examples. What is the weight of an elementary 6 He also at this time admitted that the powerful bases must be composed of two elementary volumes of oxygen and one volume of metal. The composition of the oxides of sodium, potassium, calcium, iron, zinc, and lead was, therefore, represented by the formulæ NaO,, KO2, CaO2, FeO2, PbO2, the weights of the elementary volumes of a great number of metals thus assuming a value double that which Berzelius attributed to them later. The theory of volumes, as it stood at that time, was therefore bristling with hypotheses and full of uncer tainties. And yet this conception long held its ground in science, especially in France, where at a certain period it was the fashion to express the composition of bodies in volumes,' under the impression that the substitution of volumes for atoms had the advantage of offering a representation more in accordance with facts. But in reality it was not so: the volume occupied by carbon vapour, and the degrees of condensation of the elements of carbonic acid gas, were hypothetical ideas, and these ' elementary volumes' represented the atoms themselves, at least in notation. 6 6 Berzelius recognised this fact in 1818. In his essay upon the theory of chemical proportions he modified considerably the views which he had published in 1813. The prevailing idea is no longer that of establishing the system of atomic weights upon the theory of volumes. Though still giving weight to the indications furnished by this theory, he endeavours to reconcile it with what he terms the corpuscular theory,' which is founded upon chemical proportions. The indivisible corpuscules, or the ultimate particles of bodies, are designated atoms-the most convenient term, because the one most in use. We might call them particles, molecules, or chemical equivalents, as all these terms appear to be synonymous to Berzelius. The relative weights of these atoms represent chemical proportions. The fixed proportions, which had been recognised for weights, again appeared in gaseous combinations for volumes. Thus the theory of volumes and the atomic or corpuscular theory led to the same results, as far as the ponderable relations of elements in combinations are concerned: what is called atom in one is called volume in the other. It would seem, therefore, as if we might assimilate the two notions, which indeed is necessary in the case of simple gases. Equal volumes of the latter contain the same number of atoms, under the same conditions of temperature and pressure. Berzelius observes that this law does not apply to compound gases; for, he says, it sometimes happens that a volume of a compound gas contains fewer atoms than an equal volume of a simple gas. Thus one volume of aqueous vapour contains onehalf as many atoms (compound atoms, molecules) as one volume of hydrogen. Such was the manner in which Berzelius, about 1818, expressed the atomic hypothesis, which he founded partly upon chemical proportions and partly upon a peculiar conception of the law of volumes. This conception was not a very happy one. Not to mention the difficulty which he created by applying the same term, atoms, to the ultimate indivisible particles of simple bodies and to the complex molecules of compound bodies, a confusion which had been avoided by Avogadro and Ampère, Berzelius at this time introduced into the language of science a formula which long held its ground, and which must now be considered as erroneous-namely, the proposition that equal volumes of simple gases contain the same number of atoms. We shall presently reconsider this point. We must here draw attention to the influence which the discoveries of Gay-Lussac exercised upon Berzelius in his attempt to bring the atomic hypothesis into harmony with the facts relating to the combination of gases. It is a remarkable fact that neither Dalton nor Gay-Lussac accepted the views of the Swedish chemist. The author of the atomic theory obstinately maintained his first idea of deducing atomic weights solely from the ponderable relations of elements in combinations. GayLussac, again, confined himself to the immediate consequences of his discovery, not without forcing them to some extent, in certain cases, by hypotheses upon the forms of condensation of the combining gaseous elements. He and Berzelius expressed the composition of bodies in volumes, the latter admitting that the relative weights of these volumes represented atoms, Gay-Lussac refusing to consider these weights as anything more than ponderable relations,' and inclining rather to the views of Davy. The latter, deviating to an equal extent from the profound conceptions of Dalton, and with the idea of completing them by the discoveries of the French chemist, confined himself strictly to established facts and to the consideration of 'proportional numbers.' After the ingenious but ignored attempts of Avogadro and Ampère, and the unfruitful effort of Berzelius, Dalton's conception would have been sentenced to sterility and oblivion, had it not happened that, at the period of which we are speaking, fresh discoveries and new ideas drew attention to it. We allude to Prout's hypothesis, to the discovery of the law of specific heats, and to the discovery of isomorphism. 49 CHAPTER III. PROUT'S HYPOTHESIS-LAW OF SPECIFIC HEATS ISOMORPHISM. DULONG AND PETIT-MITSCHERLICH I. WE must first return to Prout's hypothesis, not that it is of such great importance from our present point of view, but because it preceded the important discoveries which we shall presently mention. 1 The anonymous author of a memoir which appeared in 1815 upon the relations between the densities of bodies in a gaseous state and the weights of atoms, tried to prove that the densities of oxygen, nitrogen, and chlorine are integral multiples of that of hydrogen, and that the atomic weights of certain elements are similarly integral multiples of that of hydrogen. Amongst these elements we meet with some metals, the atomic weight of which had been determined by the author or by other chemists by the following excellent process: the quantities of metal were determined which, combined with oxygen, formed quantities of oxides capable of neutra 1 Annals of Philosophy, vol. vii. p. 111. E |