103 



The absence of gas-formation in lactose broth or failure to demon- 

 strate 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. When definitely positive, that is showing 

 more than 10 percent (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 examina- 

 tion 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 puri- 

 fication 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 percent (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 suffi- 

 cient. 



(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 "par- 

 tially confirmed" tests. 



The completed test is required in all cases where the result of the 

 confirmed test has been doubtful. 



IV. Modification of A. P. H. A. Method. The following pro- 

 cedure, which is a modification of the A. P. H. A. method has proved 

 very satisfactory and convenient in the hands of the author. 



