Chemical Affinity. 185 



means the equilibrium of a system the parts of which are at 

 rest. 



So long as equal amounts of A' and B', and of A and B, 

 are formed in unit time, so long is the equilibrium of the sys- 

 tem maintained. 



Let it be supposed that the molecule A consist of the atoms 

 (or molecules) a and 7, these atoms will continually perform 

 their proper vibrations within the molecule A. At certain 

 positions of « and 7 we may suppose that the force acting 

 between these two atoms is very small. Let another molecule 

 B be composed of the atoms /3 and 8, these atoms will also 

 perforin vibrations within B ; A and B will also each be pos- 

 sessed of its own proper motion. If A and B approach each 

 other at the moment when a and y and /3 and S are respec- 

 tively at those positions at which the force between them is 

 very small, then a chemical force may act between /3 and y 

 and between ex. and S which shall result in the production of 

 two new molecules A / and B'. The reverse action, resulting 

 in the production of A and B, may be supposed to take place 

 in a manner similar to that whereby A! and B' have been 

 formed. 



A similar view may be taken of the formation of an addition 

 compound. The compound molecule ABC may be split up, 

 under certain conditions, into A, B, and C, while by the simul- 

 taneous recombination of the molecules A, B, and C new com- 

 pound molecules of the form ABC may be produced. 



In order to determine the velocity of formation of the new 

 substances in such cases as the preceding, let p and q repre- 

 sent the number of the molecules of A and B in unit volume ; 

 further, let a be the number of p molecules which are so con- 

 ditioned that by coming into contact with the molecules B 

 they suffer decomposition, and let b represent the number of q 

 molecules which are similarly conditioned with regard to A . 

 In unit volume there are therefore ap molecules of the sub- 

 stance A, and bq molecules of the substance B, which by their 

 mutual meeting together are capable of being transformed into 

 new substances. The frequency of the meeting of undecom- 

 posed molecules will be represented by the product ap . bq, and 

 the velocity of formation of the new substances by the equa- 

 tion 



(p . apbq=kpq, 

 where k = (j)ab. 



If it be required that three substances A, B, and C suffer 

 mutual decomposition in order that a new compound be 

 produced, and if the number of molecules of each in unit 

 volume be p, q, and r respectively, then, expressing the proper 



