of Ediiiburgh, Session 1883 - 84 . 
443 
By this arrangement a clear view was obtained all round the wire, 
and other advantages secured. Experimenting with this apparatus, 
it was found that different kinds of dusts had different sized dark 
planes. With magnesia and other indestructible dusts, it was very 
thin j with the sulphate dust, it was much thicker; and with the sal- 
ammoniac dust, thicker still. So thick was it with the two latter 
kinds of dust, that the dark planes over the two legs expanded and 
formed one plane. As the particles could be seen streaming into the 
dark space under the wires, it was obvious that these large dark 
planes were not caused by repulsion, but by the evaporation or by the 
disintegration of the dust particles. When making the experiment 
in a mixture of different kinds of dusts, the hot wire was surrounded 
by a series of zones of different brightness, and having sharp outlines. 
The size of the different zones was determined by the temperature 
necessary to evaporate the different kinds of dust present, and out- 
side these zones was another caused by the evaporation of the water 
from the particles. 
The conclusions arrived at from these experiments are, that the 
downward dark plane is produced by the separating action of gravi- 
tation, in the space under the cold body, and that the upward dark 
plane is produced — 1st, by the separating action of gravitation ; 
2nd, by the repulsion due to heat ; 3rd, by evaporation ; and 4th, 
by disintegration. 
The effect of centrifugal force is considered. It is pointed out 
that as the air in its passage over a body such as a tube, curves as 
much in one direction as it does in another, therefore any centrifugal 
effect produced in the one part will be reversed in the other. An 
experiment is described in which an air current is caused to curve 
through 180 degrees in its passage round the edge of a thin plate, 
and without any curving in the opposite direction, but no decided 
centrifugal action could be detected. 
The motions of the dust particles produced by the repulsion of the 
hot surface suggested that electricity might play some part in these 
phenomena. Experiments were made to test this : the hot body was 
insulated, and connected with an electroscope, but no electrical 
disturbance was observed, nor could any electrification be got from 
the dust and hot air streaming up fiom the hot wires. The effects 
