DISINFECTION. 395 



the light must shine directly upon the bacteria. Consequently, 

 articles of infected clothing can thus be disinfected only when 

 they are very thin, so that the bacteria are wholly on the sur- 

 face. Such a condition is, of course, unusual and hence the 

 direct value of sunlight in disinfecting clothing is not very 

 great. At the same time it must be remembered that light is 

 hostile to bacteria, and dark rooms or stables will offer better 

 chances for their growth than light rooms. Any article which 

 can be exposed to direct sunlight may thus be readily disin- 

 fected upon its surface by a few hours' exposure. 



Cold. The application of cold is apparently useless to de- 

 stroy bacteria. Bacteria resist for hours, and even for days, 

 the extremely low temperatures of liquid air. If the action of 

 such cold be maintained for a long period it does, in the end, 

 have a considerable germicidal power. Bacteria frozen in ice 

 will remain alive and active many weeks, but after a time 

 they die. Recent experiments have shown, for example, that 

 typhoid bacilli, frozen in ice, remain alive for several weeks, 

 but eventually begin to die and, when the ice has been pre- 

 served for three months, it contains practically no live typhoid 

 bacilli. This is a matter of great practical importance in con- 

 nection with the use of ice harvested in the winter from waters 

 liable to contamination with typhoid bacilli. But while long- 

 continued freezing may in the end destroy bacterial life, the 

 use of cold is never adopted as a practical disinfectant. 



2. CHEMICAL AGENTS. 



The most common methods of disinfection are by the use of 

 chemicals. A large number of substances have been used for 

 the purpose, but we can consider only those which have proved 

 to be the most practical and efficient. 



Corrosive Sublimate. This is one of the most efficient ger- 

 micides known, and its small cost has given it very wide use. 

 When in a water solution of 1-500 it is a germicide. As an 



