TEXT-BOOK OF BACTERIOLOGY. 71 



spores will have become bacilli and that all the bacilli will have 

 been killed. Experience has shown that it is best to warm the 

 solutions to 56 or 58 C. every day for about four or five hours 

 for a week. The result is usually satisfactory. Of course this 

 method can only be used for culture fluids in which the spores 

 would in any case proceed to germination. 



These are the general principles of sterilization, and are to be 

 applied in special cases. 



We heat and thereby securely sterilize 



First. All glass and metal instruments which do not suffer from 

 the prolonged effects of high temperature for about half or three- 

 quarters of an hour in the hot-air oven at 150 C. 



Second. All culture media and similar substances for about half 

 or three-quarters of an hour at 100 C., by means of a current of 

 steam in the generator. 



Third. All substances containing albumin which do not sup- 

 port 100 C., three or four hours daily for a week at 56 or 58 C. 



When the culture solutions have been sterilized and thus the 

 first foundation laid for a pure culture, endeavor as far as possible 

 to protect them from all foreign impurities. This may be done by 

 protecting all the vessels containing the culture media from the 

 germs of foreign micro-organisms. 



A good plug of cotton-wool has proved the best and simplest 

 safeguard. The cotton-wool is a good germ filter without any 

 special preparation, which prevents almost to a certainty invasion 

 on the part of micro-organisms. 



Only the mould fungi can get through a plug of cotton-wool. 

 These disagreeable visitors are found particularly often when one 

 has put an India-rubber cap over the cotton plug to prevent evap- 

 oration of the fluid in the culture vessel. Then a sort of damp 

 chamber is formed under this cover, and any mould spores that 

 may have lain on the surface of the plug begin to germinate, send 

 their mycelia through the close complex of wool fibres, and after a 

 short time mould fibres may be seen on the under side of the plug. 

 The best protection against such uninvited guests is either to care- 

 fully drop a little solution of corrosive sublimate on the plug or 

 burn off the top end of the plug before applying the cap. 



Generally speaking, however, the simple plug of cotton-wool is 

 quite sufficient. The test-tubes, Erlenmeyer's flasks, and larger 

 glasses are provided with such plugs in advance, and they are 

 sterilized together with the glasses in the dry heat. The slight 

 scorching of the wool also serves as evidence of the sufficient steril- 

 ization of the objects in question. In addition to these precautions, 



