CONCLUDING REMARKS. 665 



FIG. 331. SIR WILLIAM THOMSON. 



which, Proteus-like, appears now in one form, now in another. These 

 physical agencies are correlated, so that no one of them can be said 

 absolutely to be the essential cause of the rest, but any one of them 

 may produce another. The transformation, however effected, always 

 takes place in definite and invariable quantitative relations. Thus, so 

 much heat will produce so much electricity ; and this quantity of elec- 

 tricity will reproduce the original quantity of heat ; and so on of the 

 rest. The relations already described as existing between heat, me- 

 chanical energy, electricity, and magnetism, for the quantitative deter- 

 mination of which science is indebted to Mr. J. P. JOULE, of Man- 

 chester, may serve as examples of these transformations. It was, 

 indeed, as we have already observed, the experiments of Mr. Joule 

 which placed the doctrine of the transmutation and indestructibility 

 of energy on a firm basis. 



To Sir William Thomson, the distinguished physicist, some of whose 

 beautiful electrical instruments were referred to on page 573, we owe 

 some researches into the subject of the transmutations of energy which 

 have conducted him to some very remarkable conclusions. Sir William 

 has not been mentioned in the preceding pages in a manner which can 

 adequately represent his signal services to science, especially to elec- 

 tricity, with which he is probably more profoundly conversant than any 

 savant living. He was the son of an eminent .professor of mathematics, 



