BACTERIA IN WATER 79 



If a large number of bacteria is present, the inference is that organic 

 material must be present in sufficient quantity to support them, hence, 

 drinking water containing large numbers of bacteria must be regarded 

 with suspicion. If further examination shows that the organisms are 

 such as are usually found in sewage water, the water is condemned for 

 drinking purposes. It is necessary to make a qualitative as well as a 

 quantitative examination, because the danger arises not so much from 

 the presence of the bacteria themselves, as from the poisonous 

 excretions of the harmful ones. Thus we find in ordinary pure water 

 such organisms as Bac. subtilis, Bac. prodigiosus, Bac. rubescens, 

 Bac. mycoides, Bac. aquatilis, also some species belonging to the 

 Coccaceae and Spirillaceae, which are quite harmless, and no count is 

 taken of them unless they are present in very large numbers. On the 

 other hand, if members of the Proteus family, or Bac. enteritidis 

 sporogenes, or Bac. coli communis are found, sewage contamination is 

 indicated, and further, it may reasonably be suspected that, in addition 

 to these, other organisms are possibly present which excrete poisonous 

 substances, the most dreaded being the bacillus of typhoid fever and 

 that of cholera. With regard to the different seasons of the year, the 

 following table is interesting as showing the difference in the number 

 of bacteria in rivers at different times of the year. 



River Thames : water collected at Hampton : 



1 c.c. WATER CONTAINED 



January, 92,000 



February, 40,000 



March, - 66,000 



April, 13,000 



May, - 1,900 



June, - 3,500 



July, - 1,070 



August, 3,000 



September, 1,740 



October, 1,130 



November, 11,700 



December, 10,600 



These figures must not be regarded as giving absolute values for 

 all rivers, for they do not always hold for the same river at different 

 places, but the relative values can be accepted for most of the rivers in 

 this country. The figures show that river water contains far more 

 bacteria in winter than during the rest of the year. This is due to the 

 fact that during the winter months, the rivers receive the washings 



