Middle of the Nineteenth Century. 241 



chemical energy of the system itself, but at the expense of the 

 thermal energy of neighbouring bodies. Now in the case of 

 the voltaic cell, the principle of Eoget, Faraday, and Joule is 

 expressed by the equation 



^ = A, 



where E denotes the available or electrical energy, which is 

 measured by the electromotive force of the cell, and where X 

 denotes the heat of the chemical reaction which supplies this 

 energy. In accordance with Thomson's principle, we must 

 replace this equation by 



F \ 4- T dE 

 ^ =X + T dT' 



which is the correct relation between the electromotive force 

 of a cell and the energy of the chemical reactions which occur 

 in it. In general the term A is much larger than the term 

 T dEjdT ; but in certain classes of cells e.g., concentration- 

 cells A is zero; in which case the whole of the electrical 

 energy is procured at the expense of the thermal energy of 

 the cells' surroundings. 



Helmholtz's memoir of 1847, to which reference has already 

 been made, bore the title, " On the Conservation of Force." It 

 was originally read to the Physical Society of Berlin*; but 

 though the younger physicists of the Society received it with 

 enthusiasm, the prejudices of the older generation prevented 

 its acceptance for the Annalen der Physik ; and it was eventually 

 published as a separate treatise.f 



In this memoir it was asserted* that the conservation of 



* On July 23rd, 1847. 



t Berlin, G. A. Reimer. English Translation in Tyndall & Francis' Scientific 

 Memoirs, p. 114. The publisher, to Helmholtz's "great surprise," gave him an 

 honorarium. Cf. Hermann von Helmholtz, by Leo Koenigsbeiger ; English 

 translation by F. A. Welby. 



j Helmholtz had been partly anticipated by "W. R. Grove, in his lectures on 

 the Correlation of Physical Forces, which were delivered in 1843 and published in 

 1846. Grove, after asserting that heat is " purely dynamical " in its nature, and 

 that the various " physical forces " may be transformed into each other, remarked : 

 " The great problem which remains to be solved, in regard to the correlation 

 of physical forces, is the establishment of their equivalent of power, or their 

 measurable relation to a given standard." 



P. 



