PHYSICAL AGENTS 121 



Dry Heat. — Dry heat is not nearly so effective as moist 

 heat as a sterihzing agent. The temperature must be higher 

 and continued longer to accomplish the same result. Thus 

 a dry heat of 150° for thirty minutes is no more efficient than 

 steam under pressure at 115° for fifteen minutes. Various 

 forms of hot-air sterilizers are made for laboratory purposes 

 (Fig. 76). On account of the greater length of time required 

 for sterilization their use is more and more restricted to 

 objects which must be used dry, as in blood and serum 

 work, for example. In practice the use of hot air in disin- 

 fecting plants is now largely restricted to objects which 

 might be injured by steam, as leather goods, furs, and certain 

 articles of furniture, but even here chemical agents are more 

 frequently used. 



Moist Heat. — Moist heat may be applied either by boil- 

 ing in water or by the use of steam at air pressure, or, for 

 rapid work and on substances that would not be injured, 

 by steam under pressure. Boiling is perhaps the best house- 

 hold method for disinfecting all material which can be so 

 treated. The method ,i^ simple, can always be made use of, 

 and is universally understood. It must be remembered that 

 all pathogenic organisms, even their spores are destroyed 

 by a few minutes' boiling. The process may be applied to 

 more resistant organisms, such as are met with in canning 

 vegetables, though the boiling must be continued for several 

 hours, or what is better, repeated on several different days. 

 This latter process, known as "discontinuous sterilization," 

 must also be applied to substances which would be injured 

 or changed in composition by too long-continued heating, ' 

 such as gelatin, milk, and certain sugars. In the laboratory 

 such materials are boiled or subjected to streaming steam 

 for half an hour on each of three successive days. In can- 

 ning vegetables the boiling should be from one to two hours 

 each day. The principle involved is that the first boiling 

 destroys the growing cells, but not all spores. Some of the 

 latter germinate by the next day and are then killed by the 

 second boiling and the remainder develop and are killed on 

 the third day. Occasionally a fourth boiling is necessary. 

 It is also true that repeated heating and cooling is more 



