The appearances may
also be examined by looking along a cylinder _toward_ the source of
light, when the dust-free spaces will appear brighter than the rest. A
rod of electric light carbon warmed and fixed horizontally across a
bell-jar full of dense smoke is very suitable for this experiment, and
by means of a lens the dust-free regions may be thus projected on to a
screen. Diminished pressure makes the coat thicker. Increased pressure
makes it thinner. In hydrogen it is thicker, and in carbonic acid
thinner, than in air. We have also succeeded in observing it in
liquids--for instance, in water holding fine rouge in suspension, the
solid body being a metal steam tube. Quantitative determinations are
now in progress.
[Illustration: Fig. 1 and Fig. 2]
Fig. 1 shows the appearance when looking along a copper or carbon rod
laterally illuminated; the paths of the dust particles are roughly
indicated. Fig. 2 shows the coat on a semi-cylinder of sheet copper
with the concave side turned toward the light.
It is difficult to give the full explanation of the dust free spaces
in a few words, but we may say roughly that there is a molecular
bombardment from all warm surfaces by means of which small suspended
bodies get driven outward and kept away from the surface.
Pages:
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65