THE MICRO-ORGANISMS PRESENT IN THE ATMOSPHERE. 
119 
Experiments in Buildings. 
I. Natural History Museum. June 14th, 1886 (Whit-Monday). 
10 litres of air yielded. 280 colonies. 
Control-tube. 49 „ 
II. Ditto. Same day. 
10 litres of air yielded. 267 colonies. 
Control-tube. 89 ,, 
III. Chemical Laboratory, Science Schools, South Kensington. October 15th, 1886. 
10 litres of air yielded. 80 colonies. 
Control-tube. 5 
IY. Ditto. October 16th, 1886. 
9 litres of air yielded . 13 colonies. 
Control-tube. 2 ,, 
Y. Ditto. October 27th, 1886. 
10 litres of air yielded. 32 colonies. 
Control-tube. 2 „ 
N.B.—The room must have been very free from aerial currents, as 
windows and door were closed and nobody was moving about. 
The two series of experiments recorded above show that the number of organisms 
gaining access to Hesse’s tubes, irrespectively of aspiration, is greater in out-door 
than in experiments within doors, this difference being obviously due to the more 
disturbed state of the external air. Thus, in the 21 experiments made in the open 
ah% the control-tube contained, on an average, 0’36 of the number of colonies found 
in the tube through which air was aspirated, whilst in the experiments made in-doors 
the proportion so found did not amount to more than 0T6. This introduces a point 
of great difficulty in the interpretation of the results obtained with Hesse’s apparatus, 
for it is obvious that the number of colonies obtained in the tube through which air 
is drawn is in excess of the number of organisms suspended in the air actually 
aspirated ; whilst, on the other hand, it is very probable that the number of colonies 
obtained in the control-tube is greater than the number of excess-organisms in the 
tube through which air is drawn. Thus, in the first open-air experiment recorded 
above, the number 158 is obviously too high, whilst 158 — 54 is probably too low, the 
truth lying somewhere between the two; but to this point I shall return later on. 
New Process. 
Already in the winter of 1885 I made some preliminary experiments on the 
bacterioscopic examination of air by aspirating a known volume through sterile glass 
