DECOMPOSITION OF HYDRIODIC ACID. — CKEIGHTON. 11 



If now, in t' e reaction under investitjation, we assume that 

 the hydriodic acid breaks down into iodine, and that this in 

 turn breaks down into a third substance, then we have a case 

 which is similar to the successive chancres whicli take place in 

 the break down of radium. Rutherford^ has shown that if in 

 such a change as this n is the amount of any substance A, in 

 this case hydriodic acid, initially present, then the amount of 

 B, in this case free iodine, at any time is given by the equation 



y=^(--^-''---^'') w 



where Xj and X, represent the rates of change of A into B and 

 of B into C, respectively, where C is the third product. 



Assuming that this third product is formed, there seem to 

 be three probable ways in which the radium may act. 



(1) The production of iodine is accelerated and also the 

 production of the new product into which the iodine is changed. 



(2) The production of iodine is unaffected, but that of the 

 third product retarded. 



(3) The production of the iodine is accelerated, while the 

 production of the third product is retarded. 



Of these three cases the two latter seem to be the least 

 probable. 



Let us apply equation (1) to the results of observation at 

 24*>C. If the second change is very slow or zero, that is, if 

 \ is negligible, the amount of free iodine at the end of time t 

 would be given by the equation 



y = n ( 1 - e^V) (2) 



Solving this equation for \, we get 



^^ log 71 -log jn-y) ^g^ 



t logio© 

 Substituting in this equation values of t and y obtained 

 from columns 1, 2, and 4 of Table II, we derive for \ values 

 which show a very satisfactory agreement, as is seen in the fol- 

 lowing table : 



1. " Radioactivity," p. 332. 



