396 Mr. W. H. Pendlebury and Miss M. Seward. 



" An Investigation of a Case of Gradual Chemical Change : 

 the Interaction of Hydrogen Chloride and Chlorate in 

 presence of Potassium Iodide." By W. H. PENDLEBURY, 

 B.A., late Scholar of Christ Church, Oxford, Assistant 

 Master of Dover College, and MARGARET SEWARD, late 

 Tutor of Somerville Hall, Oxford, Science Lecturer of 

 Holloway College. Communicated by A. VERNON HAR- 

 COURT, F.R.S. Received November 27, Read December 

 13, 1888. 



The work which we now have the honour of laying before the 

 Royal Society was undertaken at the suggestion of Mr. A. Yernon 

 Harcourt. To him we owe more than we can express, and we desire 

 here to thank him most heartily for his most valuable aid and co- 

 operation, by which many rough places in the investigation have 

 been made smooth. We also thank the Royal Society for a grant in 

 aid of the research, and the Governing Body of Christ Church for 

 the use of materials and apparatus. 



When substances which act upon each other are brought together 

 under suitable conditions, a change takes place which consists in the 

 disappearance of the original substances and the production in their 

 place of an equal weight of other substances. The change proceeds 

 till the whole of that reacting substance which was present in the 

 smallest relative quantity has disappeared. This process may take a 

 long time, as in the case which forms the subject of the present 

 investigation, or the limit may be reached so rapidly that the change 

 seems instantaneous. This difference, however, is one of degree and 

 not of kind. In the present case the masses of the substances mixed 

 together were so large relatively to the masses undergoing change 

 during the time over which the observations extended, that the masses 

 of reacting substances were practically constant. Thus it happens 

 that each set of observations was of a change proceeding with 

 constant velocity. In the second reaction studied by Messrs. Harcourt 

 and Esson 



= 2H 3 0-hI 2 , 



the amount of change occurring during each interval of time in 

 which it was estimated was a considerable fraction of the total 

 amount of potential change, as limited by the amount of hydrogen 

 dioxide taken. In this case, therefore, the observed intervals of time 

 lengthened, being the time required for the performance of the same 

 amount of chemical work with a continually diminishing amount of 

 active substance. 



