INHIBITION OF GROWTH. 177 



yeasts, too, are quite resistant to acid and can become accustomed to 

 quite strong acids; it is customary in the manufacture of industrial alcohol 

 from potatoes to prevent bacterial growth by adding about i per cent of 

 lactic acid or very active lactic bacteria together with an acid-tolerant 

 yeast, bred especially for this purpose. The acid in the stomach of man 

 and animals plays an important role as a sterilizing agent for the food. 

 Many microorganisms are killed in the stomach. In the household, 

 the natural acidity of fruit helps in keeping canned fruit, preserves and 

 jellies. Especially in heating, the acid together with the high temperature 

 has a very strong germicidal effect. Vinegar is often used to preserve 

 fruit and vegetables; in some parts of the country, meat is kept in butter- 

 milk. Benzoic and salicylic acids are often used in the preservation of 

 fruit and vegetables. Their poisonous influence is not so much due to 

 the acid reaction but to the specific chemical character of these 

 compounds. 



Of the alkalies, only one is used extensively, namely, lime; quick-lime 

 (CaO) is considered a valuable disinfectant for excreta in privy vaults; it 

 is universally applied as a white-wash in stables, barns, poultry houses and 

 similar buildings. Quite commonly, it is used as "milk of lime" (one 

 part of slaked lime with four parts of water) . It should be kept in mind 

 that the calcium oxide unites with the carbon dioxide of the air and thus 

 gradually loses its disinfecting power. 



Of the metallic salts, many are well-known germicides. The most 

 powerful disinfectant is mercuric chloride (HgCl 2 ) which is one of the 

 standard disinfectants. It is generally used in a dilution 1:1000 which is 

 sufficient to kill all vegetative cells as well as spores in a few minutes. 

 Quite commonly, hydrochloric acid or salt is added, to prevent coagula- 

 tion or precipitation of slimy or albuminous matter which would protect 

 the enclosed bacteria from immediate contact with the poison. The 

 addition of hydrochloric acid or any chloride decreases somewhat the 

 disinfectant value for bacteria suspended in distilled water because it 

 decreases the electrolytic dissociation. 



Another disinfectant of remarkable strength is silver nitrate; it is not 

 used commonly because of its high price. It also decomposes easily and 

 leaves dark spots on the skin and clothes. Of the other metallic salts 

 copper and iron sulphate are not used extensively, though recommended 

 for the disinfection of feces. Zinc sulphate may be applied to mucous 

 membrane the same as silver nitrate. Many other salts may be used 



12 



