JEAN-BAPTISTE-ANDR^ DUMAS. 551 



tliat they are present in them in equal numbers. An immediate con- 

 sequence of this mode of looking at the question has already been the 

 subject of a learned discussion on the part of Ampere," — and Avoga- 

 dro as the author subsequently adds, — " to which, however, chemists? 

 with the exception perhaps of M. Gay-Lussac, appear to have given 

 as yet but little attention. It consists in the necessity of considering 

 the molecules of the simplest gases as capable of a furtlier division, — 

 a division occurring in the moment of combination, and varying with 

 the nature of the compound." 



Here, it is obvious, are the very conceptions which form the basis 

 of our modern chemical philosophy ; and at first we are surprised that 

 they did not lead Dumas at once to the full realization of the conse- 

 quences which the doctrine of equal molecular volumes involves in the 

 interpretation of the constitution of chemical compounds, and to the 

 clear distinction between " the physically smallest particles" and " the 

 chemically smallest particles," or the molecules and the atoms, as we 

 now call the physical and the chemical units. This distinction is im- 

 plied throughout Dumas's paper already quoted, and is illustrated by a 

 striking example in the introduction to his treatise on " Chemistry 

 applied to the Arts," published two years later ; but the ground was not 

 yet prepared to receive the seed, and more than a quarter of a century 

 must pass before the full harvest of this fruitful hypothesis could be 

 reaped. 



There were, however, two imj^ortant incidental results of this in- 

 vestigation from which chemical science immediately profited. One 

 was a simple method of determining with accuracy the vapor densi- 

 ties of volatile substances which has since been known by Dumas's 

 name. The other was a radical change in the formula of the silicates. 

 On the authority of Berzelius, who based his opinion chiefly on the 

 analogy between the silicates and the sulphates, the formula SiO., had 

 been accepted as representing the constitution of silica. But from the 

 density of both the chloride and the fluoride of silicon Dumas con- 

 cluded that the formula was SiO.,, a conclusion which is now seen to be 

 in complete harmony with the scheme of allied compounds. To Ber- 

 zelius, however, the new views appeared wholly out of harmony with 

 the system of chemistry which he had so greatly assisted in develop- 

 ing, and he opposed them with the whole weight of his powerful influ- 

 ence, and so far succeeded as to prevent their general adoption for 

 many years. Still, "the new mode of looking at the constitution of 

 silicic acid slowly but surely gained ground, and it is now so firmly 

 rooted in our convictions, that the younger generation of chemists will 



