DISINFE C TION 325 



2. The bacteria serving as test objects should have equal 

 power of resistance. 



3. The numbers of bacteria used in comparative observa- 

 tion should be approximately equal. 



4. The disinfecting solution must be always used at the 

 same temperature in comparative experiments. 



5. The bacteria must be brought into contact with the 

 disinfectant with as little as possible of the nutrient material 

 carried over. (This obviously will depend upon the object 

 of the research.) 



6. After having been exposed to the disinfectant for a 

 fixed time, they should be freed from it as far as possible. 



7. They should then be returned in equal numbers to the 

 respective culture medium most favourable to the develop- 

 ment of each, and kept at the same, preferably the optimum, 

 temperature for their growth. 



8. The number of surviving bacteria capable of giving 

 rise to colonies in solid media must be estimated after the 

 lapse of equal periods of time. 



We may now turn from general principles to mention 

 shortly some of the commoner methods and substances 

 adopted to secure efficient disinfection. They are all divis- 

 ible, according to Sir George Buchanan's standard, into two 

 groups: 



1. Heat in various forms; 



2. Chemical bodies in various forms. 



It should at the outset be understood that we desire in 

 practical disinfection to inhibit or kill micro-organisms with- 

 out injury to, or destruction of, the substance harbouring 

 the germs for the time being. If this latter is of no mo- 

 ment, as in rags or carcasses, burning is the simplest and 

 most thorough treatment. But with mattresses and bed- 

 dings, bedclothes and garments, as well as with the human 

 body, it is obvious that something short of burning is 

 required. 



