ON THE CARBONIC ACID, ORGANIC MATTER, ETC., IN AIR. 
99 
We do not as yet possess sufficient data to enable us to account completely for the 
accumulation of micro-organisms in a room. It is possible that a room acts as a sort 
of trap for the particles to which bacteria or their spores are attached. This seems on 
the whole more likely than an actual multiplication of bacteria in the air or about the 
floor or furniture of a room. 
Relation of Bacteria to Moulds in various Kinds of Air. 
Up to this point we have generally considered the micro-organisms as a whole, and 
have said hut little as to the bacteria and moulds separately. It is now necessary to 
refer to these groups more particularly. 
In this connexion the most important point is the relative proportion of bacteria to 
moulds in various kinds of air. This is specially interesting, because it may furnish, 
taken in connexion with other considerations, a valuable indication of the vitiation of 
air by animal and other impurities. It must be distinctly understood that the relation 
is that observed with jelly of the composition stated previously, and rendered faintly 
alkaline after heating. 
In 167 out of 179 cases in which we observed the relation, the number of moulds is 
less than that of the bacteria. Of the exceptions, two were in the very abnormal case 
of the mill referred to above (p. 84). The rest were either in outside air or in very pure 
atmospheres (e.g., four in the Infirmary). The purer the air becomes, the more nearly, 
as a general rule, do the bacteria and moulds become equal. Thus, in outside ah’ in 
Dundee, taken in quiet open places, where there was but little traffic, there were 
only bacteria on an average to each mould, whereas in the open streets in the 
centre of the town, during dry dusty weather, the ratio was f 5 to 1. In buildings a 
much higher proportion usually prevails. 
The following Table shows the connexion of this ratio with the general state of 
purity of the ah’ in various classes of buildings :— 
Carbonic 
Acid. 
Organic 
matter. 
Total 
micro¬ 
organisms. 
Bacteria 
Moulds 
.Remarks. 
Outside air (quiet places). 
3-9 
89 
08 
2-5 
Winter. 
„ (streets) . 
31 
28 
17 5 
14-9 
April and May. 
Naturally ventilated schools :— 
Board schools. 
18-6 
162 
152' 
131-8 
Private school. 
119 
8'9 
9’ 
300 
Mechanically ventilated schools : — 
General average. 
12-3 
101 
16-58 
28-5 
* This number is probably too high, for 
Harris Academy (Board school) . 
12-8 
8-4 
160 
31- 
many of the determinations were 
Half-time School. 
10-8 
9-5 
28-0 
27- 
made during ordinary experimental 
University College. 
12-5 
12'9* 
2-8 
156 
lectures on chemistry, when reducing 
High Schoolf ....... 
13-0 
39 
36 
4- 
gases were possibly present. 
Houses:— 
One-roomed. 
112 
15-7 
60- 
49 
f The cubic space per person in this 
Two-roomed. 
9-9 
101 
46- 
20 
school is about three times as much 
Pour and more rooms. 
7-7 
45 
9- 
21 
as in any of the foregoing. 
o 2 
