96 ESSAYS BIOGRAPHICAL AND CHEMICAL 



quotation from Helmholtz, late Professor of Physics in 

 Berlin, an old and intimate friend of Lord Kelvin : 

 ' His peculiar merit consists in his method of treating 

 problems of mathematical physics. He has striven with 

 great consistency to purify the mathematical theory from 

 hypothetical assumptions, which were not a pure expres- 

 sion of the facts. In this way he has done very much to 

 destroy the old unnatural separation between experimental 

 and mathematical physics, and to reduce the latter to a 

 precise and pure expression of the laws of the phenomena. 

 He is an eminent mathematician, but the gift to translate 

 real facts into mathematical equations, and vice versa, is 

 by far more rare than that to find a solution of a given 

 mathematical problem, and in this direction Sir William 

 Thomson is most eminent and original.' When Lord 

 Kelvin began his work, the equivalence of heat and 

 energy was unrecognised; forces were distinguished as 

 ' conservative ' and ' unconservative ' ; the world was sup- 

 posed to be filled with subtle fluids and effluvia ; and it 

 must have seemed almost hopeless to seek any general 

 explanation of material phenomena. Light, heat, elec- 

 tricity, magnetism, and chemical action were all regarded 

 as distinct 'forces,' each a cause of change. Thomson, 

 and his collaborator Tait, the late Professor of Physics in 

 Edinburgh, in their Treatise on Natural PhilosopJty, did 

 much to emphasise the view that Physics deals with 

 things, not theories ; with relations, not with their mathe- 

 matical expression, equations ; and they tried successfully 

 to free the science from the bonds of formal mathematics. 

 They demonstrated that the principle of ' Least Action ' 

 is universal ; that by its help it is possible to explain the 

 motions of the planets and their satellites, of wheels, 

 lathes, machines of all kinds, of every system of which 

 we can define the moving parts and the forces which act 

 on them. 



