PROFESSOR IN BERLIN 343 



violent motion of the gas has been retransformed into heat 

 by friction. But if heat becomes latent under such slight 

 modifications as the alteration in the volume of a gas, we must 

 expect corresponding latency and disengagement of heat in 

 all the countless alterations of aggregation and density that 

 occur in almost all chemical processes. And it appears no 

 more uncertain than in the case of the latent heat of steam, 

 that all the quantities of heat as here described must be referred 

 to the bound energy comprised under Carnot's law, and are 

 therefore to be regarded as heat, which was present as such 

 in the initial states of the substance, but has no place in the 

 final states at the same temperature, and is evolved. But 

 the opposite process may equally well occur. The final states 

 may require a greater quantity of latent heat at the same tem- 

 perature, and the initial temperature may be reinstated only at 

 the cost of the heat contained in the surrounding bodies. In 

 the former case the ' heat toning ' (purely chemically developed 

 heat) will appear to be increased, in the latter to be diminished. 



' If we want to determine the largest quantity of free energy 

 that can be obtained by chemical processes, the same general 

 considerations hold good as were laid down by Carnot. Pre- 

 cautions must be taken to ensure the reversible character of 

 the entire process : i. e. the working forces must be held at 

 equilibrium by other forces which are under the control of the 

 observer, so that the entire process shall take place slowly and 

 quietly, without development of violent disturbances, in which 

 the vis viva might be converted by impact and friction into 

 heat. All friction, inelastic impact, and transfer of heat between 

 bodies of different kinds must be entirely avoided. The reversi- 

 bility of the process is conditioned by the fact that with perfect 

 equilibrium of internal and external forces, the observer has it 

 in his power to reverse the process by a slight reinforcement 

 of the latter. 



' Nor is it only in the practical task of obtaining motive power 

 for other purposes by means of chemical forces that this 

 separation between free and bound energy plays an essential 

 part : it obtains in the region of chemical phenomena also. A 

 chemical process cannot appear of itself, or go forward, un- 

 supported by external motive forces, without diminution of the 

 total sum of free energy in the co-operating bodies. 1 



