54 BACTERIA IN WATER 



external conditions. Increase of alkalinity ("01 grams of sodium 

 carbonate added to 10 c.c. of ordinary gelatine) causes the number 

 of colonies to be five or six times greater than that revealed by using 

 ordinary gelatine ; on the other hand, very slightly increasing the 

 acidity of a medium as markedly diminishes the number of bacteria. 

 Advantage is taken of this in culturing the bacillus of typhoid, which 

 is not inhibited by an acid medium. 



Water may become contaminated with pathogenic bacteria in a 

 variety of ways, as pollutions at the source, in the course, and at the 

 periphery. Gathering grounds are frequently the source of the 

 pollution. The Maidstone typhoid epidemic was an example. Here 

 some of the springs supplying the town with water were con- 

 taminated by several typhoid patients. Frequently on the gathering 

 ground one may find a number of houses the waste and refuse of 

 which will furnish ample surface pollution, which in its turn may 

 readily pass into a collecting reservoir or a well. On one occa- 

 sion the writer investigated the cause of typhoid fever in a large 

 country house in Oxfordshire, and traced it to pollution of the 

 private well by surface washings from the stable quarters. Leak- 

 age of house drains into wells is not an infrequent source of 

 contamination. 



The same cause is generally operative in cases of pollution of a 

 water supply in its course from the source to the cisterns or taps 

 at the periphery, viz., a sewer or drain leaking into the water supply. 

 Water companies and those responsible for water supply appeal- 

 frequently to hold the opinion that so long as there is sand filtration 

 or subsidence reservoirs, it is unnecessary to consider the gathering 

 ground or possible contamination during transit. But it happens 

 that a frost may completely dislocate the efficient action of a filter, 

 and times of flood may prevent proper sedimentation ; then our 

 dependence for pure water is wholly upon the gathering ground and 

 source. Hence we find water contaminated at its source by polluted 

 wells, by sewage-infected rivers and streams, by drainage of manured 

 fields, by innumerable excremental pollutions over the areas of the 

 gathering grounds, and in transit by careless laying, bad construction 

 and jointing of pipes, and close proximity of such drain pipes to the 

 water supply. 



In the third place, we may get a water infected at the periphery, 

 in the house itself. Such cases are generally due to two causes: 

 filthy cisterns and pipes or suction. Cisterns per se are more or less 

 indispensable where a constant service does not exist, but they should 

 be inspected from time to time and maintained in a cleanly condition. 

 Suction into the tap has been emphasised by Dr Vivian Poore as 

 a cause of pollution. It is liable to occur whenever a tap is left 

 turned on, and a vacuum is produced in the supply pipe by inter- 



