Chemical Affinity. 183 



of hypothesis, to the expression " coefficient of affinity." They 

 thus express themselves : — - 



" Let two bodies, A and B, be converted, by double substitu- 

 tion, into A! and B', and let A' and W be again -reconverted, 

 under the same conditions, into A and B, neither of those changes 

 will be complete. At the close of the reaction there are present 

 four bodies A, B, A', and B'; and the force which caused the for- 

 mation of A / and B' is held in equilibrium by the force which 

 caused the formation of A and B. The force causing the for- 

 mation of A' and B / increases proportionately to the coeffi- 

 cient of affinity of the reaction, but is also dependent upon 

 the quantities of A and B : we have found that this force is 

 proportional to the product of the active masses of A and B. 

 If the latter be p and q respectively, and the coefficient of affi- 

 nity be k, then the force is represented by 



k .p . q. 



This expression also evidently represents the amounts of A 

 and B transformed in unit time into A / and B'. . . . If the 

 active masses of A' and B / be p' and q f respectively, andthe 

 affinity-coefficient of the reaction A' + B / = A + B be equal to 

 ¥, then the force which tends to bring about the reformation 

 of A and B is 



k'p'q'. 



This force being held in equilibrium by the first, we get the 

 equation 



kpq = k'p r q f . 



" By experimentally determining p, q, p 1 ', q' ', the proportion 

 k : k r can be calculated; and from this the result of the reac- 

 tion for each initial condition can be determined." 



Active mass of a body is defined as the amount of that sub- 

 stance in unit volume of the chemical system which undergoes 

 change. The coefficient of affinity is dependent upon the che- 

 mical nature of A and B and upon the temperature. 



The equation expressing the conditions of equilibrium 

 (kpq = k / p'q / ) holds good only when the action of secondary 

 forces, i. e. forces whose action is to be traced to the pre- 

 sence of compounds other than A, B, A x , or B' is overlooked. 



" If the number of molecules of A, B, A' ', and B / before the 

 reaction be represented by P, Q, P', and Q', and if x be the 

 number of molecules of A and B transformed into A / and B', 

 then, supposing the total volume to remain constant during 

 the reaction, we have 



_ P-.? _ Q-.y / _P / + .7- ,_ <y+< 



V— y ? Q— y;2 7 — y j Q — y j 



02 



