rious rare metals, by means of which the molecular weights of various compounds have been satisfactorily ascertained.
M. Deville's highest title to honor, says M. Würtz, rests upon his having introduced to science the idea of dissociation as a particular method of decomposition, the understanding of which has rendered the grandest services to theoretical chemistry. Decomposition was believed, previous to his researches in this line, to be a simple phenomenon, taking place and being repeated with each body at a fixed temperature. M. Deville has shown that it is not always so, but that decomposition is accomplished in certain cases by degrees, within certain limits of temperature, in such a manner that it is arrested at a given temperature, by the establishment of an equilibrium between the decomposing body and the products of its decomposition. The principle explains a variety of occurrences among thermal phenomena that have hitherto not been accurately accounted for, and may be defined, says "Nature," "as the property of many compound bodies to undergo partial decomposition under the influence of heat in confined spaces, until the liberated gas or vapor has attained a certain tension, greater or less, according to the temperature. So long as this temperature remains constant, no further decomposition takes place, neither does any portion of the separated elements recombine. If the temperature be raised, decomposition recommences, and continues until a higher tension of the liberated gas or vapor, definite for that particular temperature, is attained. If the temperature falls, recomposition ensues, until the tension of the residual gas is reduced to that which corresponds with the lower temperature. The enunciation of this simple but far-reaching principle has thrown light upon a number of phenomena, such as the formation of minerals, the apparent volatilization of solids, etc.; and has been the fruitful source of countless novel discoveries."
Other researches with which M. Deville's name is associated are those on boron, which he prosecuted in company with Wöhler in 1857, on the preparation of silicium and its compounds with copper (1863), a new calorimeter, on the changes attendant upon the mixture of liquids (1870), and the examination of a large variety of minerals and natural products. A few years ago he was appointed on the commission to prepare the international normal standard of the measure of a metre, and acquitted himself of the task allotted to him with characteristic devotion to the interests of science.
His simplicity was the predominant quality in M. Deville's character, and distinguished him alike in his social relations and in his scientific labors. He kept away from the theoretical disputes in which many of his colleagues were too prone to indulge, finding abundant employment for his own resources "in attacking the still unsolved problems of inorganic chemistry"; and he is said to have gone so far as to declare the whole theoretical tendency of modern chemistry, the
