TESTING OF DISINFECTANTS. 525 



strength to be tested. At stated intervals (1, 2, 5, 10 minutes, etc.) 

 one large loopful of the mixture is transferred to nutrient bouillon 

 (10 c.c.), or to agar and these tubes are then set aside in the incubator. 

 The method as given is open to the objection that an apprecia 

 ble amount of the disinfectant is transferred each time to the cul- 

 ture tubes and that it may prevent growth. This is specially true 

 with substances which possess marked antiseptic properties, as mer- 

 curic chloride. Whenever possible, the disinfectant should be ren- 

 dered inert. Thus, traces of mercuric chloride can be removed by 

 precipitation with hydrogen sulphide. With other substances the 

 error is not so marked and is partly counterbalanced by keeping the 

 tubes in the incubator for many days. The important point is to in- 

 oculate into a large volume of bouillon, not less than 10 c.c., and when 

 this is done the results with mercury disinfectants are as good, if not 

 better, then when hydrogen sulphide is used. 



Laboratory work. The student will test the action of mercuric 

 chloride (1-1000), carbolic acid (5 per cent.), lysol (5 per cent.) and for- 

 maldehyde (5 per cent.) on the following organisms: Typhoid bacillus, 

 Staphylococcus pyogenes aureus, and Anthrax spores. The results 

 are to be tabulated. 



Action of mercuric chloride. A solution of mercuric chloride (1-500) 

 in distilled water is prepared. It is well to steam for a few minutes 

 all freshly prepared disinfectants in order to insure freedom from 

 error. 



a. By means of a sterile pipette 10 c.c. of this disinfectant solu- 

 tion are placed in a small, sterile Erlenmeyer flask or in an Esmarch 

 dish, and an equal volume of the bacterial suspension (p. 514) is added, 

 likewise by means of a sterile pipette. The liquid is mixed at once. At 

 intervals of 5, 10, 15, 30, 60 and 120 minutes, a large loopful of the mix- 

 ture is transplanted to at least 10 c.c. of sterile bouillon. The loop 

 used should have a clear diameter of 2 mm. The set of tubes, properly 

 labeled, are then placed in the incubator. 



b. In the above method the injurious effects of the mercury car- 

 ried over in the transplantation are largely counteracted by the 

 large volume of bouillon employed. The effects of the mercury can 

 be almost wholly done away with by passing HaS through the suspen- 

 sion. In this case, however, care must be taken to avoid two possible 

 errors. As a result of the passage of the gas, HC1 is liberated and, 

 unless it is promptly neutralized, it may affect the test-organism .- 

 Again, the precipitate of mercury sulphide tends to drag down the 



