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.