BACTERIOLOGICAL TESTING OF DISINFECTANTS 381 



occur fairly often, and substituted for the glass threads 

 small test-tubes, on the bottom of which small measured 

 drops of culture were allowed to fall and dry, the sub- 

 sequent addition of disinfectant, antidote, water, and broth, 

 being performed in the same tube. In this way the risk 

 both of pollution and of loss from washing during the 

 various manipulations is reduced, especially when a little 

 gelatine is added to the original culture, which is kept in 

 a burette at melting-point and allowed to solidify in a very 

 thin layer at the bottom of the tube. 



It must be remembered that in these, as in all bacterio- 

 logical examinations, every operation has to be controlled. 

 Thus, for instance, when organisms are prepared for test by 

 drying, a portion of the untreated organisms must be 

 inoculated into broth to make sure that at least they have 

 survived. The nature of the control varies in different 

 cases ; but no experiment is complete until a corresponding 

 inoculation has been made, embodying, as far as possible, 

 all the test conditions, except the presence of the substance 

 under examination. 



Similarly it is necessary to examine the cultures micro- 

 scopically before application of the disinfectant, and, in the 

 event of growths, to identify them both in this way and by 

 subcultures. 



It is found that the action of a disinfectant is enhanced 

 in most cases by its application at a temperature higher 

 than the optimum, even when that temperature is below 

 what would injure the organism. It is also found in 

 many cases that two separate disinfectants exercise a more 

 powerful effect, and work in much weaker solutions, than 

 either of them separately. In some cases the addition of a 

 substance possessing by itself little or no disinfectant 

 action, may greatly enhance the efficiency of a disinfectant. 

 Thus, carbolic acid solution, saturated with common salt, is 



