THE MICRO-ORGANISMS PRESENT IN THE ATMOSPHERE. 
141 
My experience is entirely in accord with Hesse’s observations on this point. It is 
obvious, however, that any individual colony in Hesse’s tubes may result either from 
the deposition of a single organism or from the deposition of an aggregate of organisms 
of one and the same kind. Now, in the new method described above, any aggregates 
of this kind would, in the process of agitating the plug with the gelatine, become 
broken up, and a number of colonies would result from that which, in Hesse’s method, 
would yield a single colony only. In comparing the two methods, there was, therefore, 
no d priori reason for anticipating that the results obtained should bear any similarity 
to each other, and, since there is nothing to render the existence of such aggregates 
improbable, it would not have been at all surprising to find the number of colonies 
yielded by the flask-method in excess, and even very largely in excess, of the 
number obtained by Hesse’s apparatus from the same volume of the same air. 
A number of experiments were carried out with this double object in view, the 
results of which are recorded below. 
I. Roof of Science Schools. September 24, 1886. Wind N.W., very gentle, but 
variable in strength and direction. No recent rain. Roads watered, pavement 
dry. 12 c- 8 C. 
No. 1 . Time. —3.1 to 3.20 p.m. 
Flask-metliod. —(Single plug.) 
Volume of air =12 litres. 
Construction of plug. —Sugared glass-wool and sugar-powder. 
Rate. —2’2 strokes per minute. (Tube exposed 19 minutes.) 
Colonies in flask, 37 j 1 ^ J^ oulds - 
Control tube, containing plug of sugared glass-wool and sugar-powder, 
exposed the whole time occupied by No. 1. 
Colonies in flask, 0. 
No. 2. Hesse’s method. 
Time. —3.30 p.m. to 4.20 p.m. 
Volume of air =12 litres. 
21 moulds. 
2 liq. 
7 moulds. 
.1 liq. 
Tube contained 71 colonies 
Control-tube contained 36 colonies 
II. Roof of Science Schools. September 25, 1886. Wind S.W., very gentle. No 
recent rain, roads watered, pavement dry, 12 0, 8 C. 
No. 1 . Time. —5.35 p.m. to 6.0 p.m. Flask method (single plug). 
Volume of air =18 litres. 
Construction of plug. —Sugared glass-wool and sugar-powder. 
