20 Mr. Crum on Testing Solutions of Bleaching Powder. 



inspection of which, the result may at once bo observed. The point 

 of saturation of tho arsenic is indicated by a blue tinge, which is 

 given to the arsenious solution by indigo. Tliis substance is not 

 affected by chlorine so long as any arsenious acid is left, after which 

 a single drop of chlorine solution causes it to disappear. 



In employing the prussiate of potash, the instruments and manipula- 

 tion are the same. Its solution is made of the same strength as the 

 arsenious solution, that is, that it should saturate an equal volume of 

 the normal solution of chlorine. Prussiate of potash has a very slight 

 action upon chloride of lime, but if previously rendered acid, it is 

 immediately changed by it, and becomes yellow. 



The prussic acid test has long been employed by my friend, Mr. 

 John Mercer of Oakenshaw, near Manchester. His test, to mark the 

 point at which the prussic acid becomes saturated, is the red oxide of 

 iron. A bit of calico dyed buff with iron, is touched with the solution 

 after each addition of the chlorine, and as soon as it ceases to become 

 blue, enough of the chlorine has been added. 



Gay Lussac's third process is that of M. Marozeau with nitrate of 

 mercury already described. It appears that Balland do Toul first recom- 

 mended this method, two years before the publication of M. Marozeau. 



Mr. John Dalton pointed out a process in 1813, which gives very 

 good results ; and, arranged as it has been by Mr. Graham, it seems to 

 be the best and most easily executed of all the tests of bleaching 

 powder. Mr. Graham directs that a few ounces of good crystals of 

 protosulphate of iron should be pounded, and dried between folds of 

 cloth. 78 grains of crystals so dried, are equivalent to 10 grains of 

 chlorine. The 78 grains are to be dissolved in 2 ounces of water, and 

 acidulated with a few drops of muriatic acid. 50 grains of the bleach- 

 ing powder to be examined, are dissolved in about 2 ounces of tepid 

 water, by rubbing them together in a mortar. The whole is then poured 

 into a graduated tube, called an alkalimeter, divided into 100 parts, 

 and filled up with water to on the scale. The solution of bleaching 

 powder being thus made up to 100 measures, is poured into the solu- 

 tion of iron until it is wholly saturated. The point of saturation is 

 discovered by means of red prussiate of potash, which gives a blue pre- 

 cipitate with protoxide of iron only, and not with salts of the peroxide. 

 A white stoneware plate is spotted over with small drops of the red 

 prussiate ; and, as soon as the iron solution ceases to produce a blue, 

 when a drop of it is applied to one of these spots, no more protosulphate 

 remains. Suppose 72 measures of the bleaching liquor to have satu- 

 rated the 78 grains of green copperas, then these 72 measures con- 

 tained 10 grains of chlorine, which is equal to 13-89 grains in the 50 

 grains of chloride of lime, or 27*78 grains of chlorine in 100 grains. 

 The calculation is simplified by at once dividing 2000 by the number 

 of measures required, thus : — 



^ = 27*78. 



