EXAMINATION OF WATER AND AIR. 947 
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. Asa matter of fact, water exami- 
nations for this purpose are usually negative. The 
varieties of bacteria most sought, except in the presence 
of acholera 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 cc. of water 25 c.c. of a 4 per cent. peptone 
nutrient bouillon is added, and the whole put in the 
