444 
Proceedings of the Royal Society 
of electrification were studied by insulating and charging the hot 
surface. The effect was found to he the opposite of the heat effect. 
If the potential is slight and the temperature high, the heat is able 
to keep the dust off the surface of the body, and the dark plane 
distinct ; but if the temperature falls, or the potential is increased, a 
point is reached when the electrical attraction overcomes the heat 
effect, and the dust particles break in upon and destroy the dark 
space. 
It was observed that after tlie dust particles were electrified they 
tended to deposit themselves on any surface near them, and experi- 
ments were made to determine the best conditions for purifying air 
in this manner. It was found to be best done by causing as rapid 
a discharge of electricity as possible, by means of points, surfaces 
being placed near them to increase the electrification of the dust, 
and to augment the rate of the currents of air which were driven 
from the points. These surfaces became places on which the dust 
deposited itself before losing its charge. A large flask was found to be 
rapidly cleared of a cloud of dust by means of a point, the dust being 
almost entirely deposited on the inside surface of the flask. If the 
end of the conductor in the flask terminated in a sphere, but little 
effect was produced. Electricity has also been found capable of 
depositing the very fine dust of the atmosphere. The air in a large 
flask was purified much more quickly by means of the electric 
discharge than it could have been by means of an air-pump and 
cotton-wool filter. 
It is shown that a wet and hot surface repels dust much more 
powerfully than a hot dry one. From this it is concluded that 
the heat and moisture in our lungs exert a protecting influence on 
the surfaces of the bronchial tubes, and tend to keep the dust in 
the air, which is ebbing and flowing through them, from coming 
into contact with their surfaces. This was illustrated by placing a 
hot and wet surface in a current of dense smoke, where it remained 
some time without receiving a speck of soot, while a similar surface, 
but cold, was blackened with the smoke. It is pointed out, that 
on account of the irregularities on the surface of the tubes and 
of the more violent movements of the air in the lungs, and on 
accou ' 1of curves and projecting edges, the protection in the lungs 
