124 Mr. W. H. Pendlebury and Miss M. Seward. [Dec. 13, 



III. " An Investigation of a Case of Gradual Chemical 

 Change." By W. H. PENDLEBURY and M. SEWARD. Com- 

 municated by A. G. VERNON HARCOURT, Esq., F.R.S. 

 Eeceived November 27, 1888. 



(Abstract.) 



The case of gradual chemical change with which the present investi- 

 gation deals is that between hydrogen chloride and potassium chlorate, 

 and also its reaction with hydrogen chlorate whether alone or in pre- 

 sence of potassium chloride. 



When dilute solutions of a chlorate (as for instance potassium 

 chlorate) and hydrogen chloride are mixed together, the liquid slowly 

 acquires a chlorous smell, and there is a gradual liberation of oxidising 

 material, chlorine, and oxides of chlorine. These immediate products 

 cannot easily be investigated, for if the mixture is left to itself so 

 that they accumulate in it, the gradual reaction first observed is 

 stopped, and there ensues decomposition of the usual complex nature 

 of these unstable solutions of chlorine and its oxygenated com- 

 pounds. 



But if a small quantity of potassium iodide is present it will be 

 decomposed by these substances, and iodine will be gradually 

 liberated as the final product of the reaction we have mentioned. 



Now Messrs. Harcourt and Esson, in their work on a gradual 

 chemical change, measured the rate at which iodine was liberated in 

 a liquid by ascertaining the time taken for a knowu quantity of 

 sodium thiosulphate added to that liquid to be entirely decomposed. 

 A small quantity of starch solution was added at the same time, and 

 served as the signal of the presence of free iodine, which meant that 

 the measured quantity of sodium thiosulphate was exhausted. The 

 observation to be made was of the interval of time which elapsed 

 between the addition of the thiosulphate and the first appearance of 

 a blue starch coloration. 



The same measurement and the same signal served our purpose. 

 The first obvious difference between the two reactions is that, whereas 

 in the former one (between hydrogen dioxide and hydrogen iodide) 

 iodine was the primary result, in the later one it is a secondary 

 result. This proved an unimportant difference. The secondary reac- 

 tion between potassium iodide and the results of the first is, by 

 comparison, instantaneous. But another difference is of great im- 

 portance. In their reaction the rate of decomposition became 

 gradually slower, as one of the substances reacting continued to 

 decrease sensibly in amount, and finally disappeared. In this reaction 

 the amount of each substance decomposed bears an infinitely small 



