Bunsen, Kirchhoff, and Roscoe.
Bunsen first distinguished himself by his classical work on the cacodyl compounds, obtained as the result of an inquiry into the nature of the so-called “fuming liquor of Cadet,” an evil-smelling, highly poisonous, inflammable liquid formed by heating arsenious oxide with an alkaline acetate. The investigation (1837–1845) is noteworthy, not only for the skill it exhibits in dealing with a difficult and highly dangerous manipulative problem, but also for the remarkable nature of its results and on account of their influence on contemporary chemical theory. The research, in the words of Berzelius, was the foundation-stone of the theory of compound radicals. The name cacodyl or kakodyl was suggested by Berzelius in allusion to the nauseous smell of the compounds of the new radical arsinedimethyl, As (CH3)2, as it was subsequently termed by Kolbe.
Bunsen greatly improved the methods of gasometric analysis; these he applied, in conjunction with Playfair, to an examination of the gaseous products of the blast furnace in the manufacture of iron, and thereby demonstrated the enormous waste of energy occasioned by allowing the gases to escape unused into the air, as was then the universal practice. This inquiry effected a revolution in the manufacture of iron as important, indeed, as that due to the introduction of the hot blast.
Bunsen devised methods for determining the solubility of gases in liquids, for ascertaining the specific gravity of gases, their rates of diffusion, and of combination or inflammation. In 1841 he invented the carbon-zinc battery, and applied it to the electrolytic production of metals, notably of magnesium, the properties of which he first accurately described. In 1844 he contrived the grease-spot photo-meter, which was long in general use for ascertaining the photometric value of illuminating gas. His methods of ascertaining the specific heats of solids and liquids were simple, ingenious, and accurate. In 1855–1863 he carried out, in conjunction with Roscoe, a long series of investigations on the chemical action of light. In 1859, in association with Kirchhoff, he devised the first methods of spectrum analysis, and explained the origin and significance of the Fraunhofer lines in the solar spectrum, thus laying the foundations of solar and stellar chemistry. The application of the spectroscope to analytical chemistry almost immediately resulted in his discovery of cæsium and rubidium.
Bunsen worked on problems of chemical geology, and made a long series of analyses of volcanic products. With Schischkoff, he examined, in 1857, the products of fired gunpowder. He effected many improvements in analytical chemistry; devised the iodiometric method of volumetric analysis, and systematised the processes of water analysis. Lastly, he invented the gas-burner—a piece of apparatus with which his name is inseparably associated, and which has been of inestimable service to operative chemistry and in the arts. Bunsen was no theorist, and purely speculative questions had little or no interest for him. At the same time he was a great teacher, and made the chemical school of Heidelberg no less famous than the schools of Giessen and Göttingen.
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The mass of material relating to the development of chemistry which has been accumulated during the past sixty years is so vast that it would be hopeless to attempt to survey it in detail within the limits of such a work as this. Nor, indeed, is this required in a history of this character. Those who desire information concerning the origin and sequence of the facts which collectively make up the superstructure of modern chemistry must be referred to the encyclopædias or larger treatises—or, preferably, to the numerous monographs, dealing with special sections, which the volume and complexity of the matter to be dealt with seem to render increasingly necessary. All we can do here is to attempt to show what has been the main outcome of this sixty years of incessant effort to elucidate the mysteries of chemical phenomena and to ascertain the nature of the conditions which control, modify, or determine them. All this effort is ultimately directed to the solution of the fundamental problem of the constitution of matter. The most significant result of this endeavour has been the elaboration and consolidation of the doctrine of chemical atoms, not necessarily of atoms in the limited Daltonian sense, but of atoms considered as associations of particles, or corpuscles—that is, of entities which may be divisible, but which, in the main, are not divided in the vast number of the transformations in which they are concerned. This modification of the original conception of Dalton has been thought by some to destroy the basis upon which his theory really rests. There is no necessity for such an assumption. So pronounced an atomist as Graham, as far back as 1863, in a suggestive paper entitled Speculative Ideas on the Constitution of Matter, enlarged the conception of the Daltonian atom in precisely the sense which recent experimental work appears to require. The present position, too, as it affects chemists, was equally well stated by Kekulé, in 1867, in the following terms:
The question whether atoms exist or not has but little significance from a chemical point; its discussion belongs rather to metaphysics. In chemistry we have only to decide whether the assumption of atoms is an hypothesis adapted to the explanation of chemical phenomena. More especially have we to consider the question whether a further development of the atomic hypothesis promises to advance our knowledge of the mechanism of chemical phenomena.
I have no hesitation in saying that, from a philosophical point of view, I do not believe in the actual existence of atoms, taking the word in its literal signification of indivisible particles of matter; I rather expect that we shall some day find for what we now call atoms a mathematico-mechanical explanation which will render an account of atomic weight, of atomicity, and of numerous other properties of the so-called atoms. As a chemist, however, I regard the assumption of atoms not only as advisable, but as absolutely necessary, in chemistry. I will even go further, and declare my belief that chemical atoms exist, provided the term be understood to denote those particles of matter which undergo no further division in chemical metamorphoses. Should the progress of science lead to a theory of the constitution of chemical atoms—important as such a knowledge might be for the general philosophy of matter—it would make but little alteration in chemistry itself. The chemical atoms will always remain the chemical unit; and for the specially chemical considerations we may always start from the constitution of atoms, and avail ourselves of the simplified expression thus obtained—that is to say, of the atomic hypothesis. We may, in fact, adopt the view of Dumas and of Faraday—that, whether matter be atomic or not, thus much is certain: that, granting it to be atomic, it would appear as it now does.[1]
[1] The Study of Chemical Composition, by Ida Freund (Cambridge University Press), 1904.
The greater part of that which follows will be devoted, therefore, to an exposition of certain of the great advances in knowledge—many of them of primary importance—which have been made during the last fifty or sixty years and which have served to strengthen this extended conception of the atomic theory, and to establish its position as an article of the scientific faith of the twentieth century.