78 MILK [vi 



ence between damp and dry heat, the former being 

 very much more deadly than the latter in its effect. In 

 order to make sure of effecting the complete destruc- 

 tion of the bacteria in any object, it is necessary to 

 submit that object to a very high temperature for 

 some time a temperature of from, say, 160 to 163 

 C. (320 to 326 Fahr.). The fact that the spores 

 possess greater powers of resistance than adult germs 

 explains the efficiency of the method largely used 

 for reducing bacterial life in such fluids as milk. 

 This consists of heating the fluid from time to time 

 at a comparatively low temperature, but sufficiently 

 high to destroy adult germ life. When such a fluid 

 is heated for the first time, many spores present in it 

 escape destruction. As, however, the fluid is kept 

 at a temperature favourable for germ development, 

 the spores in due time develop into adult organisms, 

 and are destroyed in the course of heating. Such a 

 method of treatment, which is known as intermittent 

 sterilisation, while it may be said to ensure complete 

 destruction of micro -organic life, is yet too trouble- 

 some a method to have always recourse to. It is a 

 fortunate circumstance that most pathogenic germs 

 are comparatively easily killed. Generally speaking, 

 it may be said that bacteria do not develop below 

 4 C. (39 Fahr.), and much above 50 C. (122 Fahr.). 

 By cooling a liquid containing bacteria, therefore, to 

 4 C. (39 Fahr.), bacterial development is completely 



