GUSTAV MAGNUS. 17 



in mathematical physics during the first half of this 

 century, whilst as good as nothing was known of 

 atoms ; and, for instance, hardly anything was known 

 of the extraordinary influence which heat has on mole- 

 cular forces. We now know that the expansive force of 

 gases depends on motion due to heat ; at that period 

 most physicists regarded heat as imponderable matter. 

 In reference to atoms in molecular physics, Sir W. 

 Thomson says, with much weight, that their assump- 

 tion can explain no property of the body which has 

 not previously been attributed to the atoms. Whilst 

 assenting to this opinion, I would in no way express 

 myself against the existence of atoms, but only against 

 the endeavour to deduce the principles of theoretical 

 physics from purely hypothetical assumptions as to 

 the atomic structure of bodies. We now know that 

 many of these hypotheses, which found favour in their 

 day, far overshot the mark. Mathematical physics 

 has acquired an entirely different character under the 

 hands of Gauss, of F. E. Neumann and their pupils, 

 among the Germans; as well as from those mathe- 

 maticians who in England followed Faraday's lead, 

 Stokes, W. Thomson, and Clerk-Maxwell. It is now 

 understood that mathematical physics is a purely ex- 

 perimental science ; that it has no other principles to 

 follow than those of experimental physics. In our 

 immediate experience we find bodies variously formed 



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