ANTISEPTIC AND GERMICIDAL POWER 149 



Investigations may be required to : 



(1) Determine the restraining and germicidal power of 

 different substances in solution. 



(2) Determine the germicidal power of substances when 

 volatilized. 



To determine Germicidal Power of Liquids. 



Two separate determinations may have to be made, one for 

 the bacterium and one for the spore, if spores are produced. 



In testing germicidal power it is essential that certain factors 

 be kept constant if results of any value are to be arrived at. 

 The following may be specially mentioned. 



Temperature. Disinfectants act much more powerfully at 

 higher than at lower temperatures. The relationship is an 

 orderly one and capable of being mathematically expressed. 

 Even when phenol controls, under identical temperature con- 

 ditions, are being employed, as far as possible the same tem- 

 perature should always be used, or at least only variations 

 between narrow limits. 



Number of bacteria present. Since disinfection is analogous 

 to a chemical reaction it follows that the greater the number 

 of bacteria the longer the time taken to disinfect, and this 

 applies even when the disinfectant is present in great excess. 

 When comparative experiments are undertaken as nearly as 

 possible the same amount of bacterial culture must be used for 

 the control and for the unknown disinfectant. 



The resistance of the bacterium used. Not only do different 

 bacteria differ in their resistance but considerable differences are 

 met with in different strains of the same bacillus. If controls 

 are done at the same time with the same strain variations from 

 this factor can be eliminated. 



Culture medium to be used to test viability. This is of some 

 importance since organisms damaged, but not killed, by the 

 action of the disinfectant may fail to grow if implanted into a 

 medium not perfectly suited to their requirements. 



