714 DISTRIBUTION AND HABITAT OF THE BACTERIA. 



the same manner. Hence we have the best opportunity 

 for the infection of drinking water where an imperfectly 

 covered well stands in the middle of the ordinary dirty 

 court; as a rule all dejecta and waste water are poured 

 out on the soil of these courts, and further, the arrange- 

 ments are frequently such that the superfluous water 

 employed for washing clothes, for example, flows hack 

 again into the well. At times, also, the ground water 

 which supplies the well contains numerous bacteria ; 

 this is the case, for example, when the distance from the 

 surface is slight, or when cesspools in the neighbour- 

 hoods of the wells reach as deep as the ground water, or 

 when the soil is exceptionally porous. 



Factors which The number of bacteria in a water chiefly depends on 

 number of ^ whether varieties are present which are able to multiply 

 bacteria in j n ^ e wa ter, and whether the conditions are favourable 



water. 



for their multiplication. These conditions are the more 

 favourable the higher the temperature and the longer the 

 water is stagnant and able to maintain the newly formed 

 bacteria. Hence we find the greatest numbers of 

 bacteria in stagnant water and during the summer 

 months, and the smallest numbers in wells which are 

 much used and during the cold season of the year. 

 The other conditions of the water, the amount of organic 

 material and salts present in it, are not of importance 

 for the multiplication of these special kinds of bacteria, 

 nor for the number of bacteria in any given water. It is 

 only when the true water bacteria are absent, and sapro- 

 phytes are present, which require a large quantity of 

 nutritive materials, that the differences in the chemical 

 constitution find an expression also in the number of 

 the bacteria. We cannot, therefore, draw any con- 

 clusion from the number of living bacteria in a given 

 specimen of water as to its infective power, or even as to 

 the degree of contamination ; such conclusions can only 

 be come to when we ascertain at the same time whether 

 aquatic bacteria are present, and whether the conditions, 

 such as the season of the year or the frequency with 

 which the well is used, are favourable for their multi- 

 plication before the specimen was taken. 



