Triumphs and Wonders of the 19th Century: The True Mirror of a Phenomenal Era / A volume of original, entertaining and instructive historic and descriptive writings, showing the many and marvellous achievements which distinguish an hundred years of material, intellectual, social and moral progress

The science of chemistry, as it is known to-day, had its real origin towards the end of the eighteenth century. Before and up to that time it is true there were many great workers in chemistry, whose names are associated with investigations in chemical science, such as Boyle, Stahl, Black, and Scheele. Contemporary with the close of the eighteenth century and the beginning of the nineteenth must also be mentioned particularly the names of Priestly (1733–1804), Cavendish and Humphry Davy (1778–1829). All these workers had to contend, first of all, with erroneous theories, which made it difficult to rightly interpret the data of experiment. The old theory of phlogiston produced an environment in which it was difficult for true scientific methods to survive. The great investigator, who did more than any other one man to overturn this false theory and place chemistry on a firm foundation, was Lavoisier (1743–1794). Born near the middle of the eighteenth century, his scientific activity began about 1770, and before he was twenty-five he was made a member of the French Academy of Sciences. At the age of forty he was recognized as the foremost scientist of his age.

Priestly discovered oxygen in 1774, but failed to recognize its true relations to other bodies. It was Lavoisier who discovered oxidation (1776), an achievement which meant more to chemistry than the discovery of oxygen.

The observation that metals when heated in confined air increased in weight while the volume of the confined air decreased, is the crucial experiment upon which the whole science of chemistry rests. This experiment was made most rigorously by Lavoisier, and the apparatus which he used is still preserved in the Museum of L’École des Arts et Métiers in Paris. This apparatus, simple in character and yet almost perfect in construction, has for the chemist a peculiar significance and sacredness, producing an impression similar to that inspired in the devout Christian by the relics of the Cross and the Holy Sepulchre.

In the brief space which is assigned for a discussion of the progress of chemistry during the nineteenth century, economy of words will be secured by briefly tracing some of the salient points in the progress of some of the more important branches of chemical science. In the following pages, therefore, will be found a brief statement of what has been accomplished, of the most important character, in the science of chemistry, under the following heads:—

Inorganic chemistry; physical chemistry; organic chemistry; analytical chemistry; synthetical chemistry; metallurgical chemistry; agricultural chemistry; graphic chemistry; didactic chemistry; chemistry of fermentation; and lastly electro-chemistry.

No attempt will be made in this paper to enter upon the discussion of the progress which has been made in medical, pharmaceutical, and physiological chemistry. The discussion outlined under the above heads does not by any means embrace the whole subject. It will be sufficient to indicate only the lines of progress along which the greatest advances have been made.

I. INORGANIC AND PHYSICAL CHEMISTRY.

The three propositions established by Lavoisier, which serve as the foundation for inorganic and physical chemistry, are the following:—

1. Bodies burn only in contact with pure air.

2. The air is consumed in the combustion, and the increase in weight of the burnt body is equal to the decrease in weight of the air.

3. In combustion the body is generally changed, by its combination with the pure air, into an acid, and metals are changed into metal calx.

The total number of elementary bodies known at the beginning of the century was probably less than thirty. Many had been recognized as such since remote antiquity, but none of the non-metallic elements, except oxygen and sulphur, was known, and even their properties were not established with any degree of precision.

Not only did Lavoisier establish the fundamental principles of modern chemistry, but in connection with Fourcroy (1755–1809), Berthollet (1748–1822), and Guyton de Morveau (1737–1816), laid the foundation of modern chemical nomenclature.

The contributions to chemical knowledge at this time were greatly increased by the works of the Swedish chemist, Scheele (1742–1786), and in the beginning years of the century the great work which was accomplished by Sir Humphry Davy advanced very rapidly the general knowledge of chemical science.

Davy’s first works served to elucidate the connection between electricity and chemical processes, and it was through the classical experiment with an electric current that he isolated (1807) the metals sodium and potassium, and described their properties.

This achievement of Sir Humphry Davy’s was the second great step in the progress of chemistry, after the one taken by Lavoisier. By means of the metals sodium and potassium other metallic elements were separated, notably aluminium by Wöhler (1845). Basing his work upon the above experiment, Sainte Claire Deville developed the metallurgy of aluminium (1854), and Bussy isolated magnesium (1830).

In 1811 iodine was discovered by Courtois, and its properties examined simultaneously (1814) by Davy and Gay-Lussac.

The contributions made by Berzelius (1779–1848), who was a contemporary of Davy and Gay-Lussac (1778–1850), were of the most important character. Berzelius not only added to the knowledge of inorganic chemistry but also established many of the important theories on which chemical action depends. His elaboration of the employment of the blowpipe in chemical analysis was of the greatest practical value.

In 1807 Dalton published a work entitled “New System of Chemical Philosophy,” in which was announced for the first time the law of the definite proportions of bodies forming a definite union. The atomic theory of matter was also developed by Dalton, who gave it a definite form and expression. Chemists now began to consider the elements as definite indestructible particles of matter, forming unions among themselves and with different kinds of atoms to form molecules, which were considered as the units of substances. As a result of this supposition, the development of the principle of the relative weight with which bodies combine was the logical consequence.

Now for the first time the elements began to assume not only names and descriptions of properties but also numbers, showing the relative weight of their atoms or final conditions of existence. It was only necessary, therefore, to assume the standard of comparison for any one element, in order to determine the relative weights with which it combined with others. Thus the system of atomic weights was developed.

As a result of the law of chemical action, that most elementary bodies exist in a condition where two atoms are joined together to form a molecule, it follows, that in most instances the molecular weights of the elements are double their atomic weight. There are, however, many notable exceptions to this rule.

The supposition of the existence of atoms was followed soon by another theoretical proposition, advanced by Prout (1815). Assuming that the atomic weight of hydrogen was one, Prout’s hypothesis asserted that the atomic weights of all other elementary bodies were multiples of that of hydrogen. The most rigid investigations of recent years have shown that Prout’s hypothesis is untenable; but the remarkable fact still remains, that in a great many cases the atomic weights of the elements are almost whole numbers, or differ from whole numbers by almost a half unit.

The determination of the atomic weights of the various elements during the past one hundred years has been worked on by hundreds of chemists whose names it would be impracticable to mention. The most important of them are Berzelius, Cooke, Cleve, Delafontaine, Dumas, Hermann, Marchand, Marignac (1817), Morley, Noyes, Pelouse (1807–1867), Richards, Schneider, Stas (1813–1891), and Thompson. Of all these workers Stas, a Belgian chemist, is perhaps the most renowned. Among those mentioned, Cooke, Morley, Noyes, Delafontaine, and Richards are citizens of the United States.

From the less than thirty elements which were known at the beginning of the century, there are known to-day seventy-two with certainty, and perhaps one or two more whose identity has not yet been fully established. The chemists who have become most renowned by the discovery of elementary bodies are: Cavendish, Scheele, Berzelius, Wöhler (1800–1882), Davy, Gay-Lussac, Priestly, Bunsen (b. 1811), Crookes (b. 1832), and Ramsay.

The following elements, twenty-eight in number, were known before 1800:

Four additional elements were known to exist before that date, but they had not been isolated and identified. These are:—

The following elements, forty-nine in number, have been discovered since 1800:—

The date in each case is that of the discovery. Numbers 49, 50, and 51 are not yet sufficiently well known to justify being considered elements, and are therefore properly followed by an interrogation point.

II. PHYSICAL CHEMISTRY.

In strictly physical chemistry the relations of electricity and heat to chemical action have been extensively developed during the century. The specific heats of the elements and of most of their compounds have been carefully determined, and thermo and physical chemistry under the leadership of such master minds as Berthollet, Thompson, Van’t Hoff, and Ostwald have been brought to the highest degree of perfection.

The chemist now does not consider that he knows any body until he knows thoroughly its relations to heat and to electricity. The action of light must also be included, but this subject will be more thoroughly discussed under graphic chemistry.

The nature of solutions has also been developed by the studies of Ostwald and Van’t Hoff, and as a result of these studies, a flood of light has been thrown upon the constitution of compound bodies.

In the development of physical chemistry, attention should be directed to the help afforded by Newlands (1864) and Mendelejeff (1869) and others, showing that the elements form groups which tend to recur with a periodicity which is sufficiently definite to enable the investigator to foretell to some extent the properties of the elements which have never yet been discovered, and whose existence is necessary in order to fill up the gaps in existing groups.

By this method the existence, atomic weight and properties of scandium, gallium, and germanium were foretold years before their discovery. Such actual realization of a scientific-prophetic method is one of the strongest indications of the basis of fact upon which it rests. Although a rigid application of the principles of the periodic law is not possible, yet its discovery and elaboration mark one of the great forward steps of chemical philosophy.

If we regard any material system by itself, i.e., independently of any other system or influence by which it may be surrounded, we recognize it as consisting of essentially two things,—matter and energy. A precise definition of either matter or energy is difficult, if not impossible; but what is connoted by these names is sufficiently well understood by their well-known properties. Both energy and matter are essential to each and every system. They are coexistent. In the light of human experience, we cannot conceive of one existing without the other; and in the study of any material system, consideration of one of these components without the other can only be regarded as incomplete. But, for the sake of convenience, this has been the practice, and, generally speaking, chemists have concerned themselves with matter changes of equilibria, while physicists have more especially directed their attention to energy equilibria. The object of the physical chemist is to follow equilibria changes in given systems, having due regard for both the matter and energy involved.

Berthollet may be regarded as the first true physical chemist, on account of his classical views on mass action. Largely because the time was not ripe for it, his views were not generally adopted.

A quarter of a century later (1867), Guldberg and Waage gave a precise mathematical expression of the law, but still it attracted very little attention from investigators. A tremendous impetus was given to the subject by the electrolytic dissociation theory of Arrhenius (1887), and the extension of the additive laws of gases to dilute solutions, by Van’t Hoff (1885). This was but a comparatively small field in the subject, but it stimulated activity along the whole line, the wonderful increase of our knowledge concerning the velocity or rates of reaction, the heat changes involved, and the marvelous development of electrolytic chemistry being pertinent instances.

The generalization of Gibbs, known as the phase rule (1876), which accurately states the condition for equilibrium in the system, and the Theorem of Le Chatelier (1884), that any change in the factors of equilibrium from outside is followed by a reverse change within the system, together with the mass law, now give us a consistent theoretical foundation for the subject. In general terms, it may be said that all chemistry, at least all theoretical chemistry, properly belongs to the province of physical chemistry, and the title, while in many ways convenient, is misleading.

III. ORGANIC CHEMISTRY.

Compounds containing carbon enter into all the products of a living cell. For this reason the chemistry of carbon compounds came to be known as organic chemistry. This should not be taken as a definition, however, without limitations. Many of the compounds containing carbon are not known to enter into living tissue in any way, and their connection with it is very remote and not essential. On the other hand, it should be remembered that many organic compounds, and those even of most importance, contain some other element,—nitrogen, for example,—as the significant one.

While nearly all the known elements can enter into organic compounds, the vast majority of such substances are composed of but very few. For instance, those classes of which sugar, starch, the fats, etc., are examples, contain only carbon, oxygen, and hydrogen. With nitrogen, sulphur, and phosphorus added to these elements, almost the entire range of organic chemistry is covered. Organic chemistry, therefore, differs from inorganic chemistry in that, while the number of compounds is much larger, the number of elements involved is very limited.

Berzelius may be regarded as having founded organic chemistry in the beginning of this century. As a result of his analyses of the salts of organic acids, he clearly demonstrated that the laws of definite and multiple proportions hold equally for organic compounds and for inorganic ones. The work of this master was ably furthered by Liebig (1803–1873), who devised most elegant methods for the analytical investigation of organic compounds, methods which are in use to-day without any essential change.

Very soon, however, it was found that organic compounds existed having the same percentage composition, but quite dissimilar properties, physical and chemical, as, for instance, sugar and starch. Other striking examples are Faraday’s discovery (1825) of a compound identical in composition with ethylene, but wholly different in properties; and Wöhler’s classical synthesis (1828) of urea by the transformation of ammonium cyanate. Similar facts in the domain of inorganic chemistry, though now well known, were at that time wanting, and thus this most fruitful idea, designated as isomerism, was introduced into the science.

The next great step was the introduction of the theory of radicles, first suggested tentatively by Berzelius (1810), but put forward in a definite way as one of the results of the classical investigation on benzoyl by Liebig and Wöhler (1832). That is to say, a group of elements, or radicle, can pass through a series of compounds, from one to the other, as though the group were one single element. For years this idea was the guiding principle in chemical investigations, and was most useful in aiding the classification of chemical compounds and bringing order out of the chaos of accumulating observations.

But the search for radicles was in a sense a vain one. We now know that no radicle exists as such by itself. Meanwhile, Dumas and his pupil Laurent had introduced and developed the theory of types, whereby all chemical compounds could be classified under four types, which marked a distinct step in advance. Laurent, together with his colleague Gerhardt (1816–1856), recognized the shortcomings of both the radicle and type theories in their earlier forms, and showed their inter-relation, when modified so as to do away with certain inconsistencies.

Dumas had before this demonstrated the theory of substitution (1834),—that is, that in certain compounds one or more of the elements can be driven out and replaced by others without changing the essential characteristics of the compound. For instance, chloracetic acid, in which part of the hydrogen of acetic acid has been replaced by chlorine, contains all the essential characteristics of acetic acid; in fact, some of them—its acidic properties, for example—being markedly accentuated. This theory was fiercely assailed at first, notably by Liebig. Like all theories of science, it was in the beginning pushed to the extreme, and put forward to explain things to which it was not applicable. It gradually came to demonstrate its own right to existence, largely as a result of the work of Laurent and Gerhardt, and made its influence felt in the exposition of their ideas, to which reference has just been made.

The development of these theories, about the middle of the century, was greatly hastened by the work of many brilliant investigators, notably Wurtz (1817–1884), Hofmann (1818–1892), Williamson (1824–), Kolbe (1818–1884), and Frankland (1825–) among others.

Kekulé proposed a new type, marsh gas or methane. Shortly afterwards, his well-known formula for benzene, the starting-point and foundation of the vast class of aromatic bodies, was proposed. He insisted that the time had come when chemists must ask what those ultimate particles, or atoms, of the elements themselves were doing in these compounds of various types. The answer was a grand one, and the result, our magnificent store of information concerning the constitution of organic compounds, or the way in which the atoms are connected with each other. It is not to be inferred that our knowledge on this subject, in any one case, is complete. Far from it! Much that is most interesting and important is apparently as remote from our grasp as ever. But we do know something about the general relations of the atoms in the molecule, and our knowledge, so far as it goes, is definite and precise.

Somewhat later, Van’t Hoff and Lebel, at the same time but independently, introduced the study of the space relations of organic compounds by suggesting the simplest possible space formula (the tetrahedron) for marsh gas or methane, of which all other organic compounds may, theoretically at least, be regarded as derivatives. Many inexplicable relations, especially among isomers, now became clear. The theory was at first bitterly assailed, especially by Kolbe. It found an able champion in Wislicenus (1838–), however, and has so thoroughly established itself, that it may be safely said that at the present day it is the controlling idea in the large majority of organic investigations.

The carbon atom is characterized by a wonderful facility in uniting not only with other elements, but with itself. It would even appear as though its influence in this regard extended to other elements united with it, as nitrogen, for instance, shows an unexpected ability to unite with nitrogen in organic compounds.

Further, the carbon atom is characterized by an unusually constant valency, namely, four. These two characteristics account for homology, that is, for a series of similar compounds differing in composition one from the other by—CH₂, and enables us to trace back all organic compounds to one mother substance—marsh gas or methane.

These ideas have also been more or less successfully applied to the study of the composition of inorganic compounds. The assistance organic chemistry has given to the general subject is incalculable. Finally, it may be said, that while in the nature of the case our ideas of structure in organic compounds cannot be regarded as proved, or as not subject to possible future modifications, we have, at least, a consistent theory and good working hypothesis. A homely illustration of our present ideas may be drawn from the modern high city building. The skeleton of this building is made of iron, about which are grouped the brick, stone, wood, and other materials to form a complete building. So the organic body is built on a chain or frame-work or skeleton of carbon atoms, about which are grouped the atoms of hydrogen, oxygen, and nitrogen, or radicle compounds thereof.

It is not possible here to even name some of the more eminent workers who for a quarter of a century have contributed to our knowledge of organic chemistry. This branch of chemistry has been the vogue, and has been pushed almost to the limit of possibility since 1875. Many almost unexplored fields still remain, but chemists recognize the fact that in theory and practice organic chemistry has reached a high degree of perfection, and they are returning to continue the researches in other fields which have for so long been almost neglected.

IV. ANALYTICAL CHEMISTRY.

No branch of chemical science has a more general interest for the public than that which relates to the determination of the materials of which bodies are composed, and the proportions in which they exist.

At the beginning of the century considerable progress had been made in this branch of knowledge by the researches of Boyle (1626–1691), Hoffmann, Margraff (1709–1780), Scheele and Bergmann (1735–1784). Berzelius, as has already been mentioned, had added a new and valuable factor to chemical analysis by the development of the blowpipe, and in the early part of the century mineral analysis was still further advanced by Klaproth (1743–1817), Rose (1798–1873), and many others.

No one man did so much to advance this branch of chemical science as Fresenius (1818–1897). He collated and proved all the proposed methods of analysis, both qualitative and quantitative, and out of a confused mass of material formed a logical system of procedure, which has proved invaluable to the progress of chemical science in all its branches.

The volumetric methods of analysis, which save so much time and labor without sacrificing accuracy, were developed by Gay-Lussac, Vauquelin (1763–1879), Mohr (1806–1879), Volhard, Sutton, Fehling, and Liebig.

The methods of gas analysis have been worked out chiefly by Bunsen, ably assisted by Winkler and Hempel.

The methods of determining the elementary bodies in organic compounds have been developed by Dumas, Liebig, Will, Varrentrap, and Kjeldahl, to the last of whom chemical analysis owes a debt of gratitude for the invention of a speedy and accurate method of determining nitrogen.

Not much less is the debt due to Gooch for the invention of the perforated platinum crucible, carrying an asbestos felt for securing precipitates by filtration, in a form suitable to ignition without further preparation.

Through the classic researches of Arago (1786–1853) and Biot (1774–1862), polarized light has been made a most valuable adjunct to chemical research, serving as it does to measure the quantity of various alkaloids, essential oils, and sugars.

Based on these researches of Biot and Arago, Ventzke, Soleil, Scheibler, Duboscq, Landolt, and Lippich have constructed apparatus, which have made an exact science of optical saccharimetry. Optical analysis is not without its relation to theoretical chemistry, for by it has been proved the assumption that optically active bodies contain an asymmetrical carbon atom,—that is, one which combines with four different atoms or radicles.

Electricity has become also one of the most useful factors in chemical analysis. Many metals are easily deposited by electrolytic action, and their separation and determination rendered easy and certain.

Chemical analysis has not only given us accurate knowledge of the constituents of matter, but by revealing the deportment of molecules and groups of molecules in inorganic and organic compounds, has opened up a path for organic and synthetic chemistry which otherwise must have remained forever closed.

The discovery and development of spectrum analysis is one of the great achievements of the nineteenth century in chemical science.

Wollaston, in 1802, first noticed that the spectrum of the sun’s light, when greatly magnified, was not composed of colors gradually changing from one to the other, but that the continuity of the colors was interrupted by dark bands. Fraunhofer, in 1814, had made a map of the solar spectrum, showing 576 of these dark lines. Fraunhofer was entirely ignorant of the cause of these dark lines, but when he had found them, not only in the light from the sun, but also from the moon and the fixed stars, he properly concluded that they were due to something entirely independent of the earth.

It remained for Bunsen and Kirchhoff, in 1860, to point out the fact that these dark lines were characteristic of certain chemical elements existing in the sun and its photosphere, and this fact is the foundation of spectrum analysis. The broad black band in the sun’s spectrum, called by Fraunhofer D, corresponded exactly in position and in width with the yellow band produced by a flame containing incandescent sodium. There was no doubt whatever, therefore, that the two phenomena were due to the same cause; but why in the one case should the band be black and in the other yellow? This question was answered by the discovery of the fact that a ray of light colored by incandescent sodium, passing through a luminous atmosphere of the same metal, would lose by absorption all of its yellow color, and would display a black band where before the yellow color existed.

Based upon this observation, the development of spectrum analysis went forward with amazing rapidity. The hundreds of lines in the sun’s spectrum were found to occupy exactly the position of luminous lines in the spectra of various metals, and thus it was possible for the chemist to extend his investigations beyond the limits of the earth, and distinguish the chemical elements in the sun and in the fixed stars billions of miles farther away from us than the sun itself. Celestial chemistry has thus become a fixed and definite science.

But the value of spectral examinations has extended still farther. Many luminous lines were observed in the spectrum which were not found in the spectra of any known element. The inference then logically arose that there were elements yet undiscovered to which these lines were due. From this starting point investigations proceeded which have led to the discovery of a large number of elementary bodies. Among the important elements that have been discovered by means of spectrum analysis may be mentioned: cæsium, rubidium, thallium, indium, gallium, ytterbium, and scandium.

Spectrum analysis is also extremely useful in proving the verity of supposed new elements; for if a supposed new element should be found to give a series of spectral lines coincident with those already known, it would be a positive proof of the fact that the supposed new element was but a mixture of bodies already known to exist.

V. SYNTHETICAL CHEMISTRY.

This branch of chemical science has for its object the building up of the more complex from the simpler forms of matter. In the early part of the century, Chevreul and Wöhler laid the foundation of the science by the synthesis of fatty-like bodies and urea. Berthellot and Friedel (1832–) in France, and Williamson and Frankland in England, added much to our knowledge. Kolbe, in Germany, made salicylic acid so abundantly as to banish the natural article from the market. The synthesis of coloring matters resembling indigo was also a great blow to that industry. From the products of the distillation of coal, chemists were able to make thousands of valuable bodies of the greatest utility. Many medicinal substances and nearly all the common dyes trace their origin to coal.

Fischer (b. 1852), in Germany, has contributed his remarkable results in the synthesis of sugar to the last years of the century. Lillienfeld, in Austria, has gone still further, and has built up a body which has many of the properties of protein, one of the most highly organized of organic substances.

In the inorganic world synthesis is not so difficult a matter as the vast number of compounds attest. By means of the electric furnace, Moissan, in France, has succeeded in uniting carbon with many of the metallic elements, and thus opened the path for new achievements in passing directly from inorganic to organic compounds.

The progress of chemical synthesis has already blotted out the old distinction between inorganic and organic chemistry, and we can no longer say of organic bodies that they are the products of living cells. Organic bodies are those which contain a carbon or other elementary skeleton, to which are attached the elements or groups of elements forming the complete body.

The claim which has been made that synthetical chemistry would in the near future produce the food of man, and thus relegate agriculture to the domain of the useless or forgotten arts, is, however, wholly without scientific foundation. The function of the farmer will not be usurped by the chemist. The future will see the most important contributions to chemistry coming from the field of organic chemistry, but it will also see the farmer following in the furrow, and man depending for his food on the fields of waving grain.

VI. METALLURGICAL CHEMISTRY.

This is the oldest branch of chemical science, and naturally the one which was furthest advanced at the beginning of the century. Nevertheless, the advances which the past one hundred years have seen in this science are most surprising. Gold and silver are now secured from ores so poor as to have rendered them of no value a hundred years ago. The Bessemer process of steel making (1856) has revolutionized the world, and made possible railroads and steamships. The basic Bessemer process of making steel from pig-iron rich in phosphorus, has opened up rich mines of iron ore hitherto valueless. The basic phosphatic slag, resulting from this process, is of the highest value in the fields, and has brought agriculture and metallurgy into intimate relationship. The electric furnace has made aluminium almost as cheap as iron, bulk for bulk, and electric welding bids fair to take the place of the old process, with the cheapening of metals.

VII. AGRICULTURAL CHEMISTRY.

Sir Humphry Davy, in the beginning of the century, delivered a course of lectures on the relations of chemistry to agriculture, and these were published in book form. In France, important contributions were made to agricultural chemical science by Vauquelin, Chevreul (1786–1889), and Boussingault (1802–1887), who made important researches before the middle of the century. The most important work in agricultural chemistry, however, was done by Liebig. His achievements so overshadowed those of his predecessors that he is generally regarded, although improperly, as the father of that branch of the science.

The early achievements of these workers showed the relatively small portions of the crops that were derived from the soil. The study of the ash constituents of plants laid the foundation of rational fertilizing, and the utilization of the stores of plant food preserved in great natural deposits.

Beginning with the middle of the century, the attention of agronomists was called to the desirability of utilizing the deposits of guano, found in the islands along the west coast of South America; of the deposits of phosphate rock existing in many localities; and later, of the potash salts, discovered near Stassfurt, which completed the trio of available natural foods most useful to plants.

The establishment of an agricultural experiment station by Sir John Lawes at Rothamstead (1834), before the middle of the century, set an example which has been followed by the establishment of experiment stations in all the civilized countries of the world.

Under the great stimulus given to agricultural research by these stations, progress during the latter half of the century has been very rapid. There now exist in Europe nearly one hundred stations devoted to agricultural research, and in this country the number is half as great.

Conspicuous achievements, marking the closing years of the century, have been the discovery of the methods whereby organic nitrogen is rendered suitable for plant food, and atmospheric nitrogen fixed and rendered available by leguminous plants. In the first instance, it has been established that organic nitrogen in the soil can only be utilized by plants after it has been oxidized by bacterial action. In the case of leguminous plants, nitrogen is rendered available for nutrition by means of bacteria inhabiting nodules in the roots of the legumes. These two great discoveries have proved of incalculable benefit to practical agriculture. Chemical science in its relations to agriculture has shown that the fertility of the soil may be conserved and increased, while the magnitude of the crops harvested is sustained or augmented. Thus, no matter how rapid may be the increase of population, agricultural chemistry will provide abundant food.

VIII. GRAPHIC CHEMISTRY.

The honor of discovering that prints could be made by the action of light on certain salts, such as those of silver, belongs to Daguerre, in 1839.

The fundamental principle of graphic chemistry is that metallic salts, sensitive to the light, when in contact with organic matter, suffer a complete or partial reduction and are rendered insoluble. The intensity of the reduction is measured exactly by the intensity of the light. When light is reflected from any object capable of producing different degrees of intensity, as from the hair and face of a man, the reduction of the metal is greatest by the light from that portion of the physiognomy which gives the greatest reflection. Thus, when the unreduced metallic salt is washed out, a permanent record, the negative, of the object is left.

It is a long step from the first daguerreotype to the modern photograph, but the principle of the process has remained unchanged.

Photographs in natural colors have of late years been obtained. One method is by interposing a film of metallic mercury behind the sensitive plate which must be transparent. The reflected rays of light, having different wave lengths, precipitate the metal in superimposed films, corresponding to the wave or half-wave length. When a negative thus formed is seen by reflected light, the emergent rays from the superimposed films acting as mirrors are transformed into the original colors of the photographed object.

The various methods of printing by heliotypes, photolithographs, photogravures, etc., are illustrations of the application of graphic chemistry to the arts.

IX. DIDACTIC CHEMISTRY.

The lectures of Davy and Faraday in England, of Wöhler and Liebig in Germany, of Chevreul and Dumas in France, and of Silliman (1779–1864) in this country, made the study of chemistry attractive and easy during the early part of the century.

It was noticed, however, that the students who finished these courses, while well versed in the principles of the science, were not able to apply them in practice. Towards the middle of the century, therefore, a radical change in the system of instruction was inaugurated. The student was put to work and taught to question nature for himself. The universities of France and Germany were equipped with working desks where students of chemistry put into practice at once the principles of the science which they heard elucidated in the lecture room. Cooke, at Harvard, was the chief apostle of the laboratory method in this country, and this method of instruction has now spread, until even the high and grammar schools have their chemical laboratories.

In our universities, students may now begin their chemical studies associated with laboratory practice in the first year of their course, and continue it to the end. Graduates of such courses are not only grounded in the theories of chemistry, but are thoroughly familiar with its practice. Under this system, coupled with the demand for chemical services in every branch of industry, the number of trained chemists has speedily increased. At this time (1899) there are more than four thousand trained chemists in the United States.

X. CHEMISTRY OF FERMENTATION.

Our knowledge of fermentation and bacterial action is practically all comprised in the achievements of the nineteenth century. Prior to this time it was known that fermentation took place, but its causes and character were wholly mysterious. The great work of Pasteur (1859) resulted in the fact that fermentations were chiefly caused by the activity of living cells, which have the capacity of reproduction. The most common form of fermentation is that whereby sugar is converted into alcohol and carbon dioxide. The name of the organism that produces this change is saccharomyces cerevisiae.

Another class of fermentation is seen in the process of digestion. This species of fermentation is typified by the action of sprouted barley on starch, whereby the starch is converted into sugar. The active principle of the saliva, ptyalin, has the same property, and when starchy bodies are masticated, a part, at least, of the starch which they contain is converted into sugar. The active principle of malt is known as diastase, and this, as well as ptyalin, belongs to a class of ferments which are incapable of reproduction.