EXAMINATION OF WATER AND AIR. 247 



tion, therefore, of the number of bacteria should only 

 be considered of value, except in the extreme instances 

 where enormous numbers are found, when we know 

 fairly well the conditions, chemical and physical, con- 

 cerning the supply. The examination of water, to de- 

 termine whether or not any forms of parasitic bacteria 

 or other micro-organisms are present, would be more 

 often of practical value than it is if the difficulties 

 were not so great. As a matter of fact, water exami- 

 nations for this purpose are usually negative. The 

 varieties of bacteria most sought, except in the presence 

 of a cholera epidemic, are the typhoid and colon bacilli. 

 If it were possible to readily obtain the typhoid bacilli 

 from water, when they were present in small numbers, 

 its examination for that purpose would be of much 

 greater value than it is now; but we have to remem- 

 ber that we can only examine at one time a few cubic 

 centimetres of water by bacteriological methods, and 

 that although the typhoid bacilli may be sufficiently 

 abundant in the water to give, in the quantity that we 

 ordinarily drink, a few bacilli, yet it must be a very 

 lucky chance if they happen to be in the small amount 

 which we examine. Still, further, although it is very 

 easy to isolate typhoid bacilli from water when they 

 are in considerable numbers, yet when they are a 

 very minute proportion of all the bacteria present it 

 is almost impossible not to overlook them. Many 

 attempts have been made to devise some method by 

 which the relative number of the typhoid and other 

 parasitic bacteria present in water could be increased 

 at the expense of the saprophytic bacteria. Thus to 

 100 c.c. of water 25 c.c. of a 4 per cent, peptone 

 nutrient bouillon is added, and the whole put in the 



