AIR, SOIL, AND WATER. 



147 



one having a hole in its centre 10 millimetres in diameter ; and 

 at the end B a rubber cork fits in the cylinder ; through this 

 cork a glass tube 10 mm. in diameter passes, which is plugged 

 at both ends with cotton. The cylinder and fittings are first 

 washed in alcohol and sublimate and then placed for one hour 

 in the steam chamber. 



Removing the cork of the cylinder, 50 cubic centimetres of 

 sterile gelatine in a fluid condition are introduced and rolled 

 out on the sides of the tube, after the manner of Esmarch, 

 leaving a somewhat thicker coating along the under side of the 



FIG. 76. 



cylinder. The eeroscqpe, as the cylinder and its fittings are 

 called, is placed 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 



