148 



APPENDIX. 



FIG. 77. 



outer end. The aspirator consists of two ordinary wash-bottlrs 

 connected with each other by a rubber tube, C. They are at- 

 tached to the tripod with a small hook one above the other, the 

 upper one half filled with water and slightly tilted. 



When the apparatus is wanted, the outer rubber 

 cap at the end A of the aeroscope is removed, the 

 air can then pass through the small hole in the 

 other cap. and the germs fall upon the gelatine in 

 the tube, the cotton in the small glass tube at the 

 other end preventing the germs from getting out. 

 The aspirator is set in use by tilting the upper 

 bottle so that the water flows into the lower, this 

 creates suction and draws the air through the 

 aeroscope. 



The amount entering estimated by the capacity 

 of the wash-bottle. The rate at which it enters 

 depending upon the rate of the flow of water, 

 which can be regulated. 



Hesse advises for rooms and closed spaces 1 to 5 

 litres, at the rate of 2 minutes a litre, and for open 

 spaces, 10 to 20 litres at 4 minutes a litre. Plate 

 cultures can be made from the colonies which de- 

 velop in 8 to 10 days in the cylinder. 



Petri's Method. The air pumped or sucked 

 through sand filters, and the sand then mixed with 

 gelatine. 



Sand is sterilized by -heating to redness, and 

 while still warm placed in test tubes which are 

 Sand filter then plugged. (Sand which has been passed 

 through a sieve with meshes 0.25 millimetres wide 

 is the kind required.) A glass tube 9 centimetres long is pro- 

 vided with two portions of sand each 3 cm. long and cm. apart, 

 little plates of brass gauze keeping the portions in position. 



The tube and its contents now sterilized in hot air oven at 

 150 C., the ends having first been plugged with cotton. 



One end of the tube is then fitted with a rubber cork through 

 which passes a glass tube, which is connected with an aspirator 

 (a hand-pump with a known capacity). 



