DESTROYING GERMS BY DRY HEAT. 101 



raised against the use of such appliances in exceptional cases, since 

 by this means a clear nitrate is obtainable when all other methods 

 of clarification have failed to remedy turbidity. This decision 

 must, however, be amended when it is a question of beer already 

 in good condition, this latter often suffering, under such treatment, 

 a considerable alteration (in certain circumstances) with regard to 

 its flora, apart from the depreciation of flavour already alluded to. 

 The filter removes the yeast cells, but allows the (much smaller) 

 bacteria to slip through, so that the latter appear in almost their 

 original numbers in the filtrate, where, moreover, they have free 

 play, owing to their previous competitors, the yeast cells, having 

 been got rid of. This unfavourable modification in the relative 

 condition of the two classes of organisms becomes especially 

 objectionable when a filtering material that has already been in 

 use before is employed, without having been sufficiently purified 

 in the interim. In this manner the nitrate can be actually en- 

 riched with bacteria, as the author ascertained by experiments 

 with the Enzinger filter in 1894. 



Respecting the wine filter in continually extending use in 

 cellar management, a full report can be perused in the handbook 

 issued by BABO and MACH (I.). 



76. Destroying: Germs by Dry Heat. 



Strictly speaking, the term "germ-free" should be applied 

 only to such objects as have actually been devoid of germs from 

 the beginning or have been brought into this condition by filtra- 

 tion. In the language of bacteriological practice, however, it is 

 also applied to objects wherein all the germs have been" destroyed 

 and are only present in a defunct condition. Hence it would be 

 more correct to say that the object in question is "free from 

 living germs," but this distinction, being practically unimportant, 

 is not generally drawn. 



For the destruction of germs a number of methods are available, 

 and may be classified into two principal groups : the one physical 

 and the other chemical. The former may be subdivided into 

 germ-killing by warmth, electricity, light, mechanical concussion, 

 or, finally, by gas under high pressure. We will confine our- 

 selves to the first of these five methods, the employment of the 

 remaining four being, for the purposes of the mycologist in general 

 and of the fermentation physiologist in particular, either too costly 

 or too cumbersome. In so far, however, as their influence is of 

 general biological interest, we have already reviewed them in the 

 preceding section. 



On the other hand, sterilisation by heat is the method always 

 resorted to, unless found undesirable on other grounds. Before 

 giving it more detailed consideration, we must first ascertain which 

 group of organisms exhibits the greatest tenacity of life and is 



