268 C.-E. A. WiNSLOW AND E. E. LOCHEIDGE 



With the view of securing exact quantitative resuhs, we adopted 

 the method of exposure to the acid tested, in a suspension from 

 which samples were directly plated. This process has the obvious 

 defect of permitting a certain amount of the disinfectant to be 

 carried over to the plate, where it may exert an antiseptic action. 

 We have really measured the combined disinfectant action in the 

 suspension and possible antiseptic action in the plate. The action 

 of organic matter in decreasing toxicity, shown by our experiments, 

 must greatly reduce any such action in the plate. 



The procedure in each experiment was as follows: A series of 

 bottles, each containing loo c.c. of sterile water (or peptone solution), 

 was arranged in a row, and to each bottle was added a different 

 amount of standardized acid from a graduated pipette. The amount 

 of water in each bottle was measured at the end of the experiment, 

 in order to obtain the exact strength of the solution. Immediately 

 after the addition of the acid there was added to each bottle i c.c. of 

 a fresh aqueous suspension of the bacteria tested. After standing 

 for 40 minutes, lactose agar plates were made in duplicate, from 

 the acidified bottles, and from controls with no acid. Colonies were 

 counted after 24 hours' incubation at 37° C. 



Forty minutes was selected, after some preliminary experiments, 

 as the best period of exposure to the acid, since it gave sharper 

 results than a shorter time. In the tests reported in the accompany- 

 ing tables there was not a variation from the 40 minutes of more than 

 one minute in most of the samples. The series for B. typhi in water, 

 with HCl, were also examined after 100 minutes, and after 24 hours. 

 In the sample containing 48 parts per million of sulphuric acid, 

 there was, after 40 minutes, 59.3 per cent removal of B. typhi; after 

 100 minutes, 88.15 per cent removal; after 24 hours, 100 per cent 

 removal. The sample containing 92.9 parts per million of sulphuric 

 acid removed after 40 minutes 92.97 per cent; after 100 minutes, 

 99.99+ per cent ; after 24 hours, 100 per cent. The removal was 100 

 per cent in all of the samples containing larger amounts of acid after 

 103 minutes, and in all of the samples after 24 hours. 



The temperature factor was of considerable importance, and care 

 was taken to keep conditions uniform. No agar was poured with a 

 temperature greater than 50° C. It was found that a rise in tem- 



