DISINFECTANTS 289 



tion will exercise disinfectant action within a standard 

 time. It may sometimes be necessary also to determine 

 the times in which disinfection can be effected by dilutions 

 which are incapable of disinfecting within the standard 

 time ; this, however, is not usually of great importance for 

 practical purposes, if the standard time be such as to be 

 reasonably practicable in ordinary conditions of working. 

 The exposure which represents most fairly and safely that 

 which is practicable for the majority of purposes may be 

 taken at ten minutes. 



It is preferable in these determinations to use a measured 

 quantity — say 4'5 c.c. of a twenty-four hours' culture at 

 37° C. of standard broth solution. In exact determinations 

 it is desirable to count the number of organisms present ; 

 but serviceable results may be obtained without this pre- 

 caution. The use of broth in this way unquestionably is 

 liable to affect unfavourably those disinfectants which are 

 liable to be decomposed by the substances which it con- 

 tains. On the other hand, these substances are such as 

 are extremely liable to occur in nature ; and it is in most 

 cases preferable that the action of the disinfectant on the 

 mixture of organisms and organic matters should be jointly 

 determined, rather than that any risk should be incurred of 

 exposing the organism in a non-nutrient medium capable 

 itself of exercising a detrimental action upon its resistance. 

 It is certain that results so obtained must be for all practical 

 purposes as safe as, and may be safer than, those which 

 would be had by using emulsions of bacterial growths upon 

 agar cultures in sterile distilled water. 



In examining disinfectants soluble in water, standard solu- 

 tions of the various strengths which it is proposed to apply, 

 usually from 5 per cent, to "005 per cent., are prepared, and 

 0*5 c.c. of each is added to the broth tubes, thus giving a 

 disinfectant strength of one-tenth of the strength of the 



19 



