120 ESSAYS ON BACTERIOLOGY. 



The sterilization of culture materials and apparatus 

 is accomplislied by heat, either dry or moist, accord- 

 ing to the character of the substance to be sterilized. 

 For this purpose two pieces of apparatus have come 

 into use: the steam sterilizer, similar to an ordinary 

 steam cooker, and the dry sterilizer, a simple portable 

 sheet-iron oven. The culture mediujn is placed in 

 sterilized vessels, usually ordinary chemists' glass test 

 tubes, the mouths of which are tightly jGlUed with 

 cotton, the latter having been previously subjected to 

 heat in the oven. Access of air is thus allowed, while 

 bacteria are effectually excluded. Open vessels, 

 which cannot be plugged with cotton, are placed un- 

 der a belljar inverted over a plate of glass, the whole 

 constituting what is called a moist chamber, in which 

 the growing cultures may be inspected. Other ves- 

 sels of special construction are used for special pur- 

 poses, but the test-tube method of culture is the one 

 followed in all ordinary work. If the tubes are to be 

 kept for some time either in the open air or in the in- 

 cubator, drying of their contents is prevented by rub- 

 ber caps placed over the cotton plug. The whole 

 process is indeed similar to that of cooking, sealing^ 

 and thus preserving frilit. For the cultivation of 

 bacteria at the body temperature an incubating or 

 breeding oven is used, similar to, though more ac- 

 curately constructed than, the ordinary egg hatcher. 

 This apparatus is a double-walled, felt-jacketed oven, 

 the space between the walls being filled with water to 



