative. He displayed no less zeal in the study of mathematics. For a considerable period he got private lessons from Professor Stegmann, under whose tuition he worked through analytical geometry of two and three dimensions, the Differential and Integral Calculus, and part of the Calculus of Variations.

His first scientific paper was a mathematical essay on screw surfaces, respecting which he says:—“Professor Stegmann gave me the subject of my dissertation when I took my degree: its title in English was, ‘On a Screw Surface with Inclined Generatrix, and on the Conditions of Equilibrium on such Surfaces.’ I resolved that if I could not, without the slightest aid accomplish the work from beginning to end it should not be accomplished at all. Wandering among the pine wood and pondering the subject, I became more and more master of it; and when my dissertation was handed in to the Philosophical Faculty it did not contain a thought that was not my own.”

But the man whose acquaintance at Marburg appeared to exercise most influence over his career was Dr. Knoblauch, who had just come thither from Berlin as extraordinary Professor of Physics, and who had already distinguished himself by his researches in radiant heat. He illustrated his lectures with a choice collection of apparatus brought from Berlin; and he not only suggested to Tyndall an exhaustive series of experiments bearing on a newly-discovered principle of physics, but supplied him with the necessary apparatus, and placed his own cabinet at his disposal for that purpose. The subject of investigation was diamagnetism.

Faraday’s discoveries and experiments in magnetism were then attracting the attention of the scientific world. He had shown in 1830 that by moving a magnet within the hollow of a coil of copper wire an electrical current was produced in the wire. This was a startling and pregnant discovery. Taking six hundred feet of insulated copper wire and winding it into a large vertical coil, he arranged the two ends of the wire into a small coil a little distance away from the large coil, and immediately above this small coil he suspended a balanced compass needle by a silk thread. Then, on dropping a bar magnet, or piece of iron magnetised, into the large coil, the needle, which was pointing towards the North Pole, instantly swung round, evidently impelled by magnetic force; when, again, the bar magnet was raised out of the hollow of the large coil, the needle moved round in the opposite direction; while it remained motionless so long as the bar magnet was at rest either inside or outside the coil. It thus appeared that an electrical current could be produced by the movement of the bar magnet—by dropping it into the coil or taking it out; and the current so produced he called an induced current. This operation is called magneto-electric induction. In 1845 Faraday greatly extended his magnetic discoveries. He not only established the magnetic condition of all matter by showing that every known body or thing could be influenced by magnetism, but he discovered a new property of magnetism, which he called diamagnetism. This was considered his greatest discovery.

By suspending bodies of an elongated form between the ends or poles of powerful magnets, he found that every substance was attracted or repelled from the magnetic poles; and he divided all bodies into two great classes, called magnetic and diamagnetic. The way in which a piece of iron is attracted by the poles or ends of a horseshoe magnet is a familiar illustration of the action of magnetic bodies, and the position that such bodies assume, pointing in a line from one pole to the other, he termed axial. On the other hand, diamagnetic bodies were those which, when freely suspended within the influence of the magnet, assumed a position at right angles to the line joining the poles of a magnet, or to the magnetic meridian; in other words, magnetic bodies pointed axially from pole to pole, or north and south; while diamagnetic bodies pointed east and west, or in an equatorial direction. Bismuth is a conspicuous example of diamagnetic substances. Scientific curiosity soon became excited as to the exact nature of the diamagnetic force in relation to crystals, some of which behaved in a mysterious manner between the poles of a magnet. Professor Plücker, of Bonn, discovered that some crystals formed of diamagnetic substances were not subject to the diamagnetic force; and to account for this he attributed to crystals an optical axis, upon which the poles of a magnet exercised a peculiar force. Plücker brought this theory before the British Association in 1848, and called it a new magnetic action. At the close of the same year, Faraday told the Royal Society that he had often been embarrassed by the anomalous magnetic results given by small cylinders of bismuth, and after investigation he referred these effects to the crystalline condition of the bismuth. In concluding his lecture on this subject, Faraday said:—“How rapidly the knowledge of molecular forces grows upon us, and how strikingly every investigation tends to develop more and more their importance, and their extreme attraction as an object of study. A few years ago, magnetism was to us an occult power affecting only a few bodies: now it is found to influence all bodies, and to possess the most intimate relations with electricity, heat, chemical action, light, crystallisation, and, through it, with the forces concerned in cohesion.” He thought there was in crystals a directive impelling force distinct from the magnetic and diamagnetic force.

Frequent conversations on this subject took place between Knoblauch and Tyndall in Germany during 1849. Knoblauch suggested that Tyndall should repeat the experiments of Plücker and Faraday; and as this operation was proceeding they agreed to make a joint inquiry into the deportment of crystals under the diamagnetic force. They laboured long at the problem before attaining any encouraging success. They examined the optical properties of crystals as well as made magnetic experiments with them, a great many experiments being made without discovering any new fact. Eventually, however, they found that various crystals did not act in accordance with the principles enunciated by Plücker, and the more they worked at the subject the more clearly it appeared that the deportment of certain bodies under the influence of magnetism was due, not to the presence of some force previously unknown, but to the crystalline structure of the substance under investigation, or as Tyndall put it, to peculiarities of material aggregation. For example, he showed that while a bar of iron attracted by a magnet sets itself in a line from pole to pole, an iron bar made of an aggregate of small bars sets itself in the opposite direction. Tyndall showed that the cause of the latter bar assuming an equatorial position was simply its mechanical structure, the small plates composing the “aggregated” bar setting from pole to pole. He found that the same law regulated the magnetic deportment of crystals, whose mechanism or structure, however, was generally less evident.

In 1849 eminent natural philosophers were studying this subject in England, France, and Germany, and it was expected that the investigation of diamagnetic phenomena would rapidly throw some new light upon the molecular forces which determine the conditions of the material creation. In allusion to this expectation, Tyndall said in 1850, that as nature acts by general laws, to which the terms great and small are unknown, it cannot be doubted that the modifications of magnetic force, exhibited by bits of copperas and sugar in the magnetic field, display themselves on a large scale in the crust of the earth itself, and as a lump of stratified grit, though a magnetic material, could be made, on account of its planes of stratification, to act as if it were diamagnetic, he suggested that this element might have some influence in determining the varying position of the magnetic poles of the earth—a subject which still perplexes the scientific world. Not only has the north magnetic pole gradually been changing its position, as shown by the records of three centuries, but, according to Barlow, every place has a magnetic pole and equator of its own; and according to Faraday the earth is a great magnet, whose power, as estimated by Gauss, is equal to that which would be conferred if every cubic yard of it contained six one-pound magnets; the sum of the force being thus equal to 8,464,000,000,000,000,000,000 such magnets. “The disposition of this magnetic force is not regular,” said Faraday, “nor are there any points on the surface which can be properly called poles: still the regions of polarity are in high north and south latitudes; and these are connected by lines of magnetic force (being the lines of direction), which, generally speaking, rise out of the earth in one (magnetic) hemisphere, and passing in various directions over the equatorial regions into the other hemisphere, there enter into the earth to complete the known circuit of power.”

It was in connection with his investigations on this subject that Prof. Tyndall first saw Prof. Faraday. Returning from Marburg in 1850, he called at the Royal Institution and sent in his card, together with a copy of a paper he had prepared, giving the results of his experiments on magne-crystallic action. Prof. Faraday conversed with him for half-an-hour, and being then on the point of publishing one of his papers on magne-crystallic action, he appended to it a flattering reference to the notes which Tyndall had placed in his hands.

Tyndall went back to Germany, where he worked for another year. In the beginning of 1851 he went to Berlin, where, he says, Prof. Magnus had made his name famous by physical researches of all kinds. “On April 28th, 1851, I first saw this Professor on his own doorstep in Berlin. His aspect won my immediate regard, which was strengthened to affection by our subsequent intercourse. He gave me a working place in his laboratory, and it was there I carried out my investigations on diamagnetism and magnecrystallic action published in the Philosophical Magazine for September, 1851. Among the other eminent scientific men whom I met at Berlin was Ehrenberg, with whom I had various conversations on microscopic organisms. I also made the acquaintance of Riess, the foremost exponent of frictional electricity, who more than once opposed to Faraday’s radicalism his own conservatism as regarded electric theory. Du Bois-Reymond was there at the time, full of power, both physical and mental. His fame had been everywhere noised abroad in connection with his researches on animal electricity. Du Bois-Reymond became perpetual secretary to the Academy of Sciences, Berlin. From Professor Magnus, and from Clausius, Wiedemann, and Poggendorff, I received every mark of kindness, and formed with some of them enduring friendships. Helmholtz was at this time in Königsberg. He had written his renowned essay on the “Conservation of Energy,” which I afterward translated. Helmholtz had, too, just finished his experiments on the velocity of nervous transmission, proving this velocity, which had previously been regarded as instantaneous, or, at all events, as equal to that of electricity, to be, in the nerves of the frog, only 93 ft. a second, or about one-twelfth of the velocity of sound in air of the ordinary temperature. In his own house I had the honour of an interview with Humboldt. He rallied me on having contracted the habit of smoking in Germany, his knowledge on this head being derived from my little paper on a water-jet, where the noise produced by the rupture of a film between the wet lips of a smoker is referred to. He gave me various messages to Faraday, declaring his belief that he (Faraday) had referred the annual and diurnal variation of the declination of the magnetic needle to their true cause—the variation of the magnetic condition of the oxygen of the atmosphere. I was interested to learn from Humboldt himself that, though so large a portion of his life had been spent in France, he never published a French essay without having it first revised by a Frenchman. In those days I not unfrequently found it necessary to subject myself to a process which I called depolarisation. My brain, intent on its subjects, used to acquire a set, resembling the rigid polarity of a steel magnet. It lost the pliancy needful for free conversation, and to recover this I used to walk occasionally to Charlottenburg or elsewhere. From my experiences at that time I derived the notion that hard thinking and fleet talking do not run together.”

Prof. Tyndall was exceptionally fortunate in getting so easily and so early into the friendship of such eminent men of science. In those days to form such eminent acquaintances was no small achievement for a young Irishman; but on the other hand, he had fully earned this distinction by the vigour and originality with which he attacked the latest and most perplexing problem of that time. During the five years that had elapsed since Faraday discovered diamagnetism, the subject had been investigated by the greatest scientists in England, France, and Germany, and no one had done so much to elucidate it as Prof. Tyndall. In order to master that subject he began in November, 1850, an investigation of the laws of magnetic attractions. The laws of magnetic action at distances in comparison with which the thickness of the magnet vanishes, had long been known, but the laws of magnetic action at short distances, where the thickness of the magnet comes fully into play, had not previously been subjected to reliable experiments, and were therefore at that time a perplexing matter of speculation. That desideratum he now supplied. He found, among other things, that the mutual attraction of a magnet and a sphere of soft iron, when both are separated by a small fixed distance, is directly proportional to the square of the strength of the magnet, and that the mutual attraction of a magnet of constant strength and a sphere of soft iron is inversely proportional to the distance between them.

Next year (1851) he published the results of further investigations into the relations between magnetism and diamagnetism. He found that the laws which govern magnetism and diamagnetism are identical, that the superior attraction or repulsion of a mass in any particular direction is due to the direction in which the material particles are arranged most closely together, that the forces exerted are attractive or repulsive according as the particles are magnetic or diamagnetic, and that this law is applicable to matter in general.

A paper on “The Polarity of Bismuth,” which might be regarded as a temporary instalment of his diamagnetic researches, ended with the remark that during this inquiry he had changed his mind too often to be over-confident now in the conclusion at which he had arrived. Part of the time he was a hearty subscriber to the opinion of Faraday that there existed no proof of diamagnetic polarity; and if, he said, “I now differ from that great man, it is with an honest wish to be set right, if through any unconscious bias of my own I have been led either into errors of reasoning or mis-statements of fact.”

The theory of diamagnetism was still an apple of discord in the scientific world; and although Prof. Tyndall used the language of deference rather than of doubt, he did not allow the subject to remain in a state of uncertainty. He continued his researches in Berlin, in the private laboratory of Prof. Magnus, who afforded him every possible facility for carrying on experiments, and took a lively interest in the investigation. The result was the confirmation of his previous impression that the action of crystals within the range of a magnet’s influence (technically called the “magnetic field”) was due to peculiarities of molecular arrangement. He found, for example, that a crystal of carbonate of iron, which, when suspended in the magnetic field, showed a certain deportment, could be pounded into the finest dust, and the particles could be so put together again that the mass would exhibit the same deportment as before.

Dr. Bence Jones, the Secretary of the Royal Institution, who had heard of Tyndall in Berlin in 1851, afterwards invited him to give a Friday evening lecture at the Royal Institution. “I went,” he says, “not without fear and trembling, for the Royal Institution was to me a kind of dragon’s den, where tact and strength would be necessary to save me from destruction.” The lecture, which was delivered on February 11th, 1853, was “On the Influence of Material Aggregation upon the Manifestations of Force,” and it gave a beautiful and simple exposition of the principles of magnetic and diamagnetic action discovered by himself, the chief being that the line of greatest density is that of strongest magnetic power. In the course of his lecture he pointed out that anything which increases density increases magnetic power; and upon that principle he contended that the local action of the sun upon the earth’s crust must influence in some degree the diurnal range of the magnetic needle, which Faraday, on the other hand, attributed to the modification of our atmosphere by the sun’s rays. While thus endeavouring to upset Faraday’s theory, he concluded by saying: “This evening’s discourse is, in some measure, connected with this locality, and thinking thus, I am led to inquire wherein the true value of a scientific discovery consists? Not in its immediate results alone, but in the prospect which it opens to intellectual activity, in the hopes which it excites, in the vigour which it awakens. The discovery which led to the results brought before you to-night was of this character. That magnet was the physical birthplace of these results; and if they possess any value they are to be regarded as the returning crumbs of that bread which in 1846 was cast so liberally upon the waters. I rejoice in the opportunity here afforded me of offering my tribute to the greatest worker of the age, and of laying some of the blossoms of that prolific tree which he planted at the feet of the great discoverer of diamagnetism.” At the conclusion of the lecture Faraday quitted his usual seat, and crossing the theatre to the corner where the lecturer stood, cordially shook him by the hand and congratulated him on his success. A second lecture was delivered by him on June 3rd, 1853, “On some of the Eruptive Phenomena of Iceland,” and a month later he was unanimously elected Professor of Natural Philosophy in the Royal Institution.

Some years previously he had read in a serial publication an account of Davy’s experiments on radiant heat at the Royal Institution, and he remembered ever after the longing then excited in him to be able to do something of the same kind. Now he was to occupy a position in which he should use, in his own lectures, the same apparatus of which illustrations were given in the magazine article that had fired his youthful ambition. To that position he was promoted on the recommendation of Faraday, and respecting his appointment he himself said: “I was tempted at the time to go elsewhere, but a strong attraction drew me here. It was his (Faraday’s) friendship that caused me to value my position here more highly than any other.”

While the controversy respecting magnetic and diamagnetic hypotheses was still raging, Faraday delivered a lecture at the Royal Institution early in 1855 with the express object of cautioning the investigators of scientific truths against placing too much confidence on any hypothesis. He stated that every year of increased experience had taught him more and more to distrust the theories he had once adhered to; and his present impression with regard to existing Magnetic and Electrical hypotheses was, that they were very unsatisfactory, and that the propounders of them had been following in a wrong track. As an instance of the obstacles which erroneous hypotheses throw in the way of scientific discovery, he mentioned the unsuccessful attempts that had been made in this country to educe magnetism from electricity, until Oersted showed the simple way. He said that the identity of magnetism and electricity had been strongly impressed upon the minds of all: when he came to the Royal Institution, as an assistant in the laboratory, he saw Davy, Wollaston, and Young trying by every way that suggested itself to them to produce magnetic effects from an electric current; but, having their minds diverted from the true course by their existing hypotheses, it did not occur to them to solve the point by holding a wire, through which an electric current was passing, over a suspended magnetic needle—the experiment by which Oersted afterwards proved, by the deflection of the needle, the magnetic property of an electric current.

Such cautions, however, did not deter Professor Tyndall from defending the position he had taken up with regard to magnetism and diamagnetism. He still maintained that the influence of structure was supremely important,—that under the influence of magnetism or electricity a normal diamagnetic bar always exhibits a deportment precisely antithetical to that of a normal magnetic bar; but that, by taking advantage of structure, it is possible to get diamagnetic bars which exhibit precisely the same deportment as normal magnetic ones, and magnetic bars which exhibit a deportment precisely similar to normal diamagnetic ones. He showed numerous experiments before the British Association in support of his contention that the diamagnetic force is a polar one, with a direction opposite to that of the force in ordinary magnetic bodies. Professor William Thomson, who witnessed the experiments, certified the success of every one of them; and stated that Professor Tyndall’s discoveries in this domain of science had cleared away a mass of rubbish and set things in their true light, adding that in many cases he had repeated and varied Tyndall’s experiments, and had found them to be true.

In 1855 he delivered the Bakerian lecture, in which he gave an elaborate account of his latest researches respecting the phenomena of diamagnetism. He was now firmly convinced, he said, that the force that repelled a body was similar in character to that which attracted a body; in other words, that diamagnetic bodies possess the same kind of polarity, but in the opposite direction to that of magnetic bodies. But the opponents of diamagnetic polarity, who were not yet satisfied by the evidence he adduced, said that his experiments were made with electrical conductors in which induced currents could be formed that might account for the attractions and repulsions. Professor Tyndall thought it would tend to settle the question if he were to use a new kind of apparatus that would obviate that objection. He therefore wrote to Professor Weber, of Göttingen, whom Professor William Thomson described at the time as the most profound and accurate of all experimenters, asking him to devise more delicate and powerful means than had hitherto been used in experimental tests. Weber not only devised a greatly improved apparatus, but had it constructed under his own superintendence at Leipsig.[2] With this apparatus Professor Tyndall was able to satisfy the severest conditions proposed by those who discredited the results of previous experiments. He then silenced doubt by demonstrating that magnetism and diamagnetism stand, in respect of polarity, on the same footing, with this difference, that the one polarity is the inversion of the other. This diamagnetic polarity, previously established in the case of bismuth, he showed to exist in slate, marble, calcspar, sulphur, &c. He also established the polarity of liquids, magnetic and diamagnetic. At the Royal Institution in February, 1856, he showed that prisms of the same heavy glass as that with which Faraday discovered the diamagnetic force, behaved under the magnet in the same way as bismuth; and this evidence was admitted to be conclusive by the opponents of diamagnetic polarity. The controversy thereafter subsided.

His chief papers recording his most important investigations in connection with diamagnetism were afterwards collected into a volume entitled Researches on Diamagnetism and Magnecrystallic Action.

In 1855 Professor Tyndall was appointed Examiner under the Council for Military Education, and an incident which occurred shortly afterwards illustrated the confidential relations into which his intimacy with Faraday had ripened, as well as the independence of character which distinguished both. Being strongly impressed with the advantage of increasing the knowledge of physical science given to artillery officers and engineers, Professor Tyndall advocated a more liberal recognition of scientific attainments in their examinations. At that time a committee of the British Association was endeavouring to get the British Government to recognise the claims of science; and in reply to inquiries made by that committee as to the expediency of offering inducements for the acquisition of science and of offering orders and decorations as rewards for proficiency, Professor Faraday said: “I cannot say that I have not valued such distinctions; on the contrary, I esteem them very highly; but I don’t think I have ever worked for, or sought after, them.” Lord Harrowby, in his address as President of the British Association, said that the State had till recently done absolutely nothing for the promotion of science; and it was remarked as a strange circumstance that though there were then in the Cabinet the President and President-elect of the British Association, it was considered too hazardous to apply to the Government for money for scientific purposes. While this neglect of science was being freely discussed a number of well-instructed young men were sent from Trinity College, Dublin, to compete at the Woolwich examinations in 1856 for appointments in the artillery and engineers, and their scientific knowledge appeared so creditable that Professor Tyndall thought it unnecessary to say anything about it. His colleagues, on the other hand, sent in as usual brief reports with their returns calling attention to the chief features of the examination, and a leader in the Times pointed out that the concurrent testimony of the examiners was that, both in mathematics and classics, the candidates showed a marked improvement, but that on other points they broke down. This appeared to Professor Tyndall an unjust reflection upon their scientific attainments, which were thus ignored. He accordingly wrote to the Times simply stating that “in justice to the candidates for commissions in the artillery and engineers examined by me in natural philosophy and chemistry, you will perhaps permit me to state that the general level of the answers in the last examination was much higher than that attained in the first; many of the papers returned to me gave evidence of rare ability, and if during their future career the authors of these papers continue to cultivate the powers which they have shown themselves to possess, they will, I doubt not, justify by their deeds the high opinion entertained of them.” This modest statement, intended to put the students right, put himself wrong. The Secretary of State for War promptly informed him that an examiner appointed by the Commander-in-Chief had no right to appear in the public papers as Professor Tyndall had done without the sanction of the War Office. To this reproof he at once wrote a firm but respectful reply, which, however, he submitted to Faraday before despatching it. Faraday pointed out that the consequence of sending such a reply would be dismissal. Professor Tyndall said he knew that, but he would not silently accept the reproof of the War Office. “Then send the reply,” said Faraday; and it was sent. Henceforth Professor Tyndall was in daily expectation of receiving his discharge. After a delay, the length of which surprised him, he received a reply, the contents of which still more surprised him. His explanation was “deemed perfectly satisfactory” by the Secretary for War, and he therefore continued for many years afterwards in the service of the Council for Military Education.

One of the next subjects that occupied his attention was the cleavage of slate rocks. It is a question of great importance in connection with geological problems, and hitherto only speculative solutions had been offered of what appeared to be one of the most mysterious but grandest operations of nature. For twenty years previously geologists were mostly content to accept on trust the suggestion of Professor Sedgwick, that crystalline forces had rearranged whole mountain masses so as to produce a beautiful crystalline cleavage. In 1854 Professor Tyndall visited the quarries of Cumberland and North Wales, where the question of cleavage came prominently before him. When at Penrhyn Quarry he was told that the planes of cleavage were the planes of stratification lifted up by some convulsion into an almost vertical position. But a little observation satisfied him that this view was essentially incorrect; for in certain masses of slate in which the strata were distinctly marked, the planes of cleavage were at a high angle to the planes of stratification. A little experiment, he said, demonstrated that the cleavage of slate was no more a crystalline cleavage than that of a hayrick. An elaborate examination of all the conditions of the phenomena led him to the conclusion that cleavage was the result of pressure, and that this effect of pressure was not confined to slates. In a lecture delivered in 1856 he stated that for the previous twelve months the subject had presented itself to him almost daily under one aspect or another. “I have never,” he said, “eaten a biscuit during this period in which an intellectual joy has not been superadded to the more sensual pleasure, for I have remarked in all such cases cleavage developed in the mass by the rolling-pin of the pastrycook or confectioner. I have only to break these cakes and to look at the fracture to see the laminated structure of the mass.” He exhibited some puff-paste baked under his own superintendence, and explained that while the cleavage of our hills was accidental, in the pastry it was intentional.

Among those who heard the lecture upon slaty cleavage was his friend Professor Huxley, who suggested that probably the principles then enunciated might account for the structure of glaciers, another subject that had long perplexed scientific observers. The greatest authority on glaciers at that time was Professor J. D. Forbes, of Edinburgh University, who in 1842 declared that a “glacier is an imperfect fluid or viscous body, which is urged down slopes of a certain inclination by the mutual pressure of its parts,” and who detected in glaciers a veined structure which he explained as fissures produced by particles of ice in motion sliding past each other, leaving the fissures to be filled with water and to be frozen in winter. On examining the published observations of Forbes, Professor Tyndall was struck with the probable accuracy of Professor Huxley’s suggestion, and in order to examine the matter more thoroughly, the two advocates of the cleavage theory arranged to visit together the glaciers of Grindelwald, the Aar, and the Rhone. This personal investigation and subsequent reflection confirmed Professor Tyndall in his views. He found that glaciers were formed by the property of ice which Faraday called regelation; that is, the freezing together of two pieces of ice by simple contact and slight pressure. It is the same property that enables boys to make snowballs and snow men when the snow is beginning to melt, or when the warmth of the hand raises its temperature to the point at which regelation takes place. Professor Tyndall found that when two confluent glaciers united to form a single trunk, their mutual pressure developed the veined structure in a striking degree along their line of junction. In his lectures on the subject at the Royal Institution he ingeniously illustrated the processes of Nature which make and unmake the glacier. To show that ice only becomes compressed into a solid mass at a temperature near that of freezing water, he cooled a mass of ice by exposing it to the action of the coldest freezing mixture then known. He then crushed this cooled mass of ice into fragments, and applied pressure to the fragments for the purpose of making them cohere, but they did not show the slightest cohesiveness. Very different was their action when their temperature was raised to the freezing point. When placed in a wooden cup and pressed by a hollow wooden die a size smaller than the cup, the pieces of ice became united into a compact cup of nearly transparent ice. Glaciers, he contended, were formed by a similar operation. As particles of snow or ice descend the mountain side, the pressure becomes sufficiently great to compress the particles into a mass of solid ice, which eventually assumes the magnitude of a beautiful glacier. He observed that in the laboratory of Nature it was exactly at the places where squeezing took place that the cleavage of the ice was most highly developed. In fact, he said, the association of pressure and lamination was far more distinct in the case of the glacier than in the case of the slate rock, and as it was now known that pressure caused the lamination of slate rock, he contended that it was the same cause that produced like effects in glaciers.

In a lecture delivered early in 1858, he gave an account of some beautiful phenomena of the glacier. In the preceding September and October he examined the effect of sending a beam of radiant heat through a mass of ice. When sunbeams condensed by a lens were sent through slabs of ice, the path of the beam was instantly studded with lustrous spots like brilliant stars, and “around each the ice was so liquefied as to form a beautiful flower-shaped figure, possessing six petals. From this number there was no deviation. At first the edges of the liquid leaves were clearly defined: but a continuance of the action usually caused the edges to become serrated like those of ferns. When the ice was caused to move across the beam, or the reverse, the sudden generation and crowding together of these liquid flowers, with their central spots shining with more than metallic brilliancy, was exceedingly beautiful.” By means of the electric light and a piece of ice prepared for the purpose he was able to exhibit these lovely ice-flowers to a delighted audience at the Royal Institution.

During the years 1857 and 1858 Professor Tyndall continued his observations of glacier phenomena amid the solitude of the Alps. In the summer of the latter year he betook himself to the mountains with the view of settling once for all “the rival claims of the only two theories, which then deserved serious attention, namely, those of pressure and of stratification.” Again his former views were completely confirmed. It is difficult, he said, to convey in words the force of the evidence which the glacier of Grindelwald presents to the mind of the observer who sees it; it looked like a grand laboratory experiment made by Nature herself with special reference to the point in question. The squeezing of the mass, its yielding to the force brought to bear upon it, its wrinkling and scaling off, and the appearance of the veins at the exact point where the pressure began to manifest itself, left no doubt on his mind that pressure and structure stood to each other in the relation of cause and effect.

The conclusions at which he arrived as to the structure and movement of glaciers brought him into collision with Professor Forbes, whose views, enunciated fifteen years previously, were then widely accepted as the most scientific exposition of the subject. Forbes seemed rather sensitive about his own theory, and complained that he had to some extent been misrepresented. But in the conflict of opinions Professor Tyndall invariably referred to Professor Forbes’s labours in connection with the subject in the most appreciative and complimentary language. For instance, in 1858 he said he would not content himself with saying that the book of Professor Forbes was the best that had been written upon the subject; “the qualities of mind, and the physical culture invested in that excellent work, were such as to make it, in the estimation of the physical investigator at least, outweigh all other books upon the subject taken together.” That is more generous language than Professor Forbes ever used respecting Professor Tyndall. In 1865, after the heat of controversy had been dissipated, Forbes wrote that “Dr. Tyndall’s so-called proofs that it is through ‘fracture and regelation’ that a glacier moulds itself to its bed are to my mind no proofs at all;” and that he regarded Mr. Hopkins’s mathematical demonstrations about glaciers as “irrelevant mathematical exercitations.” Nevertheless, Professor Tait, the friend and scientific biographer of Forbes, said in 1873: “To say that Forbes thoroughly explained the behaviour of glaciers would be an exaggeration; but he must be allowed the great credit of being the Copernicus or Kepler of this science.” As the subject still continues to exercise the intellect of the scientific explorers of the Alps, suffice it for the present to say that if time ratifies the position which Professor Tait has assigned to Professor Forbes, his greatest and boldest successor in the same field may be described as the Newton of glacier phenomena.