On determining the Number of Micro-organisms in Air. 455 



"A new Method of determining the Number of Micro- 

 organisms in Air." By THOMAS Caenelley, D.Sc., Pro- 

 fessor of Chemistry, and Thos. Wilson, University College, 

 Dundee. Communicated by Sir Henry Roscoe, F.R.S. 

 Received February 3 — Read February 16, 1888. 



The subject of bacteriology has of late excited considerable interest, 

 and is at present studied by a great number of investigators, both in 

 this country and on the Continent. Under these circumstances a new 

 and improved method for the bacterioscopic analysis of air will be of 

 nterest. 



There are several methods at present in use for this purpose, but 

 it will only be necessary to refer to two of these, in both of which 

 solid media are employed. 



1. Hesse's Method (' Mittheilungen aus dem Kaiserlichen Gesund- 

 heitsamte,' vol. 2, p. 182). — This is the oldest process in which a 

 solid medium is used for the nutrition of the micro-organisms, and is 

 the one which has been most commonly employed. The principle of 

 the process consists in drawing a known volume of air through a long 

 wide tube, the inside of which is coated with Koch's nutrient gela- 

 tine-peptone. As the air passes through the tube the micro-organisms 

 settle on the jelly, and in the course of a few days develop into 

 zooglea or colonies, and thus become visible to the naked eye and may 

 be counted. 



2. Br. Percy FranHand's Method (' Roy. Soc. Proc.,' vol. 41, p. 443 ; 

 'Phil. Trans.,' B, vol. 178 (1887), p. 113).— This method consists 

 essentially in aspirating a known volume of air through a small glass 

 tube containing two sterile plugs consisting either of glass-wool alone 

 or of glass-wool coated with sugar. After a given volume of air has 

 been aspirated the two plugs are transferred respectively to two flasks 

 each containing melted sterile gelatine-peptone and plugged with 

 sterile cotton-wool stoppers. The plug is carefully agitated with the 

 jelly so as to avoid any formation of froth, and when the plug has been 

 completely disintegrated and mixed with the gelatine the latter is 

 congealed so as to form an even film over the inner surface of the 

 flask. On incubating these flasks at a temperature of 20° C, the 

 colonies soon begin to appear and may be counted. 



New Method. — The new process which forms the subject of the 

 present communication is a modification of Hesse's method, in which 

 a flask is substituted for a tube. 



The flask employed is conical in form and has a capacity of about 

 half a litre. The flask is fitted with a two-holed india-rubber stopper. 

 Through one hole passes the " entrance tube " AA. This is a piece 



vol. xliv. 2 M 



