Heroes of Science: Chemists

by M. M. Pattison Muir

Two years after this Dumas employed the reaction wherein two volumes of gaseous hydrochloric acid are produced by the union of one volume of hydrogen with one volume of chlorine, as an argument which obliged him to conclude that, if Avogadro's physical hypothesis be accepted, the molecules of hydrogen and chlorine split, each into two parts, when these gases combine chemically. But Dumas did not at this time conclude that the molecular weight of hydrogen must be taken as twice its atomic weight, and that—hydrogen being the standard substance—the molecular weights of all gases must be represented by the specific gravities of these gases, referred to hydrogen as 2.

I have already shortly discussed the method for finding the relative weights of elementary atoms which is founded on Avogadro's hypothesis, and, I think, have shown that this hypothesis leads to the definition of "atom" as the smallest amount of an element in one molecule of any compound of that element (see p. 142).

This deduction from Avogadro's law is now a part and parcel of our general chemical knowledge. We wonder why it was not made by Dumas; but we must remember that a great mass of facts has been accumulated since 1826, and that this definition of "atom" has been gradually forced on chemists by the cumulative evidence of those facts.

One thing Dumas did do, for which the thanks of every chemist ought to be given him; he saw the need of a convenient method for determining the densities of compounds in the gaseous state, and he supplied this need by that simple, elegant and trustworthy method, still in constant use, known as Dumas's vapour density process.

While Dumas was working out the details of this analytical method, which was destined to be so powerful an instrument of research, Liebig was engaged in similar work; he was perfecting that process for the analysis of organic compounds which has since played so important a part in the advancement of this branch of chemical science. The processes in use during the first quarter of this century for determining the amounts of carbon, hydrogen, and oxygen in compounds of those elements, were difficult to conduct and gave untrustworthy results. Liebig adopted the principle of the method used by Lavoisier, viz. that the carbon in a compound can be oxidized, or burnt, to carbonic acid, and the hydrogen to water. He contrived a very simple apparatus wherein this burning might be effected and the products of the burning—carbonic acid and water—might be arrested and weighed. Liebig's apparatus remains now essentially as it was presented to the chemical world in 1830. Various improvements in details have been made; the introduction of gas in place of charcoal as a laboratory fuel has given the chemist a great command over the process of combustion, but in every part of the apparatus to-day made use of in the laboratory is to be traced the impress of the master's hand. A weighed quantity of the substance to be analyzed is heated with oxide of copper in a tube of hard glass; the carbon is burnt to carbonic acid and the hydrogen to water at the expense of the oxygen of the copper oxide. Attached to the combustion tube is a weighed tube containing chloride of calcium, a substance which greedily combines with water, and this tube is succeeded by a set of three or more small bulbs, blown in one piece of glass, and containing an aqueous solution of caustic potash, a substance with which carbonic acid readily enters into combination. The chloride of calcium tube and the potash bulbs are weighed before and after the experiment; the increase in weight of the former represents the amount of water, and the increase in weight of the latter the amount of carbonic acid obtained by burning a given weight of the compound under examination. As the composition of carbonic acid and of water is known, the amounts of carbon and of hydrogen in one hundred parts of the compound are easily found; the difference between the sum of these and one hundred represents the amount of oxygen in one hundred parts of the compound. If the compound should contain elements other than these three, those other elements are determined by special processes, the oxygen being always found by difference.

Soon after his settlement at Giessen Liebig turned his attention to a class of organic compounds known as the cyanates; but Wöhler—who, while Liebig was in Paris in the laboratory of Gay-Lussac, was engaged in studying the intricacies of mineral chemistry under the guidance of Berzelius—had already entered on this field of research. The two young chemists compared notes, recognized each other's powers, and became friends; this friendship strengthened as life advanced, and some of the most important papers which enriched chemical science during the next thirty years bore the joint signatures of Liebig and Wöhler.

I have already mentioned that when it was found necessary to abandon the Lavoisierian definition of organic chemistry as the chemistry of compounds containing carbon, hydrogen and oxygen, and sometimes also phosphorus or nitrogen, a definition was attempted to be based on the supposed fact that the formation of the compounds obtained from animals and plants could be accomplished only by the agency of a living organism. But the discovery made in 1828 by Wöhler, that urea—a substance specially characterized by its production in the animal economy, and in that economy only—could be built up from mineral materials, rendered this definition of organic chemistry impossible, and broke down the artificial barrier whereby naturalists attempted to separate two fields of study between which Nature made no division.

We have here another illustration of the truth of the conception which underlies so many of the recent advances of science, which is the central thought of the noble structure reared by the greatest naturalist of our time, and which is expressed by one of the profoundest students of Nature that this age has seen in the words I have already quoted from the preface to the "Lyrical Ballads," "In Nature everything is distinct, but nothing defined into absolute independent singleness."

From this time the progress of organic chemistry became rapid. Dumas continued the researches upon ethers which he had commenced at Geneva, and by the year 1829 or so he had established the relations which exist between ethers and alcohols on the one hand, and ethers and acids on the other. This research, a description of the details of which I cannot introduce here as it would involve the use of many technical terms and assume the possession by the reader of much technical knowledge, was followed by others, whereby Dumas established the existence of a series of compounds all possessed of the chemical properties of alcohol, all containing carbon, hydrogen and oxygen, but differing from one another by a constant amount of carbon and hydrogen. This discovery of a series of alcohols, distinguished by the possession of certain definite properties whereby they were marked off from all other so-called organic compounds, was as the appearance of a landmark to the traveller in a country where he is without a guide. The introduction of the comparative method of study into organic chemistry—the method, that is, which bases classification on a comparison of large groups of compounds, and which seeks to gather together those substances which are like and to separate those which are unlike—soon began to bear fruit. This method suggested to the experimenter new points of view from which to regard groups of bodies; analogies which were hidden when a few substances only were considered, became prominent as the range of view was widened. What the gentle Elia calls "fragments and scattered pieces of truth," "hints and glimpses, germs, and crude essays at a system," became important. There was work to be done, not only by the master spirits who, looking at things from a central position of vantage, saw the relative importance of the various detailed facts, but also by those who could only "beat up a little game peradventure, and leave it to knottier heads, more robust constitutions, to run it down."

Twenty years before the time of which we are now speaking Davy had decomposed the alkalis potash and soda; as he found these substances to be metallic oxides, he thought it very probable that the other well-known alkali, ammonia, would also turn out to be the oxide of a metal. By the electrolysis of salts formed by the action of ammonia on acids, using mercury as one of the poles of the battery, Davy obtained a strange-looking spongy substance which he was inclined to regard as an alloy of the metallic base of ammonia with mercury. From the results of experiments by himself and others, Davy adopted a view of this alloy which regarded it as containing a compound radicle, or group of elementary atoms which in certain definite chemical changes behaved like a single elementary atom.