368 Mr. J. P. Cooke on 



changes, which bring it into a new condition without produ- 

 cing any mechanical effect on external bodies, the amount of 

 heat evolved or absorbed, as the total result of these changes, 

 depends solely on the initial and final states of the system, 

 and is the same, whatever may be the nature or order of the 

 intermediate states. 



Principle of Maximum Work. 



III. In any chemical reaction between a system of bodies 

 not acted on by external forces, the tendency is toward that 

 condition and those products which will result in the greatest 

 evolution of heat. 



The first two of these principles are direct deductions from 

 the mechanical theory of heat ; but the third is a generaliza- 

 tion which Berthelot claims as original ; and if so, it is his 

 greatest contribution to this department of the science. In 

 the work before us the first two principles are discussed in the 

 first volume, and this discussion, together with a description 

 of the methods of experimenting and an enumeration of the 

 numerical data thus far obtained, fill nearly 600 large octavo 

 pages ; while the discussion of the third principle occupies a 

 second volume which is still larger. We will follow the same 

 order in the few remarks which the limits of a short notice 

 permit. 



As the announcement of an almost axiomatic principle of 

 thermodynamics, which every investigation of thermo-che- 

 mistry necessarily assumes, the first of these general principles 

 has an appropriate place at the opening of a discussion of the 

 subject. In regard to heat, as in regard to other manifesta- 

 tions of energy, the total effect is equal to the sum of all the 

 partial effects. But Berthelot adds to his statement of the 

 Principle of Molecular Work the remark, "This principle 

 furnishes the measure of chemical affinities." When, how- 

 ever, we come to his discussion of this general principle, we 

 are disappointed to find that the whole subject is summarily 

 dismissed without giving the reader any clear conception of 

 the distinction between the two modes of change whose results 

 are inextricably blended in all chemical processes. 



Were we able to distinguish between chemical and physical 

 change, the first principle would undoubtedly give us a mea- 

 sure of what we might then clearly define as chemical affinity 

 or chemism. Not only, however, is it at present impossible to 

 eliminate from our results the effects of physical changes, but, 

 moreover, when we study the details of the chemical processes 

 with which we are most familiar, we are surprised to find to 

 what a large extent the thermal effect depends on the changes 



