134 BACTERIA OF AIR, SOIL, AND WATER. 



remove fractions of a cubic centimetre as before, and place in the incubator. 

 Upon the second, third, and fourth days the colonies which have developed 

 should be counted by means of Lafars counter (if Petri's dishes have been 

 used) or a Wolffhiigel apparatus if glass plates were employed (see Fig. 37). 

 These counters are suitable pieces of glass upon whose surfaces are ruled 

 square centimetres and fractions thereof, which render errors in counting very 

 small indeed. 



Where a plate contains comparatively few colonies the whole number of 

 squares should be counted, but where a large number of colonies are present 

 one may count ten or fifteen representative squares at random and determine 

 approximately the total number of colonies present. All such counts are then 

 to be properly reckoned as bacteria per cubic centimetre, allowing that each 

 colony has originated from a single bacterium (this is not, however, strictly 

 the case, but where the sample has been vigorously shaken before plating, it is 

 for all practical purposes sufficiently near the truth). 



Regarding what may be considered as an impure water and one to be con- 

 demned, no hard and fast statement can be made, for under natural conditions 

 some waters contain a much higher number of bacteria than do others. So to 

 arrive at a proper basis for judging of the conditions of purity or impurity, 

 frequent repeated analyses must be made throughout a year with especial refer- 

 ence to species determination, more particularly as regards the presence of 

 bacillus coli, and the condition of the watershed rigidly examined. 



With regard to the objects with which the bacteriological 

 examination of water may be undertaken, though these may be 

 of a purely scientific character, they usually aim at contributing 

 to the settlement of questions relating to the potability of 

 waters, to their use in commerce, and to the efficiency of pro- 

 cesses undertaken for the purification of waters which have 

 undergone pollution. The last of these objects is often closely 

 associated with the first two, as the question so often arises 

 whether a purification process is so efficient as to make the 

 water again fit for use. 



Water derived from any natural source contains bacteria, 

 though, as in the case of some artesian wells and some springs, 

 the numbers may be very small, e.g. 4 to 100 per c.c. In rain, 

 snow, and ice there are often great numbers, those in the first 

 two being derived from the air. Great attention has been paid 

 to the bacterial content of wells and rivers. With regard to the 

 former, precautions are necessary in arriving at a judgment. If 

 the water in a well has been standing for some time, multiplica- 

 tion of bacteria may give a high value. To meet this difficulty, 

 if practicable the well ought to be pumped dry and then allowed 

 to fill, in order to get at what is really the important point, 



