ON THE CARBONIC ACID, ORGANIC MATTER, ETC., IN AIR. 
95 
Micro-organisms per litre. 
Before lecture. 
After lecture. 
Ventilation on 
4 
off* . . 
, . 
6 
„ on . . 
. . 
3 
,, off* . 
2-5 
1-5 
„ on . . 
3 
L5 
Large Lecture-room 
Small Lecture-room 
The observations at the end of the lectures were made after the audience had been 
in for an hour, and before they left. The fact that the numbers found were so small 
after the lecture shows that the influence of the bodies and clothing of persons of 
average cleanliness present in a room upon the number of organisms in the air is at 
least small as compared with the influence of other conditions. 
As no micro-organisms come from the breath, those which come from persons present 
in a room must arise from their clothes and skin. Hence, if we take the carbonic 
acid as a rough measure of the total impurities arising from the persons of those 
present in a room, it should be a rough measure of the micro-organisms from the 
clothes and skin. The increase of carbonic acid does not, however, as we have 
seen, run parallel with increase of micro-organisms, and this supports our previous 
conclusion that the number of micro-organisms given off by the skin and clothes 
of persons actually present in a room is small as compared with those coming from 
other sources. 
3. As the micro-organisms in the air of a room do not come to any large extent 
from the persons present at the time, they must come from the room itself. The 
circumstances in connexion with the room which are of most importance in influencing 
their number may be now considered. 
A. Cubic space .—The influence of cubic space in the naturally ventilated schools 
which may fairly be compared with one another is traced in the Table (p. 82). It will 
be seen that the organisms increase with increase of cubic space up to 250 cubic feet per 
child, when there is no further increase. The marked diminution above 300 cubic feet, 
shown in the Table, depends on the observations made in a private school which was 
scrupulously clean. The results are therefore not comparable with those obtained in 
Board schools. The diminution with cubic space below 200 cubic feet will recall the 
similar diminution with increase of carbonic acid above a certain point. Possibly 
filtration through the air-passages may be again the explanation here. This is a 
point which we intend to investigate further. 
In the case of dwelling-rooms, pp. 71-72, the micro-organisms decrease as the cubic 
space increases, but this must be largely due to the fact that other sanitary conditions 
improve as the cubic space increases. Above a thousand cubic feet there is a slight 
* Only off during the lecture. 
