178 SCIENTIFIC THOUGHT. 



abroad, I will try to sum up finally the principal points 

 in it which are of importance for the history of con- 

 temporary thought. 



Ever since the conception of energy as a quantity 

 which, like matter, is preserved in all natural processes, 

 forced itself with more or less clearness upon natural 

 philosophers, the question has been insistent as to the 

 number of different forms in which this quantity can 

 manifest itself ; and some of the earliest propounders of 

 the doctrine attempted an enumeration of the different 

 forms, mechanical energy of motion and of attraction 

 usually heading the list. When that form of energy 

 which we call heat was subjected to examination, and the 

 remarkable property formerly called latent heat defined 

 in the new terminology, the want arose of bringing about 

 some kind of connection between our ideas of motion and 

 those of heat, which were shown to be mutually con- 

 vertible quantities in nature. Before that time sound 

 and light had already yielded to the kinetic view, and an 

 enormous increase of our knowledge in acoustics and 

 optics had followed. Thus we find some of the pioneers 

 of the physical or energy view of nature notably Eankine 

 and Joule in this country, Eedtenbacher and subsequently 

 Clausius abroad engaged in translating the properties 

 of heat into mechanical analogies. 1 It was not thought 



1 Rosen berger, in his ' Geschichte 

 der Physik ' (vol. iii. p. 550, &c.), 

 gives a number of references to 

 theories mostly forgotten which 

 were published before and after 

 the year 1850. Clausius, who 

 keeps his mechanical theory of 

 heat quite separate from his kinetic 

 theory of gases (see the three 



volumes on ' Die mechanische 

 Wiirmetheorie,' 2nd ed., 1876, &c.), 

 admits, nevertheless, in a paper 

 published in 1857 (Pogg. 'Ann.,' 

 vol. c., and ' Mechan. Wiirmetheor.,' 

 vol. iii. p. 1, &c.), that "from the 

 beginning of his researches refer- 

 ring to heat he had attempted to 

 account to himself for the internal 



