256 CHEMICAL DISCOVERY AND INVENTION 



than on bacteriological processes, in which the number of 

 pathogenic organisms in measured quantities of the water can 

 be counted and their character determined. 



The softening of hard waters containing carbonates of lime 

 and magnesia can be effected by boiling the water, when the 

 bicarbonates are decomposed and carbon dioxide escapes. 

 This, however, is impracticable on a large scale, and in practice 

 such water is dealt with by the addition of slaked lime in quantity 

 which must be accurately estimated from a knowledge, obtained 

 by analysis, of the composition of the water to be treated. The 

 chemical change which occurs is represented in the following 

 equation : 



CaH 2 (C0 3 ) 2 + Ca(HO) 2 = 2CaC0 3 + 2H.O 

 calcium calcium calcium water 



bicarbonate hydroxide carbonate 

 (slaked lime) 



A precipitate is formed which consists of the lime which has 

 been added together with the lime previously held in solution, 

 both in the form of carbonate. The water is, therefore, deprived 

 of its hardness to this extent, and any hardness remaining is due 

 to the presence of lime or magnesia in the form of chloride or 

 sulphate. The latter can only be removed by the addition of 

 washing soda which consists of sodium carbonate. The process 

 of liming has the additional great advantage that the formation 

 and deposition of the fine particles of the precipitated carbonate 

 leads to removal of nearly the whole of the suspended organisms, 

 and thus reduces in a great degree the probability of the spread of 

 disease, even when the water was known to be previously 

 infected. The results recently obtained in experiments on the 

 use of lime for the purpose of water purification are described 

 with full detail in the Eleventh Report by Dr. A. C. Houston to 

 the Metropolitan Water Board, published in July, 1915. There 

 it is stated that " the credit belongs to Aberdeen of being the 

 first town in this country to show that what was proven to be 

 feasible in the laboratory could be achieved under practical 

 conditions in the case of a water supply to about 165,000 in- 

 habitants. Since writing this report the Aberdeen Town Council 

 on February 15th, 1915, decided by an almost unanimous vote 

 to adopt a threefold system of purification by (A) liming, (B) 



