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which holds true also for the silk threads, that the process 

 of desiccation tends to lower the vitality of the organism. 

 Furthermore it may be urged that the disinfectant has not 

 free access to all sides of the bacteria. 



(3.) Bacterial suspensions. This method in some of 

 its modifications is the one which is commonly employed 

 and, if used with proper precautions, yields perfectly reli- 

 able results. The first essential is to secure a suitable sus- 

 pension of the organism to be tested. For this purpose 

 the fresh growth on the surface of 3 or 4 agar tubes is care- 

 fully removed and thoroughly rubbed up in about 10 c. c. of 

 sterilized distilled water. In order to remove the coarse 

 floccules the suspension is filtered through glass wool, and 

 the filtrate immersed in a water-bath at 37.5 C. and fre- 

 quently agitated till a microscopic examination shows no 

 longer the presence of groups or masses of bacteria. In 

 this suspension now the number of bacteria present can 

 determined as already stated. 



By means of a sterilized pipette, graduated in 1-10 c. c., 

 an exact volume, 3 c. c., is transferred into each of several 

 sterilized test-tubes. To the suspension in one of these 

 tubes an equal volume of the disinfectant, of double the 

 strength to be tested, is added. At intervals of 2, 5, 10. 20, 

 30, 60 minutes etc. transfers are made to sterilized bouillon 

 or agar tubes and these are then set aside in the incubator 

 for at least one week. The inoculations should be made in 

 duplicate and 2 or 3 loopfuls used for each tube. 



The method as given is open to the objection that an 

 appreciable amount of the disinfectant is transferred each 

 time to the culture tubes and that it may prevent growth. 

 This is specially true with substances which possess 

 marked antiseptic properties, as mercuric chloride. Where 

 possible, the disinfectant should be rendered inert. Thus, 

 traces of mercuric chloride can be removed by precipita- 

 tion with hydrogen sulphide. With other substances the 

 error is not so marked and is partly counterbalanced by 



