74 MICRO-ORGANISMS IN WATER 



nation the same principle is utilised in the estimation 

 of the number of bacteria in a given volume of water ; 

 it is carried out in the following manner : About a 

 hundred test-tubes, each containing 10 c.c. of sterile 

 broth, are prepared ; into the first tube, A 15 1 c.c. of the 

 water under examination is introduced by means of a 

 pipette. Into a second broth tube, A 2 , 1 c.c. is introduced 

 from tube A 1 ; A 2 will then contain 11 c.c. of liquid, 

 which is equally divided into a series of eleven broth 

 tubes, marked respectively A 3 , A ] 3 , A 2 3 , &c. The con- 

 tents of A 3 , containing, like A 2 , 11 c.c. of liquid, is 

 then equally divided amongst eleven more tubes marked 

 A 4 , A 1 ^ &c. The number of such series of dilutions 

 that must be prepared will depend upon the number of 

 microbes supposed to be present in the water, the ob- 

 ject being to ultimately obtain a series of tubes, each of 

 wliich shall not receive more than one microbe. 



All these broth-tubes, with the exception of A 2 and 

 A 3 (the whole contents of which were divided amongst 

 the two series of tubes A ] 3 , A 2 3 , .... and A J 4 , 

 A 2 4 ....), are incubated for some days, or even 

 weeks, at from 30 to 35 C. If all the tubes subse- 

 quently exhibit turbidity, it shows that the dilution 

 lias not been carried far enough, and the process must 

 be repeated ; or, if that is impossible (which, as regards 

 water, must be the case, for the sample, after standing 

 days or weeks, is rendered absolutely worthless through 

 the multiplication of the water-bacteria in the interval), 

 a fresh sample must be collected and the process re- 

 peated de nor<>. 



If, on the other hand, in one of the several series of 

 dilutions described above some tubes become turbid, 

 whilst others remain clear, it is argued that the turbid 

 ones have received only a single microbe apiece ; and if 

 this be granted, it is obvious that from the number of 



