PHYSICAL AGENTS 119 



organ when it is desired to obtain bacteria from the interior 

 free from contamination with surface organisms. 



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. 71). 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 is 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/' 

 or '' tyndaUization,'' 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 sub- 

 jected to streaming steam for half an hour on each of three 

 successive days. In canning vegetables the boiling should 

 be from one or 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 



