Popular Science Monthly/Volume 57/May 1900/A Hundred Years of Chemistry II
|A HUNDRED YEARS OF CHEMISTRY.|
By F. W. CLARKE,
CHIEF CHEMIST, UNITED STATES GEOLOGICAL SURVEY.
IT is evident, from what has been already said, that chemistry and physics are near akin—indeed, they can hardly be separated. Avogadro's law and spectrum analysis are but two illustrations of the relationship, but many other examples are equal to them in importance. Take, for instance, the action of light upon chemical substances; it may provoke union of elements, or effect the decomposition of compounds; upon the latter phenomenon the art of photography depends. That salts of silver are chemically changed by light was the fundamental observation, and upon this fact most photographic processes, though not all, are founded. Thus light, working as a chemist in the laboratory of the photographic plate, has become the useful servant of all arts, all sciences, and all industries—an indispensable aid to invention and research. On this theme a volume might be written; a bare reference to it must be sufficient here.
Still another branch of chemistry, recently developed but essentially an extension of the theory of valence, is also due to the study of optical relations. That different crystalline bodies differ in their behavior toward polarized light has long been known, and the polariscope is recognized as an instrument of great value in chemical research. To the analysis and valuation of sugars and sirups it is most effectively applied, and commercial transactions of great magnitude depend in part upon its testimony. Here is practical utility, but the development of theory is what concerns us now.
The discovery of isomerism, of the fact that very different compounds might contain the same elements united in the same proportions, was easily interpreted by the theory of valence in a fairly complete and satisfactory way. In the structural formulæ the different atomic groupings were clearly shown, but with one essential limitation—the arrangement was in a single plane. That is, the linking of the atoms was considered, but not their relations to tri-dimensional space. For the study of reactions, for the classification of compounds, the structural symbols sufficed; but human thought is not so easily satisfied, and more was soon required. One class of isomers was unexplained, and an explanation was demanded.
A typical example of the difficulty was offered by tartaric acid, which exists in two forms differing crystallographically and optically. One form, dissolved in water, twisted a ray of polarized light to the left, the other produced a rotation to the right, while the crystals of the two acids, similar in all other respects, also showed a right- and left-handedness in the arrangement of their planes. The crystal of one variety resembled the other as would its reflection in a mirror—the same, but reversed. These differences, discovered by Pasteur as long ago as 1848, the theory of valence could not explain; to interpret them, and other similar cases, the arrangement of the atoms in space had to be considered.
In 1874 two chemists, Van t'Hoff and Lebel, working independently, offered a solution of the problem, and stereochemistry, the chemistry of molecular structure in three dimensions, was founded. They proposed, in effect, to treat the carbon atom essentially as a tetrahedron, the four angles corresponding to the four units of valence or bonds of affinity. They then studied the linking or union of such tetrahedra, and found that with their aid the formulæ for tartaric acid could be developed in different ways, showing right- and left-handed atomic groupings. Other similar compounds were equally explicable. Thus the definite conception of a tridimensional, geometric atom led to a new development of structural formulæ, from which many discoveries have already proceeded. The fruitfulness of the speculation vindicates its use, but it is only the first step in a method of research which must in time be applied to all of the chemical elements. Probably the study of crystalline form will be connected with these chemico-structural expressions, and from the union some greater generalizations will be born. From the geometry of the crystal to the geometry of the molecule there must be some legitimate transition. With all their utility, our present conceptions of chemical structure are incomplete; they represent only portions or special phases of some great general law, but so far as they go, properly used, they are valid.
But light is not the only physical force involved in chemical changes; heat and electricity are far more important. Heat, in particular, is essential to every chemical operation; it provokes combination and effects decomposition; it appears in one reaction and vanishes in another; apart from thermal phenomena the science of chemistry could not exist. From the very beginnings of chemistry this interdependence has been recognized, and its study has led to notable discoveries and to great enlargements of resource. In the theory of phlogiston the connection between heat and chemical change was crudely stated, and when Lavoisier saw that combustion was oxidation, thermochemistry began to exist.
In every chemical change a definite amount of heat is either liberated or absorbed—a distinct, measurable quantity. This fact was established by Hess in 1840, and since then the thermal values of many reactions have been determined, notably by Thomsen in Denmark and Berthelot in France. The data are already numerous, but as yet they have not been co-ordinated into any general law. They are in great measure the raw material with which some future scholar is to build. One fact, however, is already clear—namely, that the heat of formation or of combustion of any compound is conditioned by its structure. Two isomeric substances may differ widely in their calorific constants, an observation which has repeatedly been verified. Thus the conception of structure, of atomic grouping, appears again a chief factor in a set of unsolved problems.
In the relations of chemistry to heat perhaps the greatest advances have been made in the extension of our resources, particularly in regard to the development and control of temperatures. At the beginning of the century the range of temperatures available to the chemist was narrowly limited—from the freezing point of mercury at one end to the heat of a blast furnace at the other. His command of heat and cold are now vastly greater than then, and the steps which have been taken are worth tracing.
At the lower end of the scale the greatest progress has been made through the liquefaction of gases. When a liquid evaporates, heat is absorbed, or, reversely stated, cold is produced, and the more rapid the evaporation the greater is the cooling effect. A command of more volatile liquids is therefore a command of cold, and the liquefied gases represent the extreme limit of our power in that direction.
Near the beginning of the century, by combining cold and pressure, sulphurous acid and chlorine were reduced to the liquid state. In 1823 Faraday succeeded in liquefying still other gases, and in 1835 Thilorier went even further and reduced carbonic acid to a snowlike solid. Liquid chlorine, sulphurous acid, and carbonic acid, stored in strong cylinders of steel, are now commercial products, manufactured and sold in large quantities like any other merchandise. They can be transported to long distances and kept indefinitely, to the great convenience of chemists and the furtherance of research.
In 1845 Faraday published the results of further investigations, when it appeared that all but six of the known gases had been reduced to the liquid state. Through cold and pressure lower and lower temperatures were gained, each step forward having given a foothold from which a new advance was possible. In 1877 Pictet and Cailletet simultaneously succeeded in liquefying four of the supposedly permanent gases; nitrogen yielded to the attacks of Wroblevsky and Olzewski in 1883, and hydrogen alone remained unconquered. In 1898 Dewar overcame this last obstacle, and in the year following he actually reduced hydrogen to an icelike solid which melts at only eighteen degrees centigrade above the theoretical absolute zero. Every gas has been liquefied, and probably the lowest degree of cold attainable by man has been reached. Within the century the work began and ended; the future can only improve the working methods and utilize the new resources. Liquid ammonia has long been used in the manufacture of artificial ice and for direct refrigeration; liquid air, with its temperature of two hundred centigrade degrees below zero, is now almost a commercial product, obtainable in quantity, but with its possibilities of usefulness as yet practically undeveloped. The infant Hercules will doubtless find no lack of tasks to do, each one more arduous and more helpful to man than any labor of his mythical prototype.
To the chemist the possibilities thus opened are innumerable. Pictet has shown that at the low temperatures which are now easily attainable all chemical action stops, even the most energetic substances lying in contact with each other quiet and inert. A greater control of the more violent chemical reactions is therefore within reach, doubtless to be utilized in many ways yet unforeseen. At the highest temperatures, also, chemical union ceases, and compounds are decomposed; each reaction is possible only within a limited thermal range, of which the beginning and the end are measurable. From future measurements of this sort new laws will surely be discovered.
The first step in the upward scale of temperatures was taken in 1802, when Robert Hare, an American, invented the oxyhydrogen blowpipe. With this instrument platinum, hitherto infusible, was melted, a result of great importance to chemists. Apart from recent electrical applications, platinum finds its chief use in the construction of chemical apparatus, and Hare's invention was therefore of great assistance to chemical research. Later in the century electrical currents were utilized as producers of great heat, until in the very modern device of the electric furnace the range of available temperatures has been at least doubled. Temperatures of three to four thousand degrees of the centigrade scale are now at the disposal of the chemist, and these are manageable in compact apparatus at very moderate cost. Cheap aluminum is one of the products of this new instrument, and the extraordinary abrasive substance, carborundum, is another. New industries have been created by the electric furnace, and in the hands of Moissan it has yielded scientific results of great interest and remarkable variety. The rarest metals can now be separated from their oxides with perfect ease, and new compounds, obtainable in no other way, the furnace has placed at our disposal. This field of research is now barely opened; from it the twentieth century should gather a rich harvest.
With electricity also chemistry is nearly allied, and along some lines the two branches of science have been curiously intertwined. Like the other physical forces, electricity may either provoke or undo combination, and, like heat, it may itself be generated as a product of chemical action. The voltaic pile and the galvanic battery owe their currents to chemical change, and it is only since the middle of the century that any other source of electric energy has become available for practical purposes. It is not surprising, therefore, that many thinkers should have sought to identify chemical and electric force, the two have so much in common. It was with the galvanic current that Davy decomposed the alkalies, and since his day other electro-chemical decompositions have been studied in great number, to the development of important industries. To the action of the current upon metallic solutions we owe the electrotype and all our processes for electroplating, and these represent only the beginnings of usefulness. Even now, almost daily, advances are being made in the practical applications of electrolysis, and the forward movement is likely to continue throughout the coming century. From the curiously reversible chemical reactions of the secondary battery the automobile derives its power, and here again we find a field for invention so large that its limits are beyond our sight. From every peak that science can scale new ranges come into view. The solution of one problem always creates another, and this fact gives to scientific investigation its chief interest. We gain, only to see that more gain is possible; the opportunity for advance is infinite. Forever and ever thought can reach out into the unknown, and never need to weep because there are no more worlds to conquer.
It was the study of electro-chemical changes which led Berzelius to his electro-chemical theory of combination, and then to the dualistic theory, which has already been mentioned. In or about the year 1832, when the Berzelian doctrines were at the summit of their fame, Faraday showed that the chemical power of a current was directly proportioned to the quantity of electricity which passed, and this led him to believe that chemical affinity and electric energy were identical. Electrolysis, the electrical decomposition of compounds in solution, was a special object of his attention, and by quantitative methods he found that the changes produced could be stated in terms of chemical equivalents or combining numbers. One equivalent weight of zinc consumed in the galvanic battery yields a current which will deposit one equivalent of silver from its solution, or which, decomposing water, will liberate one equivalent each of oxygen and hydrogen. All electro-chemical changes followed this simple law, which gave new emphasis to the atomic theory, and furnished a new means for measuring the combining numbers.
In the early days of electro-chemistry the products of electrolysis were studied in the light of the dualistic theory. But as chemical investigation along other lines overthrew this hypothesis, a closer examination of electrolytic reactions became necessary. Electrical decompositions were dualistic in character, but the dualism was not that taught by Berzelius. When a salt, dissolved in water, is decomposed by the current it is separated into two parts, which Faraday called its ions; in Berzelian terms these were in most cases oxides, but this conclusion fitted only a part of the facts, and finally was abandoned. Whatever the ions might be, they were not ordinary oxides.
Many and long were the investigations bearing upon this subject before a satisfactory settlement was reached. The phenomena observed in solutions, raised still another question, that of the nature of solution itself, and this is not yet fully answered. Two lines of study, however, have converged, within recent years, to some remarkable conclusions, the latest large development of chemical theory.
It has long been known that solutions of salts do not freeze so easily as pure water, and also that their boiling points are higher. In 1883 Raoult discovered a remarkable relation between the freezing point of a solution and the molecular weight of the substance dissolved, a relation which has since been elaborately studied by many investigators. From either the freezing-point depression or the elevation of the boiling point the molecular weight of a soluble compound can now be calculated, and many uncertain molecular weights have thus been determined.
Another phenomenon connected with solutions, which has received much attention, is that of osmotic pressure. A salt in solution exercises a definite pressure, quantitatively measurable, which is curiously analogous to the pressure exerted by gases. In a gas the molecules are widely separated, and move about with much freedom. In a very dilute solution the molecules of a salt are similarly separated, and are also comparatively free to move. The kinetic theory of gases, therefore, is now paralleled by a kinetic theory of solutions, founded by Van t'Hoff in 1887, which is now generally accepted. All the well-established laws connecting pressure, temperature, and volume among gases find their equivalents in the phenomena exhibited by solutions. In Avogadro's law we learn that equal volumes of gases, under like conditions of temperature and pressure, contain equal numbers of molecules. According to the new generalizations, equal volumes of different solutions, if they exert the same osmotic pressure, also contain equal numbers of molecules. The parallelism is perfect. With these relations the freezing- and boiling-point phenomena are directly connected.
But, both for gases and for solutions some apparent anomalies existed. Certain compounds, when vaporized, seemed not to conform to Avogadro's law, and called for explanation. This proved to be simple, and was supplied by the fact that the anomalous compound, as such, did not exist as vapor, but was split up, dissociated, into other things. For instance, ammonium chloride, above a certain temperature, is decomposed into a mixture of two gases—hydrochloric acid and ammonia—which, on cooling, reunite and reproduce the original compound. Twice as much vapor as is required by theory, and specifically half as heavy, is produced by this transformation, which is only one of a large class, all well understood.
In the case of solutions it was found that certain compounds, notably the acids, alkalies, and metallic salts, caused a depression of freezing point which was twice as great as ought to be expected. This fact was illuminated by the phenomena observed in gases, and soon it was seen that here too a splitting up of molecules, a true dissociation, occurred. These anomalous solutions, moreover, were electrolytes—that is, they conducted electricity and underwent electrolytic decompositions—while normal substances, especially solutions of carbon compounds, such as sugar, were not.
Van t'Hoff's discoveries went far, but one more step was needed, and this was taken by Arrhenius in 1888. Electrolytic compounds, when dissolved, are actually dissociated into their ions, partially so in a strong solution, entirely so in one which is infinitely dilute, a statement which leads to some extraordinary conclusions. For instance, the ions of common salt are sodium and chlorine. In a dilute solution the salt itself ceases to exist, while atoms of sodium and atoms of chlorine wander about, chemically separated from each other but still in equilibrium. Sodium sulphate may be regarded as made up of two parts—sodium and an acid radicle which contains one atom of sulphur to four of oxygen—and these parts, its ions, are severed apart during solution to move about independent of each other.
This theory of Arrhenius, the theory of electrolytic dissociation, is supported by many facts, and fits in well with the kinetic theory of Van t'Hoff. Electrolysis is no longer to be considered as a separating process, but rather as a sorting of the ions, which receive different electrical charges and concentrate at the two electrical poles. The phenomena of freezing and boiling points in solutions, and of the absorption of heat when solid salts are dissolved, all harmonize with the conclusions which have been reached. A complete theory of solutions is yet to be proposed; but these new doctrines, which are true so far as they go, represent a long step in the right direction. A final theory will include them, but they are not likely to be set aside.
As we near the end of the century we find one more discovery to note, from a most unexpected quarter—the discovery of new gases in the atmosphere. In 1893 Lord Rayleigh was at work upon new determinations of density, with regard to the more important gases. In the case of nitrogen an anomaly appeared: nitrogen obtained from the atmosphere was found to be very slightly heavier than that prepared from chemical sources, but the difference was so slight that it might almost have been ignored. To Rayleigh, however, such a procedure was inadmissible, and he sought for an explanation of his results. Joining forces with Ramsay, the observed discrepancies were hunted down, and in 1894 the discovery of argon was announced. Ramsay soon found in certain rare minerals another new gas—helium—whose spectral lines had previously been noted in the spectrum of the sun; and still later, working with liquid air, he discovered four more of these strange elements—krypton, xenon, neon, and metargon. By extreme accuracy of measurement this chain of discovery was started, and, as some one has aptly said, it represents the triumph of the third decimal. A noble dissatisfaction with merely approximate data was the motive which initiated the work.
To the chemist these new gases are sorely puzzling. They come from a field which was thought to be exhausted, and cause us to wonder why they were not found before. The reason for the oversight is plain: the gases are devoid of chemical properties, at least none have yet been certainly observed. They are colorless, tasteless, odorless, inert; so far they have been found to be incapable of union with other elements; apart from some doubtful experiments of Berthelot, they form no chemical compounds. Under the periodic law they are difficult to classify; they seem to belong nowhere; they simply exist, unsocial, alone. Only by their density, their spectra, and some physical properties can these intractable new forms of matter be identified.
In a sketch like this a host of discoveries must remain unnoticed, and others can be barely mentioned. The isolation of fluorine and the manufacture of diamonds by Moissan, the synthesis of sugars by Fischer, the discovery of soluble forms of silver by Carey Lea—all these achievements and many more must be passed over. Something, however, needs to be said upon the utilitarian aspects of chemistry, and concerning its influence upon other sciences. Portions of this field have been touched in the preceding pages; the interdependence of chemistry and physics is already evident; other subjects now demand our attention.
Medicine and physiology are both debtors to chemistry for much of their advancement, and in more than one way. From the chemist medicine has received a host of new remedies, some new processes, and advanced methods for the diagnosis of disease. The staining of tissues for identification under the microscope is effected by chemical agents, the analysis of urine helps to identify disorders of the kidneys; nitrous oxide, chloroform, ether, and cocaine almost abolish pain. The disinfection of the sick-room and the antiseptic methods which go far toward the creation of modern surgery all depend upon chemical products whose long list increases year by year. Crude drugs are now replaced by active principles discovered in the laboratory—morphine, quinine, and the like—and instead of the bulky, nauseous draughts of olden time, the invalid is given tasteless capsules of gelatin or compressed tablets of uniform strength and more accurately graded power. A great part of physiology consists of the study of chemical processes, the transformation of compounds within the living organism, and practically all this advance is the creation of the nineteenth century. Modern bacteriology, at least in its practical applications, began with a chemical discussion between Liebig and Pasteur as to the nature of fermentation: step by step the field of exploration has enlarged; as the result of the investigations we have preventive medicine, more perfect sanitation, and antiseptic surgery. The ptomaines which cause disease and the antitoxins which prevent it are alike chemical in their nature, and were discovered by chemical methods. Physiology without chemistry could not exist; even the phenomena of respiration were meaningless before the discovery of oxygen. The human body is a chemical laboratory, and without the aid of the chemist its mysteries can not be unraveled.
To agriculture also chemistry is a potent ally, whose value can hardly be overrated. It has created fertilizers and insecticides for the use of the farmer and taught their intelligent use, and in the many experiment stations of the world it is daily discovering facts or principles which are practically applicable to agriculture. The beet-sugar industry was developed by chemical researches and chemical methods; the arts of the dairy have been chemically improved; the food of all civilized nations is better and more abundant than it was before the chemist gave his aid to its production. Adulteration, always practiced, is now easily detected by chemical analysis, and, though the evil still exists, the remedy for it is in sight. To Liebig, who gave to agricultural chemistry its first great impulse forward, mankind is indebted to an amount which is beyond all computation.
In manufactures the influence of chemistry is seen at every turn. When the century began, probably no industrial establishment in the world dreamed of maintaining a chemical laboratory; to-day, hundreds are well equipped and often heavily manned for the sole benefit of the intelligent manufacturer. Coal gas is a chemical product; its by-products are ammonia and coal tar; from the latter, as we have seen, hundreds of useful substances, the discoveries of the last half century, are prepared. Better and cheaper soap and glass owe their existence to chemical improvement in the making of alkalies; chemical bleaching has replaced the tedious action of sunlight and dew; chemical dyestuffs give our modern fabrics nearly all their hues. Metallurgy is almost wholly a group of chemical processes; every metal is extracted from its ores by methods which rest on chemical foundations; analyses of fuel, flux, and product go on side by side with the smelting. The cyanide and chlorination processes for gold, the Bessemer process for steel, are apt illustrations of the advances in chemical metallurgy; but before these come into play the dynamite of the miner, another chemical invention, must have done its work underground. For rare minerals, the mere curiosities of twenty years ago, uses have been found; from monazite we obtain the oxides which form the mantle of the Welsbach burner; from beauxite, aluminum is made. The former waste products of many an industry have also revealed unsuspected values, and chemistry has the sole honor of their discovery.
In education, chemistry has steadily grown in importance, until a single university may have need of as many as twenty chemists in its teaching staff, teaching not only what is already known, but also the art of research. As a disciplinary study, chemistry ranks high in the college curriculum, and it opens the way to a new learned profession, equal in rank with those of more ancient standing.
For the material advancement of mankind the nineteenth century has done more than all the preceding ages combined, and science has been the chief instrument of progress. Scientific methods, experimental investigation, have replaced the old empiricism, and no man can imagine where the forward movement is to end. Hitherto research, has been sporadic, individual, unorganized; but fruitful beyond all anticipation. In the future it should become more systematic, better organized, richer in facilities. Through laboratories equipped for research alone the twentieth century must work, and chemistry is entitled to its fair share of the coming opportunities. The achievements of the chemist, great as they have been during this century, are but a beginning; the larger possibilities are ahead. The greatest laws are yet undiscovered; the invitation of the unknown was never more distinct than now.