388 BACTERIA. 



contamination being only too plainly indicated by some of the 

 epidemics of typhoid fever and cholera that have from time 

 to time devastated our badly drained villages both at home 

 and in new countries. In view of all these facts, the 

 biological analysis of water has come to be a subject of im- 

 portance of the first rank, and too much stress can scarcely be 

 laid on the necessity for such analysis in order to determine 

 the suitability of any water supply for the purposes to which it 

 is to be put ; fortunately such examination is comparatively 

 easily carried out. In addition to this, however, regular 

 examination is absolutely necessary both to provide an 

 indication that no contamination is creeping in during the 

 process of distribution and to test how filter beds where such 

 are used are performing their work. Water should always 

 be examined for bacteria immediately after the sample 

 is drawn from its source, for although the amount of nutri- 

 ment for micro-organisms may be comparatively small, it is 

 all in solution, and in such a form that it can be easily 

 utilized by the organisms which in the fluid make their 

 way with the utmost rapidity from one point to another. In 

 consequence of these extremely favourable conditions their 

 rate of multiplication is most remarkable. If a sample of 

 even the purest water (containing, say, 200 germs per cc.) 

 be left to stand in a room, in which the temperature is 

 comparatively high and therefore suited for rapid growth of 

 these organisms, it may be found that in place of 200 germs 

 per cc. there may be present on the second day 5,000, on the 

 third day 20,000, whilst on the fourth, as pointed out by 

 Carl Fraenkel, they are almost innumerable. This multi- 

 plication may go on for some time, but at length there 

 comes a period at which the small quantity of food contained 

 in the water is used up, the bacteria begin to die, and the 

 number of living cultivatable organisms gradually falls until 

 eventually it may become extremely small. 



Water for analysis is collected in a sterilized Ehrlenmeyer flask, well 

 plugged with sterilized cotton wadding ; in place of this a wide-mouthed 

 stoppered bottle may be used. In either case the stopper or the wadding 

 should be covered with an indiarubber cap previously carefully steri- 

 lized in corrosive sublimate solution and boiled distilled water. If the 

 water is to be collected from a tap it should be allowed to run for several 

 minutes before any is taken ; water from the surface of a pond or a river 

 should be collected by means of large sterilized pipettes ; whereas if it is to 

 be taken from a depth or from a well a stoppered and weighted flask is 



