Page:Popular Science Monthly Volume 58.djvu/479

Rh 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 undoubtedly entitled. Both are certainly to be credited with important original work, and the method of Mayer gives a more accurate and certain measure of the constant sought than did any of the earlier experiments of the English physicist, the more exact measures of specific heats as now known being employed. Had Mayer known of Regnault's work, or had that work been completed before Mayer attempted his computations, the latter would have obtained more accurate figures than Joule secured years afterward. It was only when Prof. Henry A. Rowland 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 equivalence 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 resolvable 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 reprinted 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 Königsburg, in 1854; he at the time holding the professorship of physiology at that university. In this lecture he states his first ideas to have been published in a pamphlet, in 1847, 'On the