AIM AND PROGRESS OP PHYSICAL SCIENCE. 379 



or a certain consumption of motive force. If motive 

 power be developed it may either appear as such, or be 

 directly used up again to form other changes equivalent 

 in magnitude. The leading determinations of this equiva- 

 lency are founded on Joule's measurements of the me- 

 chanical equivalent of heat. When, by the application 

 of heat, we set a .«5team-engine in motion, heat propor- 

 tional to the work done disappears within it; in short, 

 the heat which can warm a given weight of water one 

 degree of the Centigrade scale is able, if converted into 

 work, to lift the same weight of water to a height of 

 425 metres. If we convert work into heat by friction 

 we again use, in heating a given weight of water one 

 degree Centigrade, the motive force which the same 

 quantity of water would have generated in flowing down 

 from a height of 425 metres. Chemical processes gene- 

 rate heat in definite proportion, and in like manner we 

 estimate the motive power equivalent to such chemical 

 forces; and thus the energy of the chemical force of 

 affinity is also measurable by the mechanical standard. 

 The same holds true for all the other forms of natural 

 forces, but it will not be necessary to pursue the subject 

 further here. 



It has actually been established, then, as a result of 

 these investigations, that all the forces of nature are 

 measurable by the same mechanical standard, and that 

 all pure motive forces are, as regards performance of 

 work, equivalent. And thus one great step towards the 

 solution of the comprehensive theoretical task of referring 

 all natural phenomena to motion has been accomplished. 



Whilst the foregoing considerations chiefly seek to 

 elucidate the logical value of the law of the conservation 

 of force, its actual signification in the general conception 

 of the processes of nature is expressed in the grand con- 

 nection which it establishes between the entire processes 



