298 HISTORY OF CHEMISTRY. [LECTURE XV. 



A molecule is defined as the smallest quantity of a chemical 

 substance that occurs in the free state, whether the substance 

 be elementary or compound. The determination of the mole- 

 cular weight depends essentially upon our combining the con- 

 ceptions of the physical and of the chemical molecule ; that 

 is to say, we apply the word molecule to the smallest quantity 

 of a substance which occurs free in the gaseous state, as well 

 as to the smallest quantity that enters into a reaction. 



With respect to determinations of atomic weights, it is to be 

 remarked here that the numbers proposed by Gerhardt 1 were 

 subjected to an impo r tant alteration in so far that the atomic 

 weights of all the metals were doubled, except those of the 

 monatomic ones (i.e., the alkali metals and silver). As early as 

 1840, when the atomic weights of Berzelius were still in use, 

 Regnault had proposed to halve the atomic weight of silver, 

 and, in accordance with this proposal, to assume two atoms of 

 metal in silver oxide for one atom of oxygen. 2 He afterwards 

 made a similar proposal with respect to the atomic weights of 

 potassium, sodium, and lithium. 3 The reason was, that his 

 classical experiments on specific heat had shown him that 

 Dulong and Petit's law only applied to these metals when this 

 assumption was made. .Had this proposal of Regnault's been 

 adopted at that time, our present atomic weights would (with 

 few exceptions) have been obtained. But since, following Ger- 

 hardt's lead, the atomic weights of all the metals were halved, 

 it was afterwards necessary (when the desirability of Regnault's 

 proposal had been shown upon new grounds, especially by H. 

 Rose 4 and by Cannizzaro 5 ) to double them again, with the 

 exception of those of the metals mentioned above. Cannizzaro 

 in particular, showed, in his pamphlet referred to below, that 

 the law of Dulong and Petit was a guide in the determination 



1 Compare, for example, Gerhardt, Introduction a 1'Etude de la Chimie, 

 1848, 29. 2 Ann. Chim. [2] 73, 5 ; Annalen. 36, no. 3 Ann. Chim. 

 [3] 26, 261. 4 Pogg. Ann. 100, 270. 5 Nuovo Cimento. 7, 321 ; also 

 Repert. de Chimie pure. I, 201 ; compare Suhto dj un Corso di Filosofia 

 CWmica 1858, 35. 



