872 Transactions of the American Institute. 



and well defined as to enable its votaries to determine the ultimate 

 composition of all bodies. The chemist affirms that, however inclined 

 we may be to regard a body as a whole, it is in fact composed of 

 minute parts which may be separated, and that in the great majority 

 of bodies, which are compounds, nature has herself made divisions, 

 by incorporating unlike parts which may be replaced by other unlike 

 parts. On questions relating to the actual size of these parts, their 

 form, their structure, etc., he makes no issue ; he simply asserts that 

 all these ultimate parts are permanent, and that those composed of 

 the same kind of matter are identical in size and structure. The 

 limits proposed for this paper will permit elucidation of this point 

 alone. 



The clearest conception of molecules and atoms will be arrived at 

 by examining the principal phenomena attending the mechanical mix- 

 ture and final chemical union of the lightest and the heaviest of the 

 simple gases. The electro-positive element, hydrogen, is a perma- 

 nently elastic gas, having a relative density expressed by 1. Its pro- 

 perties are in marked contrast with those of chlorine, a yellowish green 

 gas, which may be condensed into a liquid, by a pressure of about four 

 atmospheres. The density of this strong electro-negative element is 

 35.5. If two vessels of equal capacity, filled with these gases respec- 

 tively, be placed in the dark, one over the other, and a communica- 

 tion be opened between them, a mutual diffusion of the gases will com- 

 mence, the relative velocity being inversely as the square root of their 

 densities. The action continues, untraversed by the force of gravita- 

 tion, until minute portions of hydrogen and chlorine are equally dif- 

 fused throughout both receptacles. This phenomenon cannot be 

 accounted for, excepting on the supposition that minute parts of each 

 gas have undergone complete isolation. If diffusion w T ere effected 

 only through a single stratum or extremely thin layer, it would be 

 possible for two gaseous elements to retain their continuity by passing 

 each other in intertwining streams, thus forming, like threads, a warp 

 and woof; but when diffusion is in every direction, it is obvious that 

 these elements must positively separate each other, and thus be 

 divided into extremely diminutive bodies each of the same dimen- 

 sions. Let I represent the lighter gas, d the denser, and e the dimen- 

 sions or size of each isolated portion, then el and ed will denote the 

 dissimilar parts of which the whole gaseous matter is composed. As 

 the phenomenon of diffusion occurs under the conditions mentioned, 

 whatever may be the quantity of gases employed, it follows that el and 

 ed are individual volumes or molecules, invariably of the same dimen- 



