An Investigation of a Case of gradual Chemical Change. 423 



material as above, and the potassium chloride formed exercises its 

 specific effect on this reaction. 



The small quantity of potassium iodide added for the oxidising 

 material to work upon is not concerned in the primary reaction. The 

 secondary action upon it producing iodine is practically instantaneous, 

 unless its quantity is below a certain minimum. Below this there is 

 a retardation of the velocity apparent. The effect of increasing the 

 amount of this substance beyond the minimum is apparently analo- 

 gous to that of a similar increase of any neutral salt. 



The velocity of decomposition is an exponential function of the 

 temperature ; as the latter increases in arithmetical progression, the 

 former increases in geometrical progression. The velocity is about 

 doubled for a rise of 5 C. in temperature. The ratio in the 

 geometrical progression is not, however, absolutely constant, but 

 varies a little with the actual temperature. Between and 15 the 

 velocity is a little more than doubled by a rise of 5, between 20 and 

 30 a little less than doubled. 



VOL. XLV. 



