1889.] Rate of Chemical Change in Aqueous Solutions. 441 



related to the degree of mobility and diff usibility of the particles, and 

 is largely modified by the degree of stability of the mixture. 



Temperature had great effect upon the solution of potassic iodide 

 and chlorine. Heating the liquid to about 100 C. during two minutes 

 was attended by great loss of voltaic energy, considerable increase of 

 colour, and about 99'8 per cent, of the mixture was changed into 

 potassic chloride and free iodine ; the amount of change was as great 

 as that which took place during 18 days at 12 C. Similarly heating 

 the solution of potassic chloride and iodine had but little effect ; it, 

 however, slightly increased its voltaic energy and decreased its colour, 

 and so far changed it into the other mixture (?). 



Exposing the solution of potassic iodide and chlorine to diffused 

 daylight during 18 dull winter days at about 12 C. did not appear to 

 greatly alter the rate of chemical change, as shown by alterations of 

 colour and of voltaic energy. Light somewhat retarded the action. 



Strong agitation during one minute of the freshly made solution 

 appeared to slightly increase the amount of chemical change which 

 occurred during mixture. 



In making this solution the amount of chemical change which took 

 place during mixing was about 5 per cent, more if the chlorine solu- 

 tion was added to the solution of iodide than if the order of addition 

 was reversed. 



The results of the experiments show that the solution of potassic 

 iodide and chlorine was very unstable, highly sensitive to rise of tem- 

 perature, had a great tendency to lose its voltaic energy, to change its 

 chemical composition, and approach that of the other mixture ; that 

 the solution of the latter was very much more stable, and much less 

 sensitive to heat, but had a feeble tendency to absorb energy, to 

 change its chemical composition, and approach that of the solution of 

 potassic iodide arid chlorine. The effect, therefore, of heating both 

 liquids was to produce two portions possessing similar chemical com- 

 position and properties, but much more nearly resembling the chloride 

 than the iodide mixture, and consisting of about 0'23 part of potassic 

 iodide, 74'49 of potassic chloride, 126*8 of iodine, and 0'0497 of 

 chlorine. 



The collective results show that the " voltaic balance " jnethod may 

 be used to detect changes of chemical composition of aqueous solu- 

 tions, and to measure the rate of such change going on in them. 

 Although the method as described does not give the amount of change 

 which occurs during the mixing of the liquids, it gives the subsequent 

 amounts of change with a reasonable degree of accuracy. Its great 

 advantage over the colorimetric method is that it is equally applicable 

 to colourless liquids ; it is much more sensitive and exact than either 

 the colorimetric or the thermochemical method ; and it is quick and 

 easy of performance. It is at present being used to detect and measure 



2 G 2 



