286 



NATURE 



\jfuly 29, 1880 



But in actual chemical reactions the action of the 

 chemical force proper will be interfered with and com- 

 plicated by physical, or secondary forces. So much is 

 this the case, that for many years these actions were not 

 distinguished. 



One school simply measured the quantity of a substance 

 A which was needed to act on B to produce C ; the 

 oreater the quantity of A required to act on a given weight 

 of B, the greater was the affinity of A for B. With this 

 school all was chemical. With Berthollet, on the con- 

 trary, all was physical; but facts have been discovered 

 since the publication ofthe " Statique Chimique " which 

 have necessitated a reconsideration of his laws. 



Gradually the meaning of affinity has been made clear. 

 The greatest contribution towards this end is undoubtedly 

 the papers of Guldberg and Waage, whose work has been 

 sketched by the present writer in this journal (vol. xx. 

 p. 530). The Swedish naturalists disregard the action of 

 secondary forces in their method of determining the ratios 

 between the affinity coefficients of various substances. 



The importance of a measurement of the change of 

 energy accompanying the passage of a chemical system 

 from one specified state to another ; the importance, in 

 other words, of a measurement of the heat evolved or 

 absorbed in such a passage, is at once apparent. But 

 this measurement— even taken along with a general 

 knowledge of the conditions of existence of the various 

 possible systems— does not enable us certainly to predict 

 the result of the chemical action. If we had a complete 

 knowledge of the mode of variation of the potential energy 

 of a system with changes in the configuration of the 

 system, then it mii;ht be possible for mathematicians to 

 predict all possible arrangements of the system under the 

 action of specified external forces. But having made 

 heat measurements only, we are very far indeed from this 

 point. 



Indeed the fundamental assumption that chemical 

 energy is wholly potential, and depends on the arrange- 

 ment of the parts of a system, may be false ; and even 

 if this assumption be true we know nothing as yet of the 

 relation between this energy and the configuration of the 

 system. 



The heat absorbed or evolved in a chemical change 

 measures the total work done by the system in its 

 passage from one specified state to another, but it is 

 evident that it does not directly measure the true force of 

 affinity. The stress between the parts of two molecules 

 may be small, yet under certain conditions a chemical 

 change may occur ; the loss of energy in the formation of 

 the new system may be considerable, and hence the heat 

 evolved, considerable. Chemical affinity thus regarded is 

 a kind of liberating force. 



For the measurement of the ratios of the affinities of 

 various systems, Guldberg and Waage's method is to be 

 preferred to the thermal method of Berthelot. For a full 

 consideration of chemical equilibrium Berthelot's method 

 is altogether insufficient, although it has largely advanced 

 the solution of this problem. 



The method of Willard Gibbs seems the only feasible 

 one in the present state of the chemical and mathematical 

 sciences. In this method (see Nature, vol. xxi. p. 516) 

 the energy and entropy of a system are considered — the 

 stabiUty of a system depends on the component masses, 



volume, and entropy (the niagnitiidcs of the system) ; and 

 on the temperature, pressure, a.nd poli'iillal {the intensities 

 of the system). 



The stability of a system is chiefly dependent, according 

 to Berthelot, on the amount of heat evolved in the passage 

 to the given state from an initial state, and on the general 

 properties of the given system as compared with other 

 possible systems. This is evidently a much cruder 

 statement than that of Gibbs. Berthelot's principle of 

 maximum work is indeed one among many deductions 

 made by the method of the American professor. 



Both methods lead to a recognition of chemical equi- 

 librium as an outcome of chemical action ; the conditions 

 of the latter are considered before those of the former ; 

 chemical kinetics precedes chemical statics. The usual 

 method of the text-books is to make chemical equilibrium 

 all-important, and barely to mention the subject of 

 chemical kinetics. 



It is exident that the time when it will be possible to 

 treat chemical problems by a purely dynamical method is 

 yet distant. The method of Gibbs leads the way in 

 bringing chemical generalisations under the domain of 

 the principles of energy, and it does this without assump- 

 tions about the action of the parts of molecules ; the 

 method is a thermo-dynamical one. 



Berthelot's method, on the other hand, is thermo- 

 chemical ; but a thermo-chemical method seems to 

 promise the largest development in the present state of 

 the science. 



Berthelot perhaps claims too much for his method : in 

 his great work he is not always definite in his use of such 

 terms as "force," "affinity," "energy," " work ;" never- 

 theless the "Essai de Mecanique chimique" is undoubt- 

 edly a great work. To Berthelot (and to Thomsen) is 

 due the honour of having steadily pursued the thermo- 

 chemical method for many years, and of having collected 

 masses of most important facts; and he has now enriched 

 chemical science by the publication of these results in a 

 collected and systematic form, in a treatise full of original 

 ideas and suggestive of almost unhmited topics for future 

 work and discussion. What a field of work is opened 

 before one in this book ! To determine that this body is 

 produced by the action of these bodies is not enough ; 

 indeed it is scarce a beginning. Chemical science has 

 higher aims. The changes of energy which accompany 

 changes of configuration of matter must be measured; 

 the physical and chemical constants of all the products of 

 a chemical change must be determined with care, the 

 velocity of the change must be measured, and an attempt 

 must be made to apply dynamical reasoning to the results 

 thus obtained. 



The first volume of the "Essai," entitled " Calori- 

 metrie," begins with general remarks on thermo-chemical 

 work, and on affinity ; after laying down certain general 

 theorems concerning chemical reactions, and illustrating 

 the application of these in the formation of insoluble and 

 soluble salts, the formation of series of carbon com- 

 pounds, &c., a detailed account is given of experi- 

 mental calorimetric methods; this is accompanied by 

 numerous tables of specific heats, heats of combination, 

 heats of solution, heats of formation of salts in solution, 

 heats accompanying isomeric changes, &c. The second 

 volume— entitled " Mdcanique "— is concerned with a 



