1895.J Motions of and within Molecules, $c. 181 



degrees of freedom in its motion :* a supposition which ia an example 

 of how ready we are to think that Nature must work simply when she 

 works on a very small scale, and of the further error of imagining 

 that a little rigid body is something exceptionally simple the fact 

 being that a rigid body is only a figment of the imagination, and that 

 in Nature it is physically impossible. Moreover, it so happens that 

 the bodies in Nature which most nearly resemble rigid bodies, 

 namely, elastic solids, are amongst those bodies whose internal con- 

 stitution is most complex. 



Beside the Bb motions, there may be other internal events more or 

 less isolated from both the Ba and the Bb events. By two events 

 being isolated is to be understood their being unable to interchange 

 energy. We may call these in succession Be, Bd, <fec., events. 

 When they exist, the body will usually emit two or more spectra 

 under variations of the external stimulus, whether luminous or elec- 

 trical. And we mnst bear in mind that Ba events may also be the 

 source of a spectrum. 



The simplest supposition as to the interaction between the ether 

 and the molecules of matter is that which is based on Faraday's law 

 of electrolysis, which, as von Helmholtz pointed out, and as the 

 present writer had previously shown, implies that there is a certain 

 electrical charge, of the same amount in all cases, associated with 

 each chemical bond (see ' Philosophical Magazine ' for October, 1894, 

 p. 418). The approximate amount of this charge, which Helm- 

 holts 1 : designated the atom of electricity, and which the author has 

 called the electron, can be computed. According to a determination 

 made by the author in 1874, it appears to be about three-eleventhets 



* That there are few degrees of freedom in the molecule is sometimes supposed 

 to follow from the dynamical investigation ; but this appears to be a mistake. The 

 Maxwell Law of the partition of kinetic energy is only known to prevail 1 where 

 the kinetic energy is expressible as a sum of squares ; 2 where certain initial con- 

 ditions of the motions of the system of bodies have been complied with ; and 3 3 

 where the subsequent events are due exclusively to the interaction of the bodies of 

 the system. 



No one of these is known to be true of any gas ; and the second of them if ful- 

 filled initially will in general cease to prevail so soon as any agency other than the 

 dynamical action of the molecules intervenes. Radiant heat, light, electricity, 

 and many (probably all) chemical reactions, are agencies of this kind. 



That the dynamical investigation, based on data simpler than those that prevail 

 in nature, offers in a striking way an explanation of the numerical values for the 

 ratio of the two specific heats as determined by experiment in several gases, in no 

 degree proves that those simpler data are what exist in nature. Many and very 

 various phenomena of light are explainable in a very striking way by the simple 

 hypothesis that light is an undulation of transverse motions ; but it would be a 

 rash inference to conclude from this that electromagnetic waves are mere trans- 

 verse motions. The data of Nature have always to be simplified before they can be 

 used as the data of mathematical investigations. 



