18 Mr. Ceum on Testing Solutions of Bkaching Powder, 



The oldest method is that of DecroiziUcs, where the amount of 

 chlorine in a solution is measured by the quantity of indigo which 

 that solution is capable of discolouring. Welter made use of it, in 

 his researches on the nature of chloride of lime, in 1818, and con- 

 sidered it susceptible of great accuracy, by attending to certain pre- 

 cautions which he pointed out. In 1824, Gay Lussac published a 

 set of experiments on this art, to which he gave the name of Chlori- 

 metry. He also adopted the indigo test, and made every arrangement 

 for accuracy which the method would permit. A volume of chlorine 

 was taken for unity ; or, which is the same thing, a volume of liquid 

 which had absorbed its bulk of chlorine. This is formed by filling a 

 bottle with chlorine gas, inverting it in a vessel containing cream of 

 lime, and withdrawing the stopper. The chlorine is gradually 

 absorbed, and its place taken by the lime water, — which has then 

 become a solution of bleaching powder containing its own volume of 

 chlorine gas. 



To form the indigo solution, one part of the best indigo of com- 

 merce is dissolved in nine of sulphuric acid, and diluted with water 

 to such an extent, that one measure of the chlorine solution discolours 

 ten measures of it. When the two solutions are mixed together, the 

 chlorine is set free by the sulphuric acid in which the indigo is dis- 

 solved, and the indigo is immediately destroyed. The solution of 

 indigo is called " proof tincture," and the quantity of it, which an un- 

 known solution of bleaching powder is capable of discolouring, indi- 

 cates the bulk of chlorine gas which it contains. 



Much of the accuracy of this method depends upon the way in 

 which the two solutions are mixed. Thus, by pouring the chlorine 

 slowly upon the indigo, much more of it is destroyed than when the 

 indigo solution is poured into that containing the chlorine. A great 

 many trials satisfied M. Gay Lussac, that the best process is to mix 

 the two solutions rapidly together. But then several preliminary 

 trials are necessary to ascertain pretty nearly how much should be 

 employed. 



Three years after, in 1827, M. Morin, of Geneva, published experi- 

 ments on chloride of lime, and discussed the merits of the indigo test. 

 He found it impossible to have the circumstances always so much 

 alike as to produce any thing like uniform results with it. In 1831, 

 M. Marozeau corroborated the view taken by Morin, and added, (what 

 every one who has repeated the process must have noticed,) that it is 

 very difficult to observe the exact point at which the indigo is wholly 

 destroyed, from the want of a distinct line between the brown after 

 complete discoloration, and the slightly greenish tint, which M. Gay 

 Lussac indicates as the point most desirable to stop at. 



Each of these chemists proposes a substitute for the chlorimeter of 

 Gay Lussac. M. Morin would employ muriate of manganese instead 

 of indigo, but he gives no details of his process, and it would seem to 



