68 TEXT-BOOK OP BACTEEIOLOGT. 



They are double-walled cases of sheet iron, and are heated from 

 below by a powerful gas-burner. It may be seen by the thermom- 

 eter in the lid, after about ten minutes' warming, that the air inside 

 reaches a heat of about 150° C. and remains at that temperature. 



Experience and experiment have shown that spores even of 

 very high powers of resistance cannot outlirve an exposure of more 

 than half an hour in such a high teniperature. For half an hour 

 or more, then, put the larger glass and metal instruments, the ves- 

 sels for the culture media, pipettes, and syringes — in short, what- 

 ever withstands the action of a high temperature without damage 

 — into the dry box or hot-air oven. 



The nourishing liquids — the most important item to be consid- 

 ered in the entire chapter of sterilization — will not bear this treat- 

 ment. Fluids and substances which melt under the influence of 

 heat must not be exposed for any length of time to such a temper- 

 ature, since they would be affected very prejudicially, or even de- 

 stroyed by it. 



Fortunately, however, heat is able to operate much more quickly 

 and powerfully in fluids than in a dry state, as in the case of hot air. 

 Even spores which resist a temperature of 150° C. in the air for an 

 hour or more lose their vitality in a few minutes in boiling water. 



Advantage is taken of this energetic action of moist heat for 

 the sterilization of our nourishing media. Of course they can be 

 freed from any germs that they may contain by simply boiling; 

 yet certain precautionary measures are necessary in order that all 

 portions may be equally submitted to the boiling process. And 

 even then it has, in the case of numerous liquids, its decided disad- 

 vantages. Other methods were therefore resorted to. Either the 

 vessels were plunged into boiling water and kept there for a long 

 time, or steam at above 100° C. — under pressure therefore — was em- 

 ployed. 



The first-mentioned plan frequently proved inefficient, because it 

 is difficult to keep somewhat large vessels so protected from exte- 

 rior cooling — from loss of warmth in contact with the air — that the 

 desired and required temperature is kept up all the time and 

 throughout all parts of the liquid. The use of steam under pres- 

 sure has also its disadvantages. In the first place the apparatus 

 called an autoclave, intended for its application, is a very com- 

 plicated and expensive instrument, which requires particularly 

 careful treatment and attentive management, and, moreover, 

 the distribution of heat within it, as Koch has shown, is by no 

 means always equable. It is apt to have " dead corners " contain- 

 ing air that is not reached by the motionless steam, which en- 



