322 The Water Supply of London, [July, 



The amount of this contamination may be expressed by the number 

 of parts of average filtered London sewage (of the strength above 

 described), which, if contained in 100,000 parts of pure water, would 

 contaminate the latter with the same amount of combined nitrogen. 

 No contamination of this nature has yet been detected in the waters 

 supplied to the metropolis, but the investigations for its discovery 

 have only been made since February last. It will doubtless be 

 consolatory to the consumers of Thames water to know that, 

 although, according to Mr. Bateman, the population within the 

 basin of the Thames above the points at which the water is with- 

 drawn for the supply of London exceeds 1,000,000 persons, the 

 drainage of some 600,000 of whom is poured into the river, the 

 whole of this faecal matter is so completely oxidized before it reaches 

 the water-cisterns of London as to defy the detection of any trace in 

 its noxious or unoxidized condition. If the average flow of Thames 

 water just above the point at which the London Companies with- 

 draw their supply be taken at 800,000,000 of gallons daily, the 

 drainage of 600,000 people ought to produce a sewage contamina- 

 tion of 2,250 parts in 100,000. It could scarcely be expected that 

 this calculated number should approximate very closely to that ob- 

 tained by the actual analysis of Thames water, since the calculated 

 number depends upon many t contingencies, as for instance, upon 

 the volume of water actually flowing past the points of withdrawal 

 at the time the companies abstracted the water analyzed; and 

 secondly, upon the greater or less retention of the fascal matters, in 

 the sewers of the towns draining into the river. It is interesting, 

 however, to find that the sewage contamination of Thames water, as 

 determined by analysis, does not differ much from that calculated 

 according to the above data. The analytical table given above 

 shows that the average previous sewage contamination of the water 

 delivered by the five companies drawing their supply from the 

 Thames during the months of February and April, 1867, was 

 2,355 parts in 100,000 of water, the amount calculated from the 

 number of persons draining into the river being, as just mentioned, 

 2,250 parts in 100,000 of water. As summer advauces and aquatic 

 vegetation becomes vigorous in the bed of the Thames and its 

 tributaries, this coincidence of calculated and analytical results will 

 probably be disturbed, as the water-plants can scarcely fail to with- 

 draw an appreciable amount of nitrates and nitrites from the water, 

 thus diminishing the quantity of combined nitrogen and consequently 

 of previous sewage contamination as determined by analysis. 



The second important class of impurities contained in water 

 used for domestic purposes consists of certain mineral salts which 

 possess the power of decomposing soap. These substances are the 

 hardening or soap-destroying constituents of water. From a purely 

 sanitary point of view they are of less direct importance than the 



