Sterilisation Experiments on the Cambridge Water. 565 



of either taste or smell of chlorine ; in the later experiments 

 therefore no neutralisation by means of bisulphite was considered 

 necessary, especially as it was a difficult matter to keep the 

 bisulphite solution free from organisms which, of course, could 

 remain alive and active in the neutralised chlorine solution. 



It is generally accepted as indicated by Odling that dry 

 "chloride of lime" — "bleaching" powder — on being mixed with 

 water is split up into calcium chloride and calcium hypochlorite 

 to the latter of which, in the presence of a weak acid in the water, 

 the sterilising power of chloride of lime is apparently due. Probably 

 the best acid for the purpose is carbonic acid, carbon dioxide, 

 which, releasing the hypochlorous acid from its combination with 

 the base, does not cause it to break up, at once, at any rate. Thus, 

 bleaching powder on being dissolved in water is said to break up 

 into calcium hypochlorite. 



.01 

 [2Ca( =CaCL + Ca(C10)2.] 



^ClO 



This latter, in the presence of the dissolved carbon dioxide, be- 

 comes converted into calcium carbonate and active hypochlorous 

 acid 



[Ca(C10)2 + CO2 + H2O = CaCOa + 2HC10], 



the hypochlorous acid combining with organic matter directly or 

 breaking up into hydrochloric acid and oxygen which, in a nascent 

 condition, acting upon micro-organisms as does ozone speedily 

 deprives them of vitality. 



[2HCIO-2HCI + O2.] 



Organic matter, ammonium salts, and even amido-compounds, 

 according to Rideal are attacked by chlorine at once so that any 

 excess of organic matter in water militates against effective 

 sterilisation when small quantities of chlorine are used. Where, 

 however, the amount of organic matter is small and the amount 

 of carbon dioxide dissolved in the water is considerable the con- 

 ditions for sterilisation by chlorine are ideal. Only when the 

 reduction of the chlorine compounds is complete and they are 

 converted into hydrochloric acid are the best results obtained, the 

 largest amount of oxygen possible then being set free in a nascent 

 condition, and, the hydrochloric acid being tasteless or combining 

 readily with the earthy basic substances, there is little chance of 

 either disagreeable odour or taste being left in the water. In the 

 presence of ammonia, urea and similar substances, although oxygen 

 may be set free and the chlorine may combine with the nitro- 

 genous compound, a certain amount of nitrogen is liberated and 



37—2 



