The Germicidal Value 305 



One of the best methods for testing the germicidal value 

 of solutions is that suggested by Rideal and Walker.* To 

 use it, it is necessary to proceed with strict attention to 

 details. 



The advantage of this method is that it expresses the 

 germicidal value in terms of carbolic acid, as the " carbolic 

 acid coefficient," and thus makes possible an accurate com- 

 parison of germicide with germicide. 



The test culture should be grown in bouillon made accord- 

 ing to the same formula with exactly the same reaction. 

 The cultures should be grown in the thermostat at 37 C. 

 for just twenty-four hours, and, in order that they should 

 contain no clumps of bacteria, should be carefully filtered 

 through cotton-wool or glass wool just before using. The 

 transfer of the bacteria from the filtered culture to the diluted 

 germicide solutions is made with a large platinum loop, 

 about 6 mm. in diameter, slightly bent like a spoon, so as to 

 take up a large drop. Three drops are carried from the 

 culture into the germicide solution, which, no matter what 

 its dilution, should be of 5 c.c. volume, with great care that 

 the walls and mouth of the tube are not touched. The tube 

 is then well shaken from side to side, so as to distribute the 

 bacteria throughout the solution, and the tube set aside. 

 At intervals of one, three, five, ten, and fifteen minutes 

 respectively, three drops of the fluid are transferred with 

 the same platinum loop to fresh bouillon tubes. Failure 

 to grow upon such transplantation shows that the bacteria 

 have been killed. 



Gaseous Disinfection. If the germicide to be studied be 

 a gas, as in the case of sulphurous acid or formaldehyd, a 

 different method must, of course, be adopted. 



It may be sufficient to place a few test-tube cultures of 

 various bacteria, some with plugs in, some with plugs out, 

 in a closed chamber in which the gas is evolved. The germi- 

 cidal action is shown by the failure of the cultures to grow 

 upon transplantation to fresh culture media. This crude 

 method may be supplemented by an examination of the dust 

 of the room. Pledgets of sterile cotton are rubbed upon the 

 floor, washboard, or any dust-collecting surface present, and 

 subsequently dropped into culture media. Failure of growth 

 under such circumstances is very certain evidence of good 

 disinfection. These tests are, however, very severe, for in 

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