of Edinhurgli, Session 1883-84. 
447 
deposit itself on cold surfaces, the following experiments were made. 
Two mirrors, one hot the other cold, fixed face to face and at a 
distance of two or three millimetres from each other, were placed 
in a vessel filled with a dense cloud of magnesia, made by burning 
magnesium wire. After a short time the mirrors were taken out 
and examined. The hot one was quite clean, while the cold one 
was white with magnesia dust. In another experiment a cold 
metal rod was dipped into some hot magnesia powder ; when taken 
out it had a club-shaped mass of magnesia adhering to its end, 
while a hot rod attracted none. 
This tendency of dust to leave hot surfaces and attach itself to 
cold ones, explains a number of familiar things, among others it 
tells us why the walls and furniture of a stove-heated room are 
always dirtier than those of a fire-warmed one. In the one case 
the air is warmer than the surfaces, and in the other the sur- 
faces are warmer than the air. This effect of temperature is even 
necessary to explain why so much soot collects in a chimney. It 
explains something of the peculiar liquid-like movements of hot 
powders, and perhaps something of the spheroidal condition. 
For practical application, it is suggested that this effect of 
temperature might be made available in many chemical works for 
the condensation of fumes, and that it might also be used for 
trapping soot in chimneys. A small trap of this kind was shown. 
It consisted of a tall metal tube or chimney, surrounded by another 
tube slightly larger. The products of combustion are taken up the 
centre tube, and down the intervening space. The heat of the gases 
is thus made to do its own filtering. This apparatus being placed 
over a smoky lamp, it trapped out most of the soot, and deposited 
it in the inside of the outer tube. This arrangement of apparatus 
is too delicate and troublesome for general use, and it is suggested 
that as by simply cooling gases in presence of plenty of surface, 
much of its dust is deposited, it might be possible and advantageous 
under certain conditions to purify air by heating and cooling it a 
number of times, which could be done at a small expense by means 
of regenerators. 
Experiments were also made by discharging electricity into the 
smoke in a chimney. This also produced a marked diminution in 
