270 MR JOHN AITKEN ON THE 



to settle on cold surfaces, is made in the following way : — Take two narrow 

 strips of glass mirror, any substance will do, but the mirror surface shows the 

 result best. Arrange so as to hold these strips of glass face to face, and with 

 their surfaces at a distance of a few millimetres, but before putting them in 

 their places, heat one of them to a temperature of say 100° C. Have ready 

 a tall glass vessel, large enough for the glass strips to enter freely. Now 

 fill this vessel with some dust, by burning sodium or magnesium, or by shaking 

 up some calcined magnesia or other powder. By the time the air in the vessel 

 is settled and cooled, but before the dust settles, have ready the glass strips, 

 one of them hot as directed, and placed in front of the other, face to face, 

 with an air space between. Now put the mirrors into the vessel among the 

 dust. After a minute or so examine them. The following will be the result. 

 The hot one will be quite clean, while the cold one will be white with dust. 

 That the dust has no tendency to settle on the cold one, may be proved by 

 putting at the same time in the vessel another cold strip some distance from the 

 hot one, when it will be seen that this one is almost entirely free from dust, 

 depending upon whether it was a little hotter or colder than the dusty air. 



When one looks at the enormous amount of dust deposited on the cold 

 mirror in this experiment, we cannot help associating the result in some way 

 with the condensation of vapours, and it takes some time before we can arrange 

 our ideas and realise that the thick white deposit was truly thrown out of 

 suspension and settled on the mirror in the solid state, and was not in the state 

 of vapour before coming into contact with the cold glass. 



A somewhat curious experiment may be made with light calcined magnesia 

 powder, which shows the action of this force in a marked way. The magnesia 

 is heated to a good red heat in an iron vessel. If we now take a metal 

 rod 5 or 10 mm. diameter and heat it as hot as the powder. We may then 

 dip in into the powder, and stir it as much as we please, but on taking the rod 

 out, it will be found to be quite clean. But if the rod is cold, it comes 

 out of the powder with a club-shaped mass of magnesia adhering to it, 

 so thick that the magnesia-coated end is twice as thick as the rod itself. 

 If the rod is kept in the hot powder for a short time, and then taken out, 

 with its coating of powder adhering to it, whenever the powder gets outside 

 the hot vessel, and exposed to the cold, it falls away, as the inside of the 

 powder is now hotter than the outside. 



Most of us have noticed when heating powders, particularly if they are 

 light, that while they are heating they take on a peculiar semi-fluid appear- 

 ance if stirred, or if the vessel is tilted back and forwards. This I have 

 always supposed was due to the escape of occluded gases from the powder, 

 keeping it in a state of semi-suspension. Now, however, I think this peculiar 

 effect is a result of the repulsion due to heating. My reason for supposing this 





