552 
ME. JOHN AITKEN ON THE FOEMATION OF 
The above j^rinciples seemed to offer the explanation of the dust figures, but as 
reasoning on physical phenomena should always be put to the test of observation 
when possible, I prepared apparatus to repeat Dr. Russell’s experiments, and made 
arrangements for seeing the directions of the air currents and the condition of the air 
at the under and upper surfaces of the plate. For these experiments it was found 
most convenient to use small plates on which to deposit the dust figures. Metal 
jjlates were used, as they were easily prepared, and they were coated with black 
varnish to show the figures. To prevent any obstruction that might result from the 
use of three ware supports to rest the plate on, only one wire was used fixed in the 
centre of the plate. The plates were about 2’5 centims. square and 2 millims. thick. 
A thick plate is l;est, as it keeps its heat longest and gives time for observations to be 
made under fairly constant conditions. 
The object of using small plates was that the observations could be made with a 
lens of greater magnifying power than was possible with large plates. An ordinary 
glass shade 12 centims. in diameter, made of thin glass, was used for confining the 
dusty air. A thin glass receiver has the advantage of being better made, the glass 
being of more even thickness than the thick glass ones, so enabling a more perfect 
image to be obtained by the lens. For illumination a narrow strip of incandescent 
gas mantle was hung over a Bunsen burner, exposing two thicknesses of the mantle to 
the flame. Ihe Bunsen burner was enclosed in a lantern, the glass condenser of 
which was removed and its place filled with a globular flask of water. This had to 
be adopted, as the heat coming from, and through, the glass lens interfered with the 
formation of the figures. The gas mantle and flask were mounted at the same level 
as the plate, and a small hand lens was used to further concentrate the light, and by 
means of it the light could also be directed to any part of the plate where illumination 
was desired. The air was examined by means of a hand lens of as high a magnifying 
poAver as possible. 
A\hen making observations on the air currents, it was found in some cases to be an 
advantage not to use much dust, only as thick as might be called a haze, because 
Avhen the dust is dense the beam of light illuminates by reflected light the AAdrole 
interior of the receiver and makes observation difficult, Avhereas Avith few dust particles 
the illumination is confined to the part under investigation. In some cases it was 
found hest to reduce the dust to sucli an amount that the indiA'idual particles of 
magnesia could be seen in the narroAv illuminated area. 
Turning noAv to the results of the obserActions made Avith this apparatus, the 
folloAving points may be noted :—Bringing the light to bear on the under surface of 
the hot plate it is seen that there is a dust-free space beloAv the plate, the dust in the 
film of air next the plate having fallen out and been repelled by the hot air aboA’e. 
This dustless film is seen to floAv horizontally along the under surface, turn sliarpl}- round 
the edges of the plate, and Aoaa^ horizontally over the top surface, no dusty air being- 
in contact Avith the plate at any point Avhen the jjlate is first put in and fairly AA'arni 
