CHLORINE.

139

In considering the constitution of bleaching-powder it may be useful torecord that, kept under pressure or in too tightly-closed cases, it is liable tospontaneous decomposition and explosion, illustrated by the formula—•Ca(C 10 ) 2 + CaCl 2 = 2CaCl a -{- 0 2 .

When chlorine is admitted to slaked lime, chloride or hypochlorite of limeis almost immediately formed, but the formation is much retarded if an excessof the gas be present. The theory of the formation is that some of the oxygenof the lime combines with the chlorine to form hypochlorous acid, which inits turn, combining with the undecomposed lime, forms hypochlorite of lime ;and that another equivalent of the chlorine combines with the decomposedlime or calcium to form chloride of calcium. A bleaching-powder so consti-tuted should consist of—•

Hypochlorite of lime .49-31 per cent.

Chloride of calcium .38-28 ,,

Water. 12-41 „

but such a verification of theory never occurs in samples of the commercial

produdt, the closest approximation being similar to the result of the following

percentical analysis:—

Hypochlorite of lime .26-72

Adtive chloride of calcium.20-72

Excess „ „ 4-79

Hydrate of lime .30-46

Water. 17-31

IOO'OO

Bleaching-liquor is similarly formed by passing chlorine gas into a solutionof lime. If we substitute for the lime a solution of caustic soda or potash,the chlorides of these bases are obtained ; they are sometimes employed asweak bleaching agents.

Solutions of hypochlorite of potash and hypochlorite of soda are knownin commerce as Eaux de yavelle and de Labarraque. Wilson’s bleaching-liquor is a solution of hypochlorite of alumina, obtained by mixing chlorideof lime with sulphate of alumina in solution. Ramsay’s bleaching-liquor, ahypochlorite of magnesia obtained by adding sulphate of magnesia to asolution of bleaching-powder, is a very energetic preparation, and leaves thefibre more soft and supple than the hypochlorite of lime. Varrentrap’sbleaching-powder is a solution of hypochlorite of zinc.

The testing of samples of bleaching-liquors or powders is necessary in orderto ascertain the percentage they will afford of adtive chlorine and hypochlorousacid. The indigo-test is based upon the decolourising effedt of chlorine uponorganic colouring matters. This test is, however, the most inaccurate, becausethe chlorisatine resulting from the decomposition of the indigo, being in colouryellow, combines with the indigo to produce a green colour, which admits oflittle accuracy in noting the exadt period of change. The indigo-test hasgenerally given way to the method invented by Gay-Lussac, which is basedupon the property possessed by arsenious acid in a solution of hydrochloricacid of becoming oxidised to arsenic acid when in the presence of chlorineand water. One equivalent of arsenious acid thus readts with 2 equivalents°f chlorine and 2 equivalents of water to produce 1 equivalent of arsenic acid