Sterilisation Experiments on the Cambridge Water. 563 



tablet dry and to provide, when added to water, the acid radicle 

 CO necessary to liberate the feebly combined chlorine of the 

 bleaching powder. Water which may be teeming with typhoid 

 or colon bacilli becomes sterile within five minutes of the solution 

 in it of this tablet. He points out further that the addition of 

 a quarter of a grain of sodium sulphite to the pint of water so 

 treated is free from chlorine and is practically tasteless. 



[Cl^ + H2O + Na^SO, = Na^SO, + 2HC1.] 



This method of sterilisation of water has been put into practice 

 on a considerable scale in India and has been very effective in 

 preventing the spread of water-borne disease amongst troops on 

 active service. 



The relatively better results obtained in the sterilisation of 

 water than in the treatment of sewage is accounted for by the 

 fact that sewage contains such a relatively large amount of 

 organic matter and so many spore-bearing organisms. Pathogenic 

 water-borne organisms, however, are killed by chlorine with very 

 great readiness even in sewage, the "filter effluents" of the Boston 

 sewage, as pointed out by Professor E. B. Phelps*, being almost 

 sterilised by 3"5 parts per million, 9.9"2 7o of the coli organisms 

 succumbing within two hours, whilst 2"2 parts of available chlorine 

 per million is sufficient to act in the same way on the Baltimore 

 filter effluent — a somewhat weaker effluent. Although complete 

 sterilisation required a considerably larger amount of chlorine 

 there are here indications of the ease with which sterilisation 

 may be carried on in the presence of even large quantities of 

 organic matter ; water containing a small quantity of organic 

 matter is of course more readily sterilised. 



In 1905 Houston and McGowanf sterilised with chlorine the 

 whole of the Lincoln water supplying a population of about 50,000. 

 In the first instance they used one part of Chloros in 10,000 parts 

 of water. As Chloros contains 10 "/^ of available chlorine they 

 were using 10 parts chlorine per 1,000,000 of water. Later only 

 five parts per 1,000,000 and ultimately one part per 1,000,000 was 

 used. In this case, however, the sterilisation was not complete 

 though the number of Bacillus coli communis found in the treated 

 water was undoubtedly small. They found that after treatment 

 7*J'S°/^^ of the 163 samples contained no Bacillus coli in 100 c.c, 

 and that 17'7 "/^ contained Bacillus coli in 100 c.c. but not in 1 c.c. 

 Drs Houston and McGowan were of course dealing with a river 

 water and surface water from land drains of an agricultural 

 country and from a series of old sandpits containing a consider- 



* The Disinfection of Sewage and Sewage Filter Effluents, Washington, 1909. 

 + Supplement to 35th Annual Report of Local Government Board for 1905-6, 

 London, 1908. 



VOL. XV. PT. VI. 37 



