1866.] 



Lan)S of Connexion, (Sfc. 



S63 



oxides be used without, as far as we know, altering the nature of the re- 

 action, but other substances may be introduced into the solution, and 

 their influence upon the amount of change determined. 



The present paper contains an account of the methods employed for ob- 

 serving this reaction, and of the results obtained by varying two particular 

 conditions, namely the amounts of peroxide and of iodide. 



If hydric peroxide and hydric iodide, or barytic peroxide, potassic 

 iodide, and hydric chloride, be brought together in dilute solution at the 

 ordinary temperature, a gradual development of iodine takes place. If a 

 drop of a dilute solution of sodic hyposulphite be added to the mixture 

 capable of reducing to iodide all the iodine which has been formed up to 

 the time of its addition, but not all that will be formed in the course of the 

 reaction, the liquid which had become yellow becomes colourless, remains 

 colourless for a while, and then suddenly becomes yellow again. This 

 yellow colour may be exchanged for a more intense blue colour by putting 

 starch into the solution. It was found that in dilute solutions no direct 

 oxidation of hyposulphite by peroxide took place ; and that the quan- 

 tity of hyposulphite required to reduce the iodine liberated by a measure 

 of peroxide was the same, whether the hyposulphite were added little by 

 little during the course of the reaction, or after the primary reaction had 

 completed itself, and when no peroxide remained in the solution. 



The method of observation founded upon these facts w^as briefly as 

 follows : — 



Measured quantities of all the standard solutions, except that of hydric 

 peroxide, were introduced into a glass cylinder about 1 1 inches high by 3 

 broad, and water was added till the upper surface of the liquid was 

 level with a line drawn round the cylinder. This adjustment of volume 

 was made through a hole in the bung, which closed the cylinder ; two 

 other holes admitted a thermometer and an inverted funnel- tube. A cur- 

 rent of carbonic acid passing down the funnel-tube to the bottom of the 

 cylinder, and rising in large bubbles from the mouth of the funnel, served 

 at once to stir the liquid constantly, and to protect its upper surface from 

 the air. When the volume and the temperature of the solution had been 

 adjusted, a small measure of hyposulphite was first added, and then the 

 pipetteful (10 cub. centims.) of peroxide. The cylinder was placed on a 

 sheet of white paper in front of a clock beating seconds. By watching the 

 surface of the fluid and counting the seconds when the time of an observa- 

 tion was near, it was possible to note accurately the moment at which the 

 colour changed. A second small measure of hyposulphite was then intro- 

 duced, and at the proper interval a second observation was made. Each 

 such addition of hyposulphite, with the observatiou preceding and following 

 it, is spoken of as an experiment, and these experiments were continued 

 until the reducing power of the last measure of hyposulphite being greater 

 than the oxidizing power of the remaining peroxide the blue colour did not 

 return. 



