2 5 B AIDS TO BACTERIOLOGY 



from laying eggs or feeding on the refuse. Lime, which 

 has a high capacity for absorbing sulphuretted hydrogen, 

 is very well adapted as a base for a disinfectant powder. 

 This is inadmissible with phenol, but is successfully 

 employed in the preparation of cyllin and kerol powders. 

 Examination of Disinfectant Powders. Robertson and 

 Severn's process (Kept. M.O.H. Cape Colony for 1906): 

 One hundred grammes of the powder are thrown into a 

 litre stoppered measure, and made up to the mark with 

 sterile distilled water (temperature to be between 15 C. 

 and 18 C., little or no difference being noticeable be- 

 tween these limits). The cylinder is shaken violently 

 and horizontally every fifteen minutes for four hours, and 

 left then for subsidence. At the expiration of twenty- 

 four hours, some of the clear supernatant extract is drawn 

 off with a sterile 50 c.c. pipette into a sterile bottle or 

 flask. This is used to make the dilutions. (In some cases 

 a layer of the finer particles of the base will remain ob- 

 stinately on the surface of the extract, and some will 

 stick to the pipette. Quickly wipe this away with a 

 sterile cotton-wool plug.) Each 10 c.c. of the extract is 

 regarded as equal to 1 gramme of the powder. As a 

 standard, a 15 per cent, carbolic powder is taken as 1. 

 Such a powder is difficult to prepare and keep and extract, 

 owing to its deliquescence and the necessity for using 

 hot water for its complete extraction. A 10 per cent, 

 extract of a completely extracted 15 per cent, carbolic 

 acid powder can be simulated by taking 100 c.c. of a 4*7 

 per cent, solution of phenol and diluting to 313 c.c. (or 

 100 c.c. 5 per cent, phenol made up to 332 c.c.). This 

 solution is called "standard powder 1 in 10." Then a 

 17 dilution, for instance, is made by taking 100 c.c. and 

 diluting to 170 c.c. The R.-W. process is then applied 

 to extract and standard. 



Bacteriological Examination of Disinfectants. 



Few disinfectants are capable of being examined by 

 chemical means, so that the disinfectant value is revealed, 

 and, after all, the determination of the germicidal value 

 by actual experiment on living organisms is the only 

 reliable method of gauging the value. Many factors, 

 however, enter into the question, which must be re- 

 cognised: the time allowed for action, the temperature of 



