TEST FOR B COLI 303 
high pollution, such that its fitness for use as drink- 
ing water does not come into consideration. This 
applies to the routine examinations of raw water in 
connection with the control of the operation of puri- 
fication plants. 
2. When definitely negative, that is, showing no gas in forty-eight 
hours, is final and, therefore, sufficient in all cases. 
3. When doubtful, that is, showing gas less than 10 per cent (10%) 
(or none) in twenty-four hours, with gas either more or 
less than 10 per cent in forty-eight hours, must always be 
confirmed. 
B. The Partially Confirmed Test. 
1. When definitely positive, that is, showing typical plate colonies 
within twenty-four hours, is sufficient. 
(a) When applied to confirm a doubtful presumptive test in 
cases where the latter, if definitely positive, would 
have been sufficient. 
(b) In the routine examination of water-supplies where a 
sufficient number of prior examinations have estab- 
lished a satisfactory Index of the accuracy and sig- 
nificance of this test in terms of the completed test. 
2. When doubtful, that is, showing colonies of doubtful or negative 
appearance in twenty-four hours, must always be com- 
pleted. 
C. The Completed Test. 
The completed test is required as applied to the smallest gas- 
forming portion of each sample in all cases other than those 
noted as exceptions under the “ presuinptive ”’ and the “ par- 
tially confirmed ”’ tests. 
The completed test is required in all cases where the confirmed 
test has been doubtful. 
In order that tests for B. coli may have a quantitative significance, 
the following general principles and rules should be observed: 
Ordinarily not less than three portions of each sample should be 
tested, the portions being even decimal multiples of fractions of a cubic 
centimeter; for example, 10 c.c., 1 cc, 0.1 cc. .01 ce. ete. It is 
essential that the dilutions should be such that the largest amount 
gives a positive test (unless the water is such as to give negative tests 
in 10 c.c.), and the smallest dilution, a negative result. To insure this 
result, it is often necessary to plant four or five dilutions, especially in 
