MODERN PHYSICS. 145 



Again, the spectroscope reveals to us vibrations 

 of the ether, which are connected in some way with 

 the vibrations of the molecules of gas, whose spectrum 

 we are observing. It seems clear that the law of 

 equal partition does not apply to these, and yet, 

 if wo are to suppose that the ether vibrations are 

 duo to actual vibrations of the atoms which con- 

 stitute a molecule, why does it not apply ? Where 

 does the condition come in which leads to failure in 

 the proof ? Or, again, is it, as has been suggested, the 

 fact that the complex spectrum of a gas represents 

 the terms of a Fourier Series, into which some 

 elaborate vibration of the atoms is resolved by the 

 ether ? or is the spectrum duo simply to electro- 

 magnetic vibrations on the surface of the molecules 

 vibrations whose period is determined chiefly by the 

 size and sliape of the molecule, but in which the 

 atoms of which it is composed take part ? There are 

 grave difficulties in the way of either of these ex- 

 planations, but we must not let our dread of the tusk 

 which remains to be done blind our eyes to the great- 

 ness of Maxwell's work. 



One other important paper, and a number of 

 shorter articles, remain to be mentioned 



The Boltzmann-Maxwell law applies only to cases 

 in which the temperature is uniform throughout. In 

 a paper published in the Philosophical Transactions 

 for 1879, on " Stresses in Rarefied Gases Arising from 

 Inequalities of Temperature," Maxwell deals, among 

 other matters, with the theory of the radiometer, He 

 shows that the observed motions will not take place 

 unless gas, in contact with a solid, can slide along 

 i 





