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472 of Force,' in which he 'endeavored to ascertain all the relations between the different natural processes.' In his lecture of 1854, he credits earlier writers on the subject, in the following order: Carnot (1824), Mayer (1842), Colding (1843), Joule (1843), and states that he was awakened to this work by the last-named.

To the Carnot law, Helmholtz gives the following expression: 'Heat only when passing from a warmer to a colder body, and then only partially, can be converted into mechanical work.'

This is obviously no other than the essence of the principle as not only asserted, but actually proved, a quarter of a century before Carnot by Benjamin Thompson and Humphry Davy, by direct experiment, so far as it is an assertion of the convertibility of the two energies. Helmholtz acknowledges the indebtedness of the scientific world to Mayer, whose paper 'On the Forces of Inorganic Nature' had been printed in 1842, that 'On Organic Motion and Nutrition' in 1845, and that 'On Celestial Dynamics' in 1848; while his paper 'On the Mechanical Equivalent of Heat' was not published until 1851.

Helmholtz concludes: 'Thus the thread which was spun in darkness by those who sought a perpetual motion has conducted us to a universal law of nature which radiates light into the distant nights of the beginning and to the end of the history of the universe.'

Dr. W. B. Carpenter, in a lecture before the Royal Society, published later in their Transactions for 1851, summarized the work in this field, to his date, under the title 'The Correlation of the Vital and Physical Forces,' and showed, probably for the first time in this field, the unity of the principle of equivalence of energies in organic and vital, as well as in inorganic and lifeless nature. He attributes to Dr. Mayer, of Heilbronn, the first annunciation of the great principle of 'Conservation of Force,' in its then broadest form, in his paper of 1845, already mentioned; while Carpenter considers his own paper of 1850 'On the Mutual Relations of the Vital Physical Forces, as the first announcement of the extension of the law beyond the latter class of phenomena into the range of vital energies. It is in his lecture on this subject that Carpenter states the fact, since recognized perhaps most explicitly, among contemporary writers, by Haeckel, that 'what the germ supplies is not the force but the directive agency.' 'The actual constructive force is supplied by heat.' Even 'the life of man, of any of the higher animals, consists in the manifestation of forces of various kinds, of which the organism is the instrument,' and, further: 'during the whole life of the animal, the organism is restoring to the world around it both the materials and the forces which it draws from it.'