80 PRACTICAL BACTERIOLOGY 



sublimate solution 1 to 1000, and sterilize in the steam sterilizer 

 The cover-glasses and other material likely to contain infectious 

 material must also be further sterilized when of no further use. 



METHODS OF EXAMINING AIR, WATER, 

 AND EARTH. 



CXLVII. AIR ORDINARY METHOD. 



1. Liquefy 10 c.c. of sterile gelatine, pour on a sterile plate or in 

 a Petri-dish, and set aside to cool. 



2. Remove the cover of the dish or plate, leaving the gelatine 

 exposed for one hour. 



3. Replace the cover, set aside, and examine any future develop- 

 ment. 



Agar can be used in place of gelatine. This method yields only 

 qualitative results. 



CXLVIII. AIR HESSE'S METHOD. 



1. A sterilized glass cylinder, 70 c. long and 3 to 5 c. in diameter, 

 containing 50 c.c. of sterile gelatine adhering to the inner surfaces, 

 prepared according to the Roll Culture process (see 114, p. 61), with 

 one end of the cylinder closed by two elastic caps, the inner one 

 having a central orifice, and the other with a cork and glass tube 

 plugged with cotton wool. 



2. The prepared apparatus is fitted on a tripod. The cotton wool 

 plug is removed, and the small tube connected with an aspirator 

 capable of drawing air through the cylinder at a velocity of half a 

 litre per minute. 



3. Before the aspirator is finally adjusted, the outer rubber cap at 

 the opposite end of the cylinder is removed, when any germs passing 

 into the cylinder adhere to the gelatine. 



About 20 litres of air is the limit tested by this method. (For 

 illustration of apparatus, see Fig. 24, p. 81.) 



CXLIX. AIR PETBI'S METHOD. 



By this method a greater quantity of air can be tested than by 

 Hesse's method. 



1. Prepare a glass tube 9 c. long and 1*5 c. in diameter, containing 



