1908] on the Scientific WorTc of Lord Kelvin. 21i) 



temperature), remained for Gibbs and Helmholtz twenty-four years 

 afterwards. In another paper of the same date, on absohite electric 

 measurement, Thomson discusses Joule's thermal determination of 

 absolute electric resistance of 1846, which afterwards proved to be 

 more correct than the earlier values of the ohm. 



Most interesting in connection with modern ideas is an abstract of 

 February 5, 1852,* again mainly expounding Joule's inspiring results 

 and views on the transformations of energy. Thomson estimates 

 from Liebig's data that about one-thousandth part of the total solar 

 radiation incident on forest land is absorbed usefully by the trees, 

 that being the amount recoverable as heat by their combustion. An 

 intention to discuss these matters in connection with Carnot's principle, 

 dealing also with the wave-lengths of the radiation, does not appear 

 to have been fulfilled. Passing on to animal work, he estimates, after 

 Joule, that as much as one-sixth of the energy of the food consumed 

 can go directly into mechanical power. Then, relying on Carnot's 

 principle, and Joule's discoveries regarding the heat of electrolysis 

 and of electromagnetism, he proceeds to argue that " it is nearly certain 

 that when an animal works against resisting forces, there is not a 

 conversion of heat into external mechanical effect, but the full thermal 

 equivalent of the chemical force is never iwoduced — in other words, 

 that the animal body does not act as a thermodynamic engine ; and 

 very probable that the chemical forces produce the external mechanical 

 effects through electrical means." 



Here he is emerging from the narrower theory of heat to the 

 general theory of available energy, where heat is not the intermediary 

 towards mechanical power ; and we shall see presently how quickly 

 he progressed in it. When it is recalled that at the time all this was 

 going on, or immediately after, he was also laying the dynamical 

 foundations of the phenomena of induced electric currents, including 

 for example, the calculation of the period of the vibrations produced 

 by electric discharges, the activity may well seem unprecedented ; 

 adequate exposition of the results had to fall behind. 



The next stage (1855) in this series of investigations, the develop- 

 ment of the ideas expressed in the extract just quoted, seems to 

 demand special attention, for it is surely nothing less than the laying 

 down of the precise laws of the all-embracing modern science of Free 

 or Available Energy. The evolution of this generalisation can, as it 

 happens, be traced. The memoir on 'A Mathematical Theory of 

 Magnetism ' has been already alluded to. In it, as everywhere else 

 in Thomson's dynamical writings, the conservation of the potential 

 energy, used there in the manner of Lagrange and Green and Mac- 

 Cullagh and Helmholtz, in the sense of a potential of mechanical 

 forces, is employed to determine the essential relations between 



Loc. cit., p. 505. 



