Mr. M. M. P. Muir on Chemical Notation. 17 



must regard chemical action as constantly taking place. To 

 use Dr. Mills's words (Phil. Mag. Jan. 1876), "Chemical 

 action can begin, because it never has ceased." 



38. Suppose, however, that the molecules of a given sub- 

 stance (A) come into contact with those of another substance, 

 (B), and that these substances are capable of exercising chemi- 

 cal action upon each other, we shall have an interchange of 

 "atoms" between the molecules A and B; at any moment, 

 therefore, we shall have four kinds of molecules present — A and 

 B, the generating substances, and A / and B x , the products. We 

 shall have an exchange of atoms taking place between A and 

 B, an exchange of atoms taking place between A / and W (and 

 possibly an exchange of atoms taking place between A and A', 

 and between B andB'&c). Now the final condition of atomic 

 distribution among the reacting molecules will depend chiefly 

 upon the rate of atomic interchange; if more changes take place 

 between the atoms constituting the molecules A and B than 

 between the atoms constituting the molecules A' and B / in 

 equal times, it is clear that an increase in the number of the 

 molecules A' and B / will take place ; and conversely, if more 

 atomic changes take place between A' and B' than between A 

 and B in equal times, the number of A and B molecules will 

 increase ; in other words, the decomposition of A and B will 

 cease. Again, if by any means it is possible to remove the 

 newly formed molecules A! and W from the sphere of action, 

 then the atomic interchange represented by the equation 

 A + B = A / + B / will proceed until A and B have entirely 

 disappeared, and the sum of the weights of A / and B / is equal 

 to the original sum of the weights of A and B. Such an action 

 as this very generally takes place in ordinary chemical reac- 

 tions. For instance, the equation 



BaCl 2 + Na 2 S0 4 = 2NaCl + BaS0 4 



represents the final atomic arrangement of the molecules taking 

 part in the reaction ; but we must imagine that a secondary 

 reaction, symbolized thus, 



2NaCl + Ba S0 4 = Na 2 S0 4 + BaC] 2 , 



also takes place, but only to a very limited extent, because the 

 molecule BaS0 4 is removed from the sphere of action almost 

 as quickly as it is produced. It is true, as has been urged, 

 that the equation Zn + H 2 S0 4 = Zn S0 4 + H 2 has never been 

 rigorously realized in practice. Why? Because, before the 

 atomic interchanges symbolized are completed, so many mole- 

 cules having the composition ZnS0 4 have been formed, and 

 have not been removed, that the completion of the change be- 

 comes impossible. Remove the products of the action, how- 

 Phil. Mag. S. 5. Vol. 2. No. 8. July 1876. C 



