INHIBITION OF GROWTH 293 



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 occasionally for disinfecting purposes, though 

 the expense or undesirable qualities prevent their common application. 



The alcohols are well known for their poisonous effects, but the 

 value of ethyl alcohol as a disinfectant is usually overestimated. It 

 takes quite strong alcoholic solutions, more than 20 per cent, to kill 

 certain yeasts and the spores of some bacteria in less than a day, 

 and a complete sterilization by alcohol in a few minutes cannot al- 

 ways be guaranteed even with 50 to 60 per cent solution. It has 

 already been mentioned that desiccated organisms are very resistant 

 to concentrated alcohol, more so than to a 50 per cent mixture. 

 Methyl alcohol is weaker, the higher alcohols, especially amyl alcohol, 

 are stronger disinfectants than ethyl alcohol. They all give good 

 results in the presence of water while the absolute alcohols have 

 scarcely any effect upon desiccated bacteria. None of these alcohols 

 in whatever concentration they may be used, can be relied upon to 

 kill bacterial spores. 



Stronger germicidal effects can be obtained by the alcohols of the 

 benzol group, of which phenol or so-called carbolic acid (CeH 5 OH) 

 is the simplest representative. Phenol, like ethyl alcohol, is not as 

 effective as is commonly believed. It is applied in solutions from .5 per 

 cent to 5 per cent ordinarily, but it usually takes a long time even for 

 the 5 per cent solution to kill vegetative cells as Bact. tuberculosis or 

 B. coli; it is inefficient against anthrax spores. More powerful are 

 the higher cyclic alcohols, of which the cresols are examples. They are 

 used extensively as disinfectants and antiseptics. They are, together 

 with phenol, coal-tar constituents and are sold commercially under many 

 different names, either pure or mixed with soap or other disinfectants 

 which make them emulsify readily in water. The cresols are almost 

 insoluble in water, and not as effective in solutions as they are in 



