-524 BACTERIOLOGY. 



cohol. Finally, it is transferred to a tube of nutrient bouillon (10 c.c.) 

 and then set aside in the incubator for a week or more. Similar tests 

 with exposures of 5, 10, 30 and 60 minutes are made. 



The objections to this method are threefold and have already 

 been incidentally mentioned. In the first place, the bacteria on the 

 thread may not be evenly exposed to the action of the disinfectant; 

 secondly, the disinfectant itself may be transferred to the nutrient 

 medium; and lastly, a small amount of the disinfectant may remain 

 in chemical combination with the silk thread and by its presence in- 

 hibit the development of the already attenuated organism. The at- 

 tempt is made to obviate the second objection by washing* the threads. 

 This may be successful in some cases, but in others it fails. The third 

 objection is especially true of mercuric chloride which is apparently 

 held fast by the fiber and can only be removed by the action of hydro- 

 gen sulphide (Geppert). 



2. Muslin squares. The preparation of these specimens is given 

 on p. 517. They are used chiefly as test-objects in experiments on the 

 disinfection of rooms. 



3. Cover-glasses. This method was introduced by Geppert and 

 has been used by Spirig and others. To test a disinfectant a dry 

 cover-glass, streaked with the organism in the manner described on 

 p. 517, is immersed in it for a given length of time as in the case of 

 the silk threads. It is then removed with sterilized forceps and 

 washed in a large volume of sterilized water for about i-i hour. The 

 cover-glass is then placed in sterilized bouillon which is set aside in 

 the incubator. 



The advantages of this method are (1) that a thin film of evenly 

 spread bacteria is employed; and, (2) that the cover-glass does not 

 unite with the disinfectant, as is the case with the silk threads. It is 

 open to the objection, which holds true also for the silk threads, that 

 the process of desiccation tends to lower the vitality of the organ- 

 ism. Furthermore, it may be urged that the disinfectant has not free 

 access to all sides of the bacteria. 



4. Bacterial suspensions. This method in some of its modifica- 

 tions is the one which is commonly employed, and, if used with proper 

 precautions, it will yield perfectly reliable results. The first essen- 

 tial is to secure a suitable suspension of the organism to be tested. 

 Directions for doing this are given on p. 514. 



In general the procedure consists in adding to a given volume of 

 the suspension an equal volume of the disinfectant of double the 



