MOLECULAR CONSTITUTION OP BODIES. 435 



To shew that all the molecules of the same substance are equal in mass, 

 we may refer to the methods of dialysis introduced by Graham, by which two 

 gases of different densities may be separated by percolation through a porous 

 plug. 



If in a single gas there were molecules of different masses, the same process 

 of dialysis, repeated a sufficient number of times, would furnish us with two 

 portions of the gas, in one of which the average mass of the molecules would 

 be greater than in the other. The density and the combining weight of these 

 two portions would be different. Now, it may be said that no one has carried 

 out this experiment in a sufficiently elaborate manner for every chemical sub- 

 stance. But the processes of nature are continually carrying out experiments 

 of the same kind ; and if there were molecules of the same substance nearly 

 alike, but differing slightly in mass, the greater molecules would be selected in 

 preference to form one compound, and the smaller to form another. But hy- 

 drogen is of the same density, whether we obtain it from water or from a 

 hydrocarbon, so that neither oxygen nor carbon can find in hydrogen molecules 

 greater or smaller than the average. 



The estimates which have been made of the actual size of molecules are 

 founded on a comparison of the volumes of bodies in the liquid or solid state, 

 with their volumes in the gaseous state. In the study of molecular volumes 

 we meet with "many difficulties, but at the same time there are a sufficient 

 number of consistent results to make the study a hopeful one. 



The theory of the possible vibrations of a molecule has not yet been studied 

 as it ought, with the help of a continual comparison between the dynamical 

 theory and the evidence of the spectroscope. An intelligent student, armed 

 with the calculus and the spectroscope, can hardly fail to discover some important 

 fact about the internal constitution of a molecule. 



The observed transparency of gases may seem hardly consistent with the 

 results of molecular investigations. 



A model of the molecules of a gas consisting of marbles scattered at dis- 

 tances bearing the proper proportion to their diameters, would allow very little 

 light to penetrate through a hundred feet. 



But if we remember the small size of the molecules compared with the 

 length of a wave of light, we may apply certain theoretical investigations of 

 Lord Kayleigh's about the mutual action between waves and small spheres, 

 which shew that the transparency of the atmosphere, if affected only by the 



552 



