34 A MANUAL OF BACTERIOLOGY 



a series of holes of different sizes; by this means 

 the tubes or flasks are placed in a vertical posi- 

 tion in the steriliser. It may be stated that all 

 good hot-air sterilisers should allow the tubes to 

 be placed in a vertical rather than a horizontal 

 position. By this means the heated air rises in the 

 inverted tubes, flasks, etc., and the current so 

 formed (in each tube, etc.), destroys all the microbes 

 and spores present therein. 



The hot-air sterilisers of Koch, Muencke, Pasteur, 

 and Klein are all good sterilisers. Dr. Klein's con- 

 sists of an iron chamber with double wall and 

 double folding-doors. In the inner chamber are 

 placed the test-tubes in a horizontal position, and 

 the cotton-wool above them. After closing the doors 

 the steriliser is heated by a Fletcher's gas-burner. 



' Test-tubes (to be sterilised) should be exposed 

 to the full heat of the chamber for several hours. 

 After this they should be taken out of the steriliser 

 while hot, plugged with sterilised cotton-wool, and 

 then reheated for a few hours longer. Beakers and 

 glass funnels may also be sterilised in the hot-air 

 steriliser, or by being heated over a Bun sen flame. 

 To prepare sterilised cotton-wool, place the wool in 

 a loose condition, and heat it in the hot-air steri- 

 liser to a temperature of about 150C. for several 

 hours on several successive days.' 1 Over-heating 

 the cotton-wool in the hot-air steriliser to the above 

 temperature until singed has proved invariably and 

 absolutely safe for all cultivations. 



1 See Dr. A. B. Griffiths' book, Researches on Micro-Organisms, 

 p. 14(Bailliere&Co.). 



