172 SCIENTIFIC THOUGHT. 



It measured chemical reactions by what is termed their 

 heat - toning, i.e., by the amount of heat developed, 

 and culminated in the celebrated third law of thermo- 

 chemistry viz., that such reactions take place as are 

 accompanied by the greatest amount of energy liberated 

 in the form of heat. Now, although this contains an 

 adequate description of a very large number of reactions 

 that take place at the temperatures at which we operate 

 in our laboratories, the rule is by no means universal, 

 and it required a great amount of ingenuity to ex- 

 plain away the many exceptions which presented them- 

 selves. The rule needed to be modified or amplified. 

 The measurement of the energy of a chemical process by 

 the heat-toning was not the only instance in which the 

 thermal side of a phenomenon had been considered a 

 sufficient means of measuring. In an allied department, 

 that of electrolysis, Helmholtz had suggested, as early 

 as 1847, that the electro-motive force of a galvanic cell 

 may be measured by the heat-toning of the chemical pro- 

 cesses which produce the current, and for a long time 

 this was considered to be a correct expression of facts. 

 In consequence, however, of some discrepancies which 

 had presented themselves, Helmholtz himself was induced, 

 about 1881, to examine the subject more thoroughly. 

 He arrived at the conclusion that the heat-toning is not 

 always a correct measure ; and at the same time he intro- 

 duced a more adequate and generally applicable method 

 of measurement. In fact, he arrived at the conception 

 of available or useful energy for processes which take 

 place at constant temperature. To this quantity, which 

 decides in which direction a reaction takes place (tempera- 



