140 AGEICULTUEAL AND INDUSTRIAL BACTEEIOLOGY 



8. Economy. — The ideal disinfectant should be low in 

 cost. Certain valuable disinfectants, because they are so 

 expensive, can be used only in limited quantities and for 

 special purposes. The high cost of salts of silver, for 

 example, limit the use of silver almost entirely to medi- 

 cine. If the entire water supply of a city is to be steri- 

 lized, obviously a disinfectant must be chosen which is 

 relatively inexpensive. 



9. Power to Remove Dirt and Grease. — This is related to 

 power of penetration. A film of oil or grease over the 

 surface of certain materials may AvhoUy prevent the 

 action of many disinfectants. Those which have power 

 to dissolve or remove grease and other kinds of dirt are 

 naturally more efficient. 



10. Deodorizing Power. — A disinfectant which can com- 

 bine with and destroy malodorous substances is prefer- 

 able to one which does not interfere with them. 



Effect of Hydrogen Ion Concentration upon Death 

 Rates. — The addition of acid to any nonalkaline solution 

 containing bacteria (with the consequent increase in hydro- 

 gen ion concentration) increases markedly the rate of 

 death. 



Bacteria which grow well in a relatively high hydrogen 

 ion concentration are termed acidophiles. The ability of 

 these organisms to grow in the presence of high concentra- 

 tions of acid is sometimes utilized in their isolation. 



Acids are most commonly used as preservatives. The 

 most important are the acetic (usually in the form of vine- 

 gar) and the lactic acids, the latter the preservative agent 

 in sour milk, sauerkraut, and silage. 



Strong acids are those which ionize most completely. 

 "Weak acids are those which ionize poorly. Lactic and 

 acetic acids are relatively weak acids, and must be present 

 in relatively large amounts in order to secure a high con- 

 centration of hydrogen ions. For example, tenth normal 



