THE MICROBES OF THE AIR 269 



containing two small porous plugs placed one in 

 front of the other. The first plug consists of glass- 

 wool coated with sugar, whilst the second contains, 

 in addition, a layer, inch in thickness, of fine 

 sugar-powder. The microbes, suspended in the 

 aspirated air, are deposited on these plugs, which are 

 introduced into separate flasks, each containing a 

 a small quantity of melted nutrient gelatine. Each 

 flask is then agitated until the plug is disintegrated, 

 and since the sugar-coating of the glass-wool dis- 

 solves in the liquid gelatine, the microbes become 

 immediately detached. The gelatine is now allowed 

 to solidify, forming a thin film over the inner sur- 

 faces of the flasks. The flasks are finally placed in 

 an incubator ; and in a few days colonies derived 

 from the microbes, which were collected by the 

 plugs make their appearance and can be counted 

 and further studied (Fig. 54). 



The author 1 has examined the air of Lincoln, 

 Paris, and London. The methods used for estimating 

 the number of microbian colonies in a known volume 

 of air were those of Hesse and Frankland. Before 

 August 6th, 1888, Hesse's method was used, while 

 after that date Frankland's method was substituted 

 for that of Hesse. The average number of colonies 

 in three gallons (fifteen litres) of air are given in 

 the following tables : 



1 Proceedings of Royal Society of Edinburgh, vol. xvii. 

 p. 265 ; and Researches on Micro-Organisms, p. 59. 



