ATO]\rS AND SUNBEAMS. 129 



this is not so. We must carefully observe tbat there is another kind 

 of molecular motion besides that which the molecule possesses as a 

 whole. \\\' have hitherto been occupied only with the movements of 

 each molecule as a little projectile pursuing' its zigzag course, each 

 turn of the zigzag being the result of an encounter with some similar 

 molecule belonging to the same medium. IJut we have now to observe 

 that the molecule itself. is by no means to be regarded as a sim[)le 

 rigid particle; indeed, if it were so it is certain that we should receive 

 no heat at all from the sun. We have the best reasons for believing 

 that the molecule of matter, so far from resembling a simple rigid ])ar- 

 ticle, is an elal)orate structure, whose parts are in some degree capable 

 of independent movement. It will not, indeed, be necessary for us to 

 adopt the splendid hypothesis of Lord Kelvin, which supposes that 

 molecules of matter are merely vortex rings in that perfect fluid, the 

 ;ether. It seems difdcult to doubt that this doctrine represents the 

 facts, but if anyone should reject it, then I have only to say that its 

 assumption is not lequiied for our present argument. All that is 

 necessary for us is to regard each molecule as somewhat resembling an 

 elastic structure made of parts which can (piiver like springs, and so 

 arranged as to be susceptible of many different modes of vibration. 

 We are to suppose that each molecule, in addition to the energy which 

 it possesses in virtue of its movement of translation as a whole, has 

 also a store of energy corresponding to the oscillations of its electric 

 springs. We can, in fact, in some cases determine the ratio which 

 exists between the amount of energy which is, on the average, pos- 

 sessed by molecules in consequence of their velocities of translation, 

 and the amount of energy whicli they possess in consequence of the 

 vibrations by which their several parts are animated. It is these 

 internal molecular vibrations which are of essential imi)ortance in our 

 present inquiry. It is believed that the radiation of light or of heat 

 generally takes rise in the impulses given to jcther by the internal 

 molecular vibrations. Do we not know that the essential character- 

 istic of those jetherial movements which correspond to radiant light 

 and heat is that they have the nature of oscillations' Such could not 

 be imparted by mere rectilinear movements of the molecules as a whole. 

 They must be due to those internal oscillations by which the actual 

 molecules are animated. 



No doubt it is difficult to realize that much can be learned with 

 regard to the performances that actually go on in the internal parts of 

 a molecule, especially when it is remembered that each molecule in its 

 entirety is so extremely minute as to be entirely beyond the reach of 

 our organs of sense. It is nevertheless impossible to doubt that the 

 statements Just made correspond to the veritable facts of nature. It 

 would be impracticable here to go into any complete detail with regard 

 to the evidence on this subject; 1 can only sketch an outline of it. Let 

 us take, perhaps as the simplest case, that presented by hydrogen. 

 SM 93 9 



