236 PROCEEDINGS OP THE AMERICAN ACADEMY, 



sary to attribute it to free spaces within the carbou dioxide molecule. 

 For it is not improbable that the inherent density of massive atoms like 

 those of mercury may be considerably greater than that of light atoms, 

 such as those of carbon and oxygen. 



In closing, we desire to point out that the principle here established, 

 that atoms and molecules are of the same order of magnitude, and that 

 no considerable free interatomic spaces exist within the molecule is in 

 accordance with the remarkable fact that the molecular cross-section of 

 most comparatively simple molecules is approximately an additive prop- 

 erty calculable from certain constant values of the atomic cross-section.* 

 This fact would be unintelligible, were the principle not correct ; for if 

 considerable space existed between the atoms, it is not to be supposed 

 that those spaces would be the same in entirely dissimilar molecules, — 

 that, for example, the space between the hydrogen and chlorine atoms 

 in hydrochloric acid would have any relation to the space between the 

 atoms in the elementary gases hydrogen and chlorine. 



* See 0. E. Meyer, Kinetische Tbeorie der Gase, p. 209. 



Rogers Laboratory of Physics, 

 September, 189G. 



