THE LAW OF SUBSTANCE. 471 



cisely the idea now held by Haeckel and other contemporary men of 

 science. 



Mayer accepted the principle and, basing his computations on the 

 then accepted values of the specific heat of air, determined an equally 

 approximate mechanical equivalent. Joule followed, in 1845-49, and 

 later, determining this equivalent by admirable direct experiment. 

 English writers have sometimes insisted upon assigning all credit to the 

 latter for this determination; but Tyndall is less insular in his attitude 

 and frankly and cordially gives Mayer the credit to which he is un- 

 doubtedly entitled. Both are certainly to be credited with important 

 original work, and the method of Mayer gives a more accurate and cer- 

 tain measure of the constant sought than did any of the earlier experi- 

 ments of the English physicist, the more exact measures of specific heats 

 as now known being employed. Had Mayer known of Kegnault's work, 

 or had that work been completed before Mayer attempted his computa- 

 tions, the latter would have obtained more accurate figures than Joule 

 secured years afterward. It was only when Prof. Henry A. Row- 

 land took up the task and performed his marvelously fine work that an 

 acceptable valuation was secured. 



Meantime, Helmholtz had accepted and applied the law of equiva- 

 lence of the energies broadly, as holding in all physical phenomena; but 

 he was distinctly anticipated by Grove, the English physicist, who in 

 January, 1842, in a lecture before the London Institution, asserted that 

 'Heat, light, electricity, magnetism, motion and chemical affinity are all 

 convertible material affections' and that 'all these affections are resolv- 

 able into one, namely motion.'* This thesis he enforced then and 

 thenceforward continuously. In 1862, he summarized his work in a 

 published study of 'The Correlation of the Physical Forces,' later re- 

 printed by Youmans in his famous collection of similar papers of 1864. 

 His paper concludes with an excellent bibliography, in which he shows 

 the origin of the now unquestioned view of authority in the minds of 

 the old Greeks, and its gradual establishment by observation, experience 

 and, finally, by experiment in the nineteenth century. 



Helmholtz's lecture 'On the Interaction of the Natural Forces' was 

 delivered at Konigsburg,.in 1854; he at the time holding the professor- 

 ship of physiology at that university. In this lecture he states his first 

 ideas to have been published in a pamphlet, in 1847, 'On the Conserva- 



* Perhaps the best presentation of the work of the earlier men of science, rec- 

 ognizing these great and fundamental truths, is that of Prof. Edward L. You- 

 mans, the founder of the Popular Science Monthly and one of the most 

 broad-minded and far-seeing men of his time, who, in his 'Correlation and Con- 

 servation of Forces,' published by the Appletons in 1865, brought together the rec- 

 ords of the great pioneers in this evolution of the scientific basis of all natural 

 science, including the papers of Grove, Helmholtz, Mayer, Faraday, Liebig and 

 Carpenter. 



