ON THE CARBONIC ACID, ORGANIC MATTER, ETC., IN AIR. 
101 
their actual distance. Our own observations are in accordance with his on this point. 
He has also made a number of very interesting observations in this connexion on the 
relative ease with which moulds and bacteria penetrate fine pores. 
In consequence of the relative lightness of moulds, the ratio of bacteria to moulds 
tends to diminish when the air of a room remains at rest. We had a room kept closed 
for two days, after ventilation with outside air ; the ratio was then found to be 
•41 ... . . 2'5 
—= -, whereas in outside air the ratio about that time was —. 
The same thing is exhibited by the ratio of bacteria to moulds on still and windy 
days respectively, as proved by the following ratios, which are the mean results of all 
our available data. 
Still, damp days . 
Windy, damp days 
Still, dry days 
Windy, dry days . 
Bacteria ’36 
Moulds - 37 
55 
55 
5 5 
•63 
•5 
f7_ 
— -27 
10-6 
— -75 
= 1-3 
= 2T> 
= 14-1 
These results are all from observations in Dundee only. They show not only the 
effect of wind, but also that of dryness and dampness of weather. Other things being 
equal, there are thus fewer bacteria in the air on damp or still days than on dry or 
windy days. The moulds do not seem to be affected by wind or dryness to anything 
like the same extent. 
The relative lightness of moulds and bacteria is also shown by some observations 
made at the top and at the foot of the Clock Tower at Westminster. The results 
were as follows :— 
Bacteria 
T °P : Moulds' 
First observation. 
Second observation. 
ll 
O 
Bacteria 
Bottom: Mouldg - 
nr ' 40 
S =' 81 
At the time this experiment was made the number of moulds in the air was 
exceptionally large. 
Standards of Purity. 
It will be convenient, and, we trust, serviceable, to give at this place what we would 
propose as standards of purity. 
