152 DISINFECTION AND STERILIZATION 



be used in several times the ordinary strength. Sohds 

 require relatively more of a given solution than liquids. 



7. State of the disinfectant, whether solid, liquid or gas, 

 and whether it is ionized or not. Solutions penetrate best 

 and are therefore more quickly active and more efficient. 



8. The solvent. Water is the best solvent to use. Strong 

 alcohol (90 per cent. +) diminishes the effect of carbolic 

 acid, formaldehyde and bichloride of mercury. Oil has a 

 similar effect. The action is probably to prevent the pene- 

 tration of the disinfectant. 



9. Strength of solntion. The stronger the solution, the 

 more rapid and more certain the action, for the same rea- 

 son as mentioned under .5. In fact, every disinfectant has 

 a strength below the lethal at which it stimulates bacterial 

 growth. 



10. Addition of salts. Common salt favors the action of 

 bichloride of mercury and also of carbolic acid. Other salts 

 may hinder by precipitating the disinfectant. 



11. Temperature. Chemical disinfectants, as a rule, fol- 

 low the general law that chemical action increases with the 

 temperature, up to the point where the heat of itself is 

 sufficient to kill. 



12. Time of action. It is scarcely necessary to point out 

 that a certain length of time is necessary for any disinfec- 

 tant to act. One may touch a red hot stove and not be 

 burned. All the above-mentioned conditions are influenced 

 by the time of action. 



PRACTICAL STERILIZATION AND DISINFECTION. 



The methods for sterilizing in the laboratory have been 

 discussed and will be referred to again in the next chapter. 



In practical disinfection it is a good plan always to pro- 

 ceed a^ though spores were present even if the organism is 

 known. Hence use an abundance of the agent and apply it 

 as long as practicable. Also it is best to secure the chemical 

 substances used as such and not depend on patented mi.vtnres 

 purporting to contain them. As a rule the latter are more 

 expensim in proportion to the results secured. 



