DECOMPOSITION OF H f DRIODIC ACID. CREIGHTON. 7 



it was under observation. Similar hydnodic acid solutions 

 made up with the less pure water were subjected to the influ- 

 ence of sunliu^ht, and in that case also the iodine increases at 

 first, reaches a maximum after some days, and finally disappears. 

 Hence the effect of impure water is of the same luiture 

 whether the solution be left in the dark or acted on by the 

 sunlight. It will be seen later that in certain circumstances 

 radium has the same effect on solutions made up with 

 pure water. 



It would seem that the effect of the small amount of 

 impurity in the water is to cause the iodine, by some sort of 

 catalytic action to change into a third iodine product in addi- 

 tion to the hydriodic acid and free iodine, which alone we 

 might at first expect. In the case of solutions made up with 

 the purer water, where the iodine content tends towards a 

 constant asymptotic value, as given in the third column of the 

 above table the simplest explanation is that a third product is 

 not being formed, and that we have there the ordinary 

 equilibrium between the hydriodic acid, the hydrogen and the 

 iodine. If the third product is still being formed, two sugges- 

 tions present themselves to account for the continued constancy 

 of the amount of free iodine present : (I) that the rate of 

 formation of the third product is very small, but that in time 

 the numbers in the last column of the table would begin to 

 decrease also; (2) that the whole system reaches a state of 

 equilibrium, and the iodine content will be constant however 

 long the time. The former suggestion is the more probable 

 one, since it is likely that by a more careful distillation of the 

 water we have not got rid entirely of the cause of the trouble, 

 but cm\y reduced it in amount. 



This, however, is not the only effect of the impurity in the 

 water ; it also accelerates the rate of accumulation of iodine. 

 This is evident from the fact that the maximum value reached 

 in the case of the less pure water is greater than the asymptotic 

 value approached in the case of the more pure sample. As it 



