ROUTINE PROCEDURES 305 
application of the theory of probability. To apply this theory to a 
correct mathematical solution of any considerable series of results 
involves, however, mathematical calculations so complex as to be im- 
practicable of application in general practice. The simpler method 
given is, therefore, considered preferable, since it is easily applied and 
the results so expressed are readily comprehensible. 
In order that the results as reported may be checked and carefully 
valuated, it is necessary that the report should show not only the aver- 
age number of B. cols per cubic centimeter, but also the number of sam- 
ples examined; and for each dilution, the total number of tests made, 
and the number (or per cent) positive. 
RovutrintE PROCEDURES FOR EXAMINATION OF SAMPLES OF WATER 
First Day. 
1. Prepare dilutions as required. 
2. Make two (2) gelatin plates from each dilution, and incubate at 
20° C. 
3. Make two (2) agar plates from each dilution, and incubate at 37° C. 
4. Inoculate lactose broth fermentation tubes with appropriate 
amounts for B. coli tests, inoculating two (2) tubes with each 
amount. 
Note. Where repeated tests are made of water from the same 
source as is customary in the control of public supplies, it is 
not necessary to make duplicate plates or fermentation tubes 
in each dilution. It is sufficient, in such circumstances, to 
make duplicate plates only from thé dilution which will most 
probably give from 25 to 250 colonies per plate. 
Second Day. 
1. Count the agar plates made on the first day. 
2. Record the number of additional fermentation tubes which show 
10 per cent (10%) or more of gas. 
Note. In cases only the presumptive test for B. colt is required 
fermentation tubes showing more than 10 per cent (10%) 
of gas at this time may be discarded. 
Third Day. 
1. Count gelatin plates made on the first day. 
2. Record the additional number of fermentation tubes which show 
10 per cent or more of gas. 
