THE MICROBES OF THE AIR 267 



water is always fixed uppermost when the air is 

 passing through the tube. The down-flow of water 

 causes the air to pass slowly through the tube when 

 the outer cap (C) has been removed ; and as the 

 flasks are of known capacity, two, five, ten, or more 

 litres of air may be aspirated through the tube. 

 After this the cap is replaced, and the tube is then 

 removed to a warm situation for several days, in 

 order that colonies may develop. 



Before introducing the nutrient gelatine, the 

 tube, caps, and plug are sterilised by means of a 

 solution of mercuric chloride, and finally with 



FIG. 53. GRIFFITHS' MODIFICATION OF HESSE'S AEROSCOPE. 



alcohol. After this treatment, 50 cc. of melted 

 nutrient gelatine are poured into the tube, which is 

 then sterilised in a steamer by the discontinuous 

 method. 



The author has made a modification of Hesse's 

 apparatus (Fig. 53), by substituting a small exhaust 

 pump of known capacity for the aspirator. This 

 modification is far handier and occupies less space 

 than Hesse's aeroscope ; while it gives results which 

 agree with those obtained with the original apparatus. 

 The late Dr. T. Carnelley 1 also modified Hesse's 



1 Report of British Association, 1887, p. 654. 



