An Investigation of a Case of gradual Chemical Chanye. 399 



In our ordinary mode of working the reaction batween chlorate 

 and chloride occurred in presence of iodide. In order to examine 

 the reaction when only chlorate and chloride were present and the 

 products of their reaction were not at once reduced, and thus 

 removed, the following experiments were made. Through a vessel 

 containing a mixture of hydrogen chloride and potassium chlorate, 

 kept at a constant temperature of 30, a certain volume of air could 

 be drawn at a fixed rate. The air, thus charged with a part of what- 

 ever gas was liberated in the mixture, was drawn through a series of 

 washing-tubes containing potassium iodide. The liberated iodine 

 was determined at the end of equal intervals of time. It was found 

 that comparatively little oxidising gas was evolved. At the end of 

 20 hours the amount of gas dissolved in the mixture, capable of 

 liberating iodine, was determined, and this quantity also was found 

 to be very small. The remarkable diminution in the rate of forma- 

 tion of oxidising substance when no iodide was present will be 

 evident when it is stated that whereas, in the presence of iodide, the 

 change proceeded at such a rate that in 20 hours the amount of 

 iodine set free would have corresponded to 6700 c.c. of the standard 

 thiosulphate, the oxidising material formed in absence of an iodide 

 only set free iodine corresponding to 100 c.c. thiosulphate. 



The action was also found to be reversible in sunlight. Some of 

 the mixture of potassium chlorate and hydrogen chloride, which had 

 acquired a deep yellow colour, was exposed for a short time to bright 

 sunlight; the solution became colourless, and was found to liberate 

 no iodine. In our experiments, in presence of an iodide, we found 

 sunlight to have no effect upon the rate of change. 



It would thus appear that when the oxidising substance is produced 

 in presence of an iodide it does its oxidising work at once and is re- 

 moved, and the change proceeds uniformly. In the absence of an 

 iodide, however, the oxidising substance accumulates in the liquid, 

 and its further production is impeded probably by the occurrence of a 

 reverse action. 



Though the potassium iodide appears thus to be a necessary ingre- 

 dient of the mixture if the change is to proceed at a uniform rate, it 

 does not take part in the primary reaction ; for otherwise variation in 

 the amount of potassium iodide in the mixture, other things being 

 unaltered, would produce very marked differences in the rate. That 

 this is not the case was proved by an experiment, the results of which 

 are shown in the following table : 



