TEXT-BOOK OF BACTERIOLOGY. 67 



of bacteria is of little importance. For if one of these substances 

 be added to a culture medium, it (the solution) will indeed become 

 germ-free, but as it is impossible to remove the germicide again, the 

 culture soil becomes permanently unsuited for bacterium culture; 

 besides all of which, the further changes which the action of the 

 chemical agent may produce in the solution must be considered. 

 The effects of these disinfecting substances are such that neither 

 the vessels in which the culture media are held nor the implements 

 with which we inoculate bacteria can be brought into contact with 

 them without incurring the danger of failure in the attempted 

 breeding experiment. 



It is necessarj', then, to adopt some plan of destroying bacterial 

 germs without so affecting the substances containing them as to 

 make them valueless for further use : the culture media must be 

 germ-free, but not barren — i.e., they must retain their nourishing 

 qualities. 



The only means which can fulfil the requirements is heat in its 

 various forms. Even the spores cannot resist the continued action 

 of high temperatures, and the modern process of sterilization is 

 based on the utilization of this fact. 



Dry heat may be employed. Place the object which it is desired 

 to sterilize in a flame, and in a short time everj' trace of organic 

 life will be destroyed. 



Of course this process is only applicable now and then, and solely 

 in the case of incombustible matters. The platinum wire, for in- 

 stance, with which the bacteria are transferred must in each and 

 every case be heated in the flame of the Bunsen burner or spirit- 

 lamp before being used. 



For the other metal implements — inoculating needles, knives, 

 scissors, etc. — a direct heating in the flame will also be found the 

 quickest and simplest way of insuring thorough sterilization. It is 

 by no means necessary to make the instruments red-hot; if they 

 are moved backward and forward perhaps for a minute over the 

 Bunsen burner, it is all that is necessary. Even such a heating de- 

 stroys the temper and sharpness of the blades quickly enough, but 

 this disadvantage is submitted to willingly for the certainty with 

 which sterilization is accomplished. 



Wherever, for any particular reason, the direct application of 

 the flame is impossible, whether the objects are too large, too 

 numerous, or too combustible, or whether they are in some other 

 way difflcult to manage, we have special apparatus for the appli- 

 cation of dry heat which insure the equable and regular action of 

 high temperatures. 



