1888 - 89 .] Mr John Aitken on Dust Particles. 
165 
from the two side lights get collected in the upper space. From 
the tests it would appear that this hot polluted air had remained all 
night, as on testing next morning it had lost only about one half of 
its particles. 
By way of illustration, I have made some tests of the air in an 
ordinary room while gas was burning, but where there are no venti- 
lators over the gas. As will be seen from the next numbers in the 
table, the contrast is very marked, and show that the ventilators in 
the Meeting-Boom of the Boyal Society do their work efficiently. 
The results of this series of tests are given in the table, and show in 
a marked way the effect of burning gas on the purity of the air of 
an ordinary room such as we are accustomed to live in. These tests 
were made here in a room 24 feet long by 17 broad and 13 high. 
There is a fireplace, in which a good fire was burning during these 
experiments ; the room has two windows and two doors, and the 
polluting effect of four jets of gas was tested. 
The air outside was tested at one o’clock, and also at three o’clock, 
when the tests began, and the numbers are entered in the table. 
Before the gas was lighted, the air in the room was tested at a height 
of 4 feet from the floor, and near the ceiling, and was found to have 
very nearly the same number of particles as the outside air, as will 
be seen from the figures in the table. The air near the ceiling 
seemed to have slightly less than that near the floor. This point I 
checked twice, and always found the higher air the purer. The 
lower number in the air from near the ceiling would partly be due 
to the air losing some of its particles in the pipe by which it was 
drawn down to the apparatus. 
After these tests were made the gas was lighted at 3 p.m. The 
rapidity with which the products spread through the room was very 
remarkable. By the time I had lighted all the jets and turned to the 
apparatus, the number in the air from near the ceiling had increased 
to many times its original amount. Lower down, also, the number 
rose very rapidly. After an hour the air was again tested, and found 
to be greatly polluted by the products of combustion, as will be seen 
from the numbers given in the table taken at 4 p.m. The air was 
again tested at 5 p.m., and, as will be seen, the impurity in the air near 
the ceiling had again greatly increased. From the table it might 
be thought that the air at 4 feet from the floor, when tested at 
