ESMARCH'S COUNTER. 185 



ably not always the case, as there may exist clumps of 

 bacteria which represent hundreds or even thousands of 

 individuals, but which still give rise to but a single 

 colony this is usually estimated as a single organism in 

 the water under analysis. 



Where grounds exist for suspecting the presence of 

 these clumps, they may in part be broken up by shaking 

 the original water with sterilized sand. 



What has been said for the bacteriological examina- 

 tion of water, holds good for all fluids which are to be 

 subjected to this form of analysis. 



In considering water from a bacteriological standpoint, 

 it must always be borne in mind that comparisons of the 

 water with any general fixed standard are not of much 

 value, for just as normal waters from different sources are 

 seen to present differences in their chemical composition 

 without being unfit for use, so may the number of 

 bacteria per volume in water from one source always be 

 greater or smaller than that from another locality, and yet 

 no differences can be seen to result from their employ- 

 ment. For this reason the proper study of any water, 

 from this point of view, means the establishment of what 

 may be termed its normal proportion of bacteria, as well 

 as a study of the organisms most commonly present. For 

 this purpose experiments covering a long period of time, 

 made at short intervals, must be conducted, and from 

 these observations the means for that water at the dif- 

 ferent seasons of the year calculated. Marked devia- 

 tions from these means, either in quantity or quality of 

 the bacteria present, are the only comparisons that are of 

 any value. 



A sudden variation from the normal mean in the 

 number of bacteria in any water calls at once for a 



