3. The lactose broth fermentation tubes thus inoculated shall be incubated until gas formation is noted; the incubation not to exceed 48 hours. The agar slants shall be incubated at 37° C. for 48 hours, when a microscopic examination shall be made of at least one culture, selecting one which corresponds to one of the lactose broth fermentation tubes which has shown gas-formation. The formation of gas in lactose broth and the demonstration of non-spore-forming bacilli in the agar culture shall be considered a satisfactory completed test, demonstrating the presence of a member of the B. coli group. The absence of gas-formation in lactose broth or failure to demonstrate non-spore-forming bacilli in a gas-forming culture constitutes a negative test. APPLICATION OF PRESUMPTIVE, PARTIALLY CONFIRMED, AND COMPLETED TESTS. A. The Presumptive Test. 1. When definitely positive, that is showing more than 10 per cent. (10%) of gas in 24 hours, is sufficient: (a) As applied to all except the smallest gas-forming portion of each sample in all examinations. (b) As applied to the smallest gas-forming portion in the examination of sewage or of water showing relatively high pollution, such that its fitness for use as drinking water does not come into consideration. This applies to the routine examinations of raw water in connection with control of the operation of purification plants. 2. When definitely negative, that is showing no gas in 48 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 24 hours, with gas either more or less than 10 per cent. in 48 hours, must always be confirmed. B. The Partially Confirmed Test. 1. When definitely positive, that is, showing typical plate colonies within 24 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 established a satisfactory index of the accuracy and significance of this test in terms of the completed test. 2. When doubtful, that is, showing colonies of doubtful or negative appearance in 24 hours, must always be completed. 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 “presumptive” and the “partially confirmed” tests. The completed test is required in _all_ cases where the result of the confirmed test has been doubtful. 9. EXPRESSION OF RESULTS.