104 EXAMINATION OF AIR AND WATER 



appearing. A simple calculation will give the number of organisms 

 per metre cube, the standard usually employed. Thus, if 25 litres 

 of air are passed through the apparatus and i c.c. of the bouillon 

 gives, say, 5 colonies, then 



25 litres of air contain 5 x 10 aerobic organisms 



and 



, r ■ , v. \ ^ . • 5 X lox 1000 .. , . 



I metre cube of air (1000 litres) contains - — ■ = 2000 aerobic 



25 



organisms per metre cube. 



As a control the glass wool plug should, in all cases, be carefully 

 withdrawn from the tube by means of a sterile needle or forceps, 

 and dropped into a tube of melted gelatine. From this an ordinary 

 plate cultivation should be made. If the filtration has been 

 effective, no growth whatever should appear upon the plate. The 

 withdrawal of the plug should be made through the end of the 

 tube which was attached to the aspirator, in order to avoid the 

 picking up of any chance organism which may have been left on 

 the tube walls on the removal of the salt, and the orifice and end 

 portion of the tube should be well heated in the flame and allowed 

 to cool before the withdrawal is proceeded with. 



Aspirating apparatus. — A convenient form is shown with the 

 filtering tube in position, on Plate 13. It consists of a portable 

 tripod stand with head, from the under side of which are sus- 

 pended 2 chains of about 8 inches in length, and furnished at 

 the ends with 2 large rings oi \\ inches in diameter. From 

 these latter are suspended, the one below the other, 2 vessels of 

 exactly 5 litres capacity, and furnished with chains terminating 

 with large S hooks as shown. The vessels are furnished at 

 the top with an opening of f of an inch in diameter, to which is 

 fitted a I -hole india-rubber stopper, and at the bottom with 

 straight-way taps of about \ of an inch bore. Through this stopper 

 a short length of tightly fitting glass tubing is run, and to the upper 

 extremity of this a piece of india-rubber tubing of some 2 or 3 feet 

 in length is attached. This tubing should lead through a hole in 

 the supporting plate of the tripod, upon the upper surface of which 

 is fixed a large spring pinchcock for the purpose of holding in 

 position the tube containing the filtering medium. When required 

 for use, the upper vessel is filled with water to its utmost capacity, 

 the india-rubber stopper pressed tightly home so as to ensure an 

 air-tight joint, and the free end of the tubing slipped on to the lower 

 extremity of the filtering tube, which is then fixed by the spring 

 pinchcock in a position approaching the vertical. The top of the 



