102 BACTERIOLOGY 



plates, or a roll-culture is made after Esmarch's method in 

 the vessel itself. 



Petri's Method. A sand-filter is prepared and carefully 

 sterilised. This consists of a tube 8 or 9 cm. long and 1-5 

 cm. in diameter, into which sand is introduced after one 

 end has been closed with wire netting. When the layer of 

 sand has reached a depth of 3 cm. another wire netting is 

 laid on it, [another layer of sand introduced, and a third 

 netting last of all], so that the tube is now provided with two 

 sand-filters kept together by wire gauze. Quartz-sand, each 

 grain of which is to ^ mm. in size, is the best. About 50 

 to 100 litres of air are drawn through with a water aspirator 



Sand-filter -=^^ IPS^Win- netting 



Aspirator tube 



Receiving bottle 

 I 



FIG. 33. PETRI'S SAND-FILTERING APPARATUS. 



at the rate of some 10 litres per minute, the quantity being 

 determined by means of a gas meter. When aspiration of 

 the air is concluded, each sand-filter is partitioned out 

 separately into several glass capsules prepared with nutrient 

 gelatine or some other solid culture medium. The second 

 layer of the filter should be free from germs. 



Miquel uses powdered sodium sulphate to absorb the 

 microbes instead of sand. 



Tyndall's method, &c. Sterilised cotton wool is used 

 for absorbing the micro-organisms, instead of air-filters con- 

 sisting of substances in the form of powder, and is then 

 transferred to gelatine, and plate cultures made therefrom. 



