54 JOURNAL AND PROCEEDINGS. 



with slaked lime until no more of the chlorine is absorbed by it. 

 The bleaching powder is then stored away for future use. 



When sulphur dioxide is used as the bleaching agent, the 

 necessary supply is obtained by burning sulphur, although in the 

 chemical laboratory it is more frequently obtained by action of sul- 

 phuric acid on metallic copper. 



The bleaching powder prepared in the manner above described 

 is the source from which chlorine for bleaching purposes is obtained. 

 The reactions by which the chlorine is liberated from the powder 

 may be referred to at this, point. There is some difference of opin- 

 ion as to the exact composition of bleaching powder, and until this 

 is positively settled it will be difficult, if not impossible, to give cor- 

 rectly the reactions which result in the liberation of chlorine from it. 



On the one hand it is claimed that it is a mixture of two salts^ 

 calcic chloride and calcic hypochlorite, of which the formulas are 

 Ca CI2 and Ca (C10)2 respectively. On the other hand it is claimed 

 that it is a single chemical compound of the formula Ca CI2O, which 

 it will be observed gives the same percentage of the elements pres- 

 ent as the mixture in the first case. The argument usually ad- 

 vanced in support of the latter view does not hold in the case of the 

 powder as we ordinarily know it, and as that is the form in which it 

 is used, it may be assumed that it is of the composition first stated. 

 Of the salts contained in the mixture, namely, calcic chloride and 

 calcic hypochlorite, the former is a comparatively stable salt, while 

 the latter is very unstable, decomposing in contact with the air, by 

 reason of the influence of the carbon dioxide contained therein, 

 yielding among other products hypochlorous acid and free chlorine ; 

 hence the peculiar smell of bleaching powder. This also explains 

 how it is that bleaching will take place in an aqueous solution of 

 the powder, although very slowly, since the decomposition goes on 

 very slowly. 



The decomposition of the powder may be effected rapidly by 

 the addition of a small quantity of sulphuric acid to the solution 

 of the powder. For the purpose of bleaching, however, this is not 

 to be recommended, for although the process is more quickly com- 

 pleted, the action of the acid injures the material of the cloth to 

 such an extent as to overcome the advantages ot rapid bleaching. 



The hberation of the chlorine may be readily explained by the 

 use of chemical equations. Simultaneously two reactions take place 



