DISINFECTION AND DISINFECTANTS 261 



has been said to increase its germicidal power, and there- 

 fore to give a low coefficient for the sample. But, in 

 fact, the amounts found are too small to have appreciable 

 influence. More important is the quantity of cresols 

 usually found in carbolic acid crystals. As cresol has 

 approximately three times the germicidal power of 

 phenol, the error from this may be considerable. The 

 solidifying-point of the crystals should be not less than 

 40 C. Ainslie Walker and Weiss (Journ. Franklin Inst., 

 1912, 101) say the bromine titration method does not 

 yield trustworthy results as regards the purity of the 

 phenol, but it may be employed for checking the strength 

 of the 5 per cent, stock solution prepared from pure 

 phenol. The 5 per cent, stock solution should be kept 

 in a closely-stoppered bottle in a dark cupboard. The 

 culture must be a Lemco broth one, twenty-four hours 

 old. The broth should be made as follows: 



Lemco . . . . 20 grammes. 



Peptone 20 



Sodium chloride . . . . 10 



Water 1 litre. 



Boil the mixture for thirty minutes, and standardise to 

 a reaction of +1-5 per cent. (4-15, Eyre's scale), using 

 phenolphthalein. 



When the cholera vibrio or diphtheria bacillus is the 

 test organism, a neutral or alkaline broth must be used 

 (' Bacteriological Examination of Disinfectants,' 1907, 

 p. 25), and for the latter a forty-eight hour old culture is 

 necessary (ibid., 17). Hewlett and Norman Hall (Journ. 

 Hygiene, 1912, p. 473) have shown that when working with 

 anthrax spores agar is preferable to nutrient broth for 

 the subcultures, as the latter does not show a life very 

 often when agar does. 



Before addition to the medication tubes the culture 

 should be freed from clumps. This may be attained by 

 shaking the culture and allowing to settle for twenty 

 minutes before the test, when the culture required is 

 pipetted from the top half, or it may be filtered through 

 filter-paper. 



The general laboratory procedure is to keep a stock 

 culture on agar, and subculture on to a fresh agar slope 

 at the end of a month From the twenty-four hour 



