164 Proceedings of Royal Society of Edinburgh. [sess. 
slight difference in the direction of the wind, but more probably to 
the dampness of the air loading the particles, and preventing them 
from rising so quickly on the 16th. The relative numbers for Edin- 
burgh and Glasgow cannot be estimated from the figures in the table. 
The day when a large number was counted in Edinburgh was dull, 
with little wind; whereas all the Glasgow observations were made 
when there was more wind and the air clear. In a still damp day, 
or a foggy one, the number for Glasgow will probably be many 
times greater than any yet obtained. 
The next set of observations entered in the table refer to some 
tests made in the Meeting-Room of the Royal Society on the evening 
this paper was read. The first observations were taken shortly before 
the meeting began at 8 p.m. The air was tested at a height of 
about 4 feet from the floor, and also near the ceiling, the air being 
drawn down through a pipe by means of the gasometer. Near the 
floor the air before the meeting began had 275,000 particles per c.c., 
or very little above that of the air outside on that day. Near the 
ceiling the number was 3,000,000. After an hour and three 
quarters the numbers were found to have increased to 400,000 
near the floor, and 3,500,000 near the ceiling. These tests were 
checked the following day, and the numbers are entered in the table. 
It will be seen that the results were somewhat similar. The increase 
in the numbers was very much the same, though in the latter case the 
gas had been burning a little longer than in the former. It is, of 
course, the increase in number, and not the numbers themselves, that 
have to be noted here. 
It may be as well to state that the Meeting-Room of the Royal 
Society is lighted by means of two sets of open lights, and each set 
is provided with a ventilator immediately over it. This accounts for 
the small increase in the number of particles while the gas was burn- 
ing. Most of the increase which took place was probably due to two 
side lights which have no ventilators over them. It will be noticed 
that there was a large number of particles in the air near the ceiling 
of the Meeting-Room, not only while the gas was burning, but also 
on the morning after the meeting. The cause of these large 
numbers is greatly due to the ventilators projecting downwards to 
some distance from the ceiling ; the air near the ceiling does not 
therefore get the benefit of the circulation, and the particles rising 
