An Investigation of a Case of gradual Chemical Change. 403 



The observations were made by looking down on the column of 

 fluid and watching the appearance of the disk forming its upper sur- 

 face. As soon as the change is complete a blue shade shoots 

 rapidly across the brightly illuminated disk, and there is no difficulty 

 in ascertaining the exact second of the change ; the observer listens 

 to the beat of the clock and counts the seconds whilst watching the 

 disk. As soon as the blue colour has appeared, the minute and 

 second are noted, and a drop of thiosulphate is brought into the 

 cylinder. The end of the tube charged with a drop is plunged 

 into the liquid through the opening for that purpose, and moved up 

 and down, active stirring being carried on by means of the bubbles of 

 carbon dioxide. The intervals date from one appearance of the colour 

 to the next reappearance, and as the rate is not affected by the presence 

 of a small amount of iodine or a small diminution in the amount of 

 iodide, it is clear that the fact of the addition and admixture of the 

 thiosulphate not following immediately upon the appearance of the 

 blue colour, does not disturb the uniformity of the rate of change. 



The potassium chlorate employed in our experiments was purified 

 by recrystallisation. 



The hydrogen chlorate was prepared by the ca/ntious addition of 

 sulphuric acid to a solution of pure barium chlorate until no milki- 

 ness was produced either by further additions of hydrogen sulphate 

 or barium chlorate. The barium sulphate precipitate was then filtered 

 off. The solution of hydrogen chlorate thus obtained contained no 

 chloride. 



The potassium iodide solution was prepared by dissolving 80 grams 

 of recrystallised and fused potassium iodide in 8 litres of water. 



The starch solution employed was prepared by adding a magma of 

 starch and water, containing about 5 grams of starch to about 300 c.c. 

 of boiling water, and allowing the whole to boil briskly for a few 

 minutes. When cool, the liquid was transferred to a cylinder and 

 covered over. On standing, the upper part of the liquid become per- 

 fectly clear ; of this 10 c.c. were taken by means of a pipette. 



The hydrogen chloride solution was prepared by diluting pure 

 acid till 100 c.c. contained 18*823 grams of hydrogen chloride. 



It may be of interest here to give the details of an actual experi- 

 ment. 



Taken 30 grams of potassium chlorate, 200 c.e. hydrochloric acid 

 (containing 18'823 grams hydrogen chloride in 100 c.c.), 10 c.c. of 

 a clear starch solution, and 10 c.c. of a solution of potassium 

 iodide (containing O'Ol gram potassium iodide per c.c.). A few drops 

 of a dilute solution of thiosulphate were added to discharge the 

 colour due to the iodine liberated before it was possible to com- 

 mence observations. 



The temperature throughout was 20. 



