EXAMINATION OF AIR, SOIL, AND WATER 217 



upon an ordinary photographer's tripod and the glass tube, 

 which passes through the rubber cork, connected with an 

 aspirator, the cotton having first been removed from its outer 

 end. The aspirator consists of two ordinary wash-bottles 

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

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

 the upper one half filled with water and slightly tilted. 



Fig. 131 . Hesse's apparatus for collecting bacteria from the air (McFarland). 



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 gelatin 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. 



