418 



Mr. W. H. Peiidlebury and Miss M. Seward. 



great enough, a retardation following any diminution of the iodide 

 beyond this minimum necessary, and our experiments fully bear this 

 out. The reason probably is that there is a tendency for molecules 

 of oxidising material to begin to accumulate in the liquid if they do 

 not immediately find molecules of potassium iodide to react with. 

 In other words, whereas we are accustomed to consider the second 

 reaction between chlorine or oxides of chlorine and potassium 

 iodide, to be instantaneous, this is true only when the amount of 

 potassium iodide present is beyond a certain minimum. In the 

 mixture above the minimum is between 0*03784 m.g.m.' and 

 0'0473 m.g.m. per c.c., and after that the rate of decomposition 

 remains practically stationary until the amount present is 0' 11 352 

 m.g.m. per c.c. 



The following table shows the effect of variation of iodide by larger 

 quantities at a time, beginning with about half the usual quantity 

 0*76 m.g.m. per c.c. 



Table XII. 



There is a slight increase at the beginning of the series when the 

 quantity of iodide is doubled ; after this the rate remains practically 

 stationary for several increments. There is, however, a marked in- 

 crease between z = 3 and z = 11, but not so rapid as at the beginning 

 of the series. The reason for the indistinctness of form is evidently 

 the fact that the effect of variation of the iodide is within the limits of 

 experimental error. It was, therefore, deemed advisable to vary the 

 iodide by larger quantities. In the following series single grams of 

 the substance were introduced into the cylinder, one after the other. 



