1886.] The Distribution of Micro-organisms in Air. 513 



uniformly restricted the rate to the 1 litre in three minutes, and can 

 fully confirm the fact of the remarkably complete deposition of, the 

 organisms in the front part of the tube, as well as of their being 

 almost uniformly found on the bottom. In conducting experiments in 

 the open air, I have made, with few exceptions, the invariable prac- 

 tice of directing the aperture of the tube at an angle of about 135° to 

 the direction of the wind, so as to avoid currents of air penetrating 

 into the tube irrespectively of the action of the aspirator. 



In commencing the experiment the outer unperf orated india-rubber 

 cap is removed and carefully folded up, so that its inner surface is not 

 exposed to the air, and as soon as the experiment is over it is carefully 

 replaced and wired on. The tube is kept in a chamber at 20 — 25° C. 

 for incubation in the position in which it was used, and in the course 

 of a few days the organisms which have been arrested by the gelatine 

 are readily distinguishable by the colonies, visible to the naked eye or 

 by means of a low power, to which they give rise. Unless the number 

 of organisms which have fallen on the gelatine is very great, each 

 colony will consist of a pure cultivation of a particular organism, and 

 can be further examined as desired. 



From the number of colonies found in a given volume of air the 

 number in any standard volume, say 100 litres, can be calculated. 



Examination of Air by Gelatine- surf ace Exposure. 



In addition to the tube experiments above described, I have con- 

 sidered it advisable to make simultaneous tests by exposing for a definite 

 period of time a surface of nutrient gelatine of definite area as originally 

 recommended by Koch (" Mittheilungen Kaiserl. Gesundheitsamte," 

 Berlin, 1881), as in this manner an idea is obtained, not of the 

 number of organisms contained in a given volume of air, but of what 

 for many purposes is more important, of the number falling on a given 

 surface in a definite time. 



For this purpose small circular glass dishes rather less than 1 inch 

 in height and about 3 inches in diameter, and provided with a glass 

 cover fitting loosely and overlapping like the lid of a pill-box, were 

 filled to a depth of about one-third of an inch with nutrient gelatine 

 and sterilised for fifteen minutes on three successive days in the 

 steamer. As long as the covers are on, the gelatine in these dishes 

 remains sterile for practically an indefinite length of time, and can be 

 transported without danger in a tin box. 



In using these for experiment, the lid is removed and placed with 

 its mouth downwards on a clean surface, and then after the desired 

 exposure replaced on the dish. 



VOL. XL. 



