PREVENTIVE MEDICINE 209 



action. Care must be taken that the solution is not stronger than 

 1 per cent, nor the powder imperfectly mixed, else corrosion of the 

 cloth is sure to occur, and even at this strength some bleaching 

 will take place. 



Bleaching powder is a valuable disinfectant for polluted water 

 (see Disinfection of Water). 



When bleaching powder is mixed with water it splits up into 

 calcium chloride and calcium hypochlorite : 



2CaOCl 2 = CaCl 2 + Ca(ClO) 2 



The hypochlorite is acted upon by the CO 2 of the air and water 

 with the formation of calcium carbonate and hypochlorous acid : 

 Ca(ClO) 2 + CO 2 + H 2 O = CaCO 3 + 2HC1O 



When the hypochlorous acid comes in contact with organic 

 matter it splits up into HC1 and O. It should be well understood 

 that " chloride of lime " acts by oxidation, not by chlorination. 



The calcium chloride that is left behind is a deliquescent powder, 

 so that after disinfection is complete it should be washed away. It 

 also has an errosive action on metal pipes ; for this reason traps and 

 pipes should be well flushed out following it use. 



LIME (Calcium Oxide or Quicklime, CaO). Quicklime results 

 from the ignition of the carbonate of lime. It is a greyish- white 

 substance. It combines with water with the evolution of great 

 heat, swells up and crumbles into a fine powder, calcium hydroxide 

 or " slaked lime " Ca(OH) 2 . Freshly burned quicklime is an 

 absorbent, and dehydrates organic matter. It is a disinfectant of 

 some value, but its action on sporulating organisms is uncertain. 

 When quicklime is left exposed to the air it absorbs moisture and 

 becomes " air-slaked " and practically useless. Freshly burned 

 lime should always be used, and lime that slakes badly is of inferior 

 value. Quicklime is used in practical disinfection, chiefly in 

 connection with the disinfection of faecal and other organic matter; 

 it should be well mixed with it and left for several hours. 



SLAKED LIME (Calcium hydrate, Ca(OH) 2 ). The most 

 common form in which lime is used is as milk of lime or whitewash. 

 It must be prepared from fresh quicklime of good quality. Quick- 

 lime is converted into the hydrate of lime by adding approximately 

 its own weight of water, roughly a pint of water to two pounds of 

 lime. Milk of lime is made by adding four parts by volume of 

 water to one of slaked lime. Milk of lime when it is freshly 

 prepared has undoubtedly a caustic action and is therefore a dis- 

 infectant. Whitewash is made by diluting milk of lime with water. 

 Freshly prepared whitewash has a certain germicidal action, but its 

 potency is greatly increased by the addition of a disinfectant of 

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