THE METHODS OF DISINFECTION. 85 



forming definite chemical compounds with the bacteria cells, 

 which are thus rendered innocuous. They must, therefore, 

 come into direct contact with the bacteria themselves, and in 

 the combination so formed they and the bacteria change their 

 chemical properties. 



In the choice of chemical disinfectants, one must be guided 

 by the species of bacteria to be destroyed, by the number of 

 these bacteria, by the nature of the media containing the 

 germs, by the substances to be disinfected, and by the quan- 

 tity of material to be disinfected. All chemicals so used 

 must be of a definite strength, and must be made to act for 

 a specified time, the quantity and strength of the disinfec- 

 tant and the time varying with the nature of the chemical 

 and the species of bacteria to be destroyed. 



To test the germicidal power of substances, the following is 

 a convenient method : To young bouillon cultures of the 

 bacteria to be acted upon sufficient of the chemical to be 

 tested is added to make the proper dilution, and at given 

 intervals of time a few droplets of this mixture are inocu- 

 lated on sterile agar and gelatin tubes and the result carefully 

 noted for instance, supposing it is intended to test the germi- 

 cidal power of carbolic acid toward some pus organism. To 

 9.90 c.c. of a bouillon culture of a pus germ in a test-tube 

 0.10 c.c. of carbolic acid is added, making the mixture a 

 1 per cent, carbolic acid dilution. The tube is well shaken, 

 and at the end of one minute a few droplets of the mixture 

 are taken on a sterile platinum needle and inoculated on a 

 fresh tube of gelatin or agar, another fresh agar tube is 

 inoculated with the mixture in the same manner after two 

 minutes, another again after five minutes, and a fourth in 

 fifteen minutes, a fifth in a half hour, a sixth in an hour, 

 and a seventh in two hours. The growth in the inoculated 

 tubes would indicate the action of 1 per cent, carbolic acid 

 upon the given pus germ in the specified time. The same 

 process is repeated, using a 2 per cent, dilution of the car- 

 bolic acid, and next a 3 per cent., a 4 per cent., and a 5 per 

 cent, dilution. Higher than 5 per cent, solutions of carbolic 

 acid are not possible with any but water too hot for this work. 



