5()L' CALCIUM. 



rivals the chlorine compounds in bleaching power. But when 

 the replacing bodies differ so much from each other as chlorine 

 and oxygen, it is not to be expected that the resulting com- 

 pounds will exhibit the closest analogy in properties. 



CHLORIMETRY. 



The bleaching power of chloride of lime is often estimated by 

 the quantity of a solution of sulphate of indigo, which a known 

 weight of chloride can discolour or render yellow. But as the 

 indigo solution alters by keeping, this method is not unobjec- 

 tionable. Several exact methods have been proposed, of which 

 that in which sulphate of iron is used appears to be entitled to 

 preference. This method reposes upon the circumstance that 

 the chlorine of chloride of lime converts a salt of the protoxide 

 into a salt of the peroxide of iron; half an equivalent, or 221.3 

 parts of chlorine, effecting that change upon a whole equivalent, 

 or 1728 parts of cr. protosulphate of iron. Protoxide of iron is 

 convertible into peroxide by half an equivalent of oxygen, which 

 the half equivalent of chlorine may be supposed to supply, by 

 decomposing water, in becoming hydrochloric acid. It follows, 

 by proportion, that 10 grains of chlorine are capable of peroxi- 

 dising 78.1 grains of cr. sulphate of iron. 



A few ounces of good crystals of protosulphate of iron are 

 reduced to powder, and dried by strong pressure between folds 

 of cloth ; the salt may afterwards be preserved in a bottle with- 

 out change. In a chlorimetric experiment /8 grains (equiva- 

 lent to 10 grains of chlorine) of this salt are dissolved in about 

 two ounces of water, which may be acidulated by a few drops 

 of sulphuric or hydrochloric acid. Fifty grains of the chloride 

 of lime to be examined are dissolved in about two ounces of 

 tepid water, by rubbing them together in a mortar, and the 

 whole poured into the alkalimeter (page 466); which is after- 

 wards filled up to on the scale, by the addition of water, and 

 the whole mixed by inverting the alkalimeter upon the palm of 

 the hand. The solution of chloride of lime, being thus made 

 up to 1 00 measures, is poured gradually into the sulphate of 

 iron, till the latter is completely peroxidised, and the number 

 of measures of chloride required to produce that effect ob- 

 served. The change in the degree of oxidation of the iron 

 solution is discovered by means of red prussiate of potash,, 

 which gives a precipitate of prussian blue with a salt of the 



