100 
PROF. T. CARNELLEY, MR. J. S. HALDANE, AND DR, A. M. ANDERSON 
Bacteria 
The explanation of the ratio increasing with the vitiation of the air is 
Moulds 
that moulds come mostly from the outside air. When the air in a room becomes 
vitiated the bacteria increase largely, while the number of moulds is affected to a 
relatively much less extent, if at all. 
This is well illustrated by some observations made in the half-time school already 
referred to. The unoccupied room was first well ventilated by means of the fans, and 
the carbonic acid, micro-organisms, and organic matter determined. The children 
were then admitted, and the determination made at the end of half an hour, and 
again at the end of an hour. The results were as follows :— 
Bovs’ School f Beginning of Il0Ur 
noys ocnooi, I MiddL 
1.ret day ^ End ” 
{ Beginning of hour 
Middle ,, 
End _ „ 
f Beginning of hour 
Girls’ School Middle ,, 
[End 
Carbonic Acid. 
Organic matter. 
Bacteria. 
Moulds. 
4-0 
5-1 
4 
2 
11-4 
6-4 
Not deter 
mined 
111 
73 
56 
2 
3-9 
2-8 
0 
3 
17-1 
4-4 
Not deter mined 
15-1 
2-8 
14 
0 
5-0 
4-0 
0 
3 
12-3 
6-2 
Not determined 
11-4 
3-9 
8 
4 
Thus in three experiments the number of bacteria was much greater at the end of 
the hour than at the beginning, whereas the number of moulds had remained prac¬ 
tically constant. 
The effect of stirring up dust is to increase the ratio. The bacteria are increased, 
while the moulds are little affected. Thus, in the High School the micro-organisms were 
determined just before and just after the boys had raised a cloud of dust by stamping 
on the floor. Before stamping, 10 bacteria and 1 mould were found ; after stamping, 
150 bacteria and 0 moulds. Again, in one of the wards in the Infirmary, where the 
beds had just been made, the ratio was — x -, while the next highest ratio observed in 
any of the other wards was f. No dust had been raised in any of the latter wards, 
except what was due to ordinary movements about the wards. 
Relative lightness of moulds and- bacteria .—The ratio of bacteria to moulds is 
considerably affected if the air remains quiet for any length of time, as the bacteria 
(or rather the particles to which they are attached), as a rule, settle out much more 
rapidly than moulds. In fact, the moulds settle out so slowly that we have never 
noticed the effects of their subsidence in the course of our observations. 
The relative lightness of moulds as compared with the particles to which bacteria are 
attached has already been observed by Hesse (‘Mitth. a. d. Iv. Gesundheitsamte/vol. 2, 
p. 186), who found that moulds, as a rule, penetrate much further into the tubes 
before settling down on the jelly than bacteria do. He gives many measurements of 
