MICRO-ORGANISMS IN WATER 



FIG. 3. HOT-AIR STERILISER. 



commonly used for bacteriological purposes. A ther- 

 mometer is fitted into A, and by means of the perfo- 

 rated slide B, together with the 

 gas-regulator placed in C, the 

 temperature can be kept under 

 strict control. This is of course 

 of importance in the case of some 

 substances, such as milk, where 

 it is desired that a particular 

 temperature should not be ex- 

 ceeded. Test - tubes, pipettes, 

 glass plates, and other glass ves- 

 sels, and certain pieces of appa- 

 ratus as well as cotton-wool, are 

 all sterilised by means of dry 

 heat. Glass vessels should be 

 exposed to 150 9 C. for two or 

 more hours, and should be allowed to cool in the oven 

 to avoid the risk of their cracking by being too sud- 

 denly chilled. In practice it is a good plan to place 

 some loose cotton-wool in a beaker in the oven along* 

 with the glass vessels that are being sterilised, for 

 when the former becomes slightly browned it may be 

 taken as a sign that the sterilisation of the objects is 

 complete, and the gas may be turned out. 



It is particularly important to bear in mind that the 

 temperature is by no means uniform throughout such 

 ovens, and care must be taken that the objects are so 

 placed as to be really exposed to the desired tempera- 

 ture. 



(3) Sterilisation ly means of filtration. In order to 

 deprive a liquid which is not viscid of micro-organisms, 

 it may be made to pass through cylinders constructed 

 either of unglazed porcelain (Chamberland) or of baked 

 infusorial earth (Berkefeld). In the case of water, 



