222 Proceedings of Royal Society of Edinburgh. [sess. 
than the air in the room the dust was deposited upon it. If it was 
desired to collect the dust in the outer air, the plate of glass was 
attached to the outside of the window pane, hut kept at a distance 
of two or three millimetres from it. It was held in its place by the 
india-rubber solution, four pieces of sheet india-rubber being fixed at 
the comers to keep the plate away from the window glass. By this 
arrangement the outside air circulated in the space between the 
outside plate and the window, and as this air got heated on the 
window pane it was warmer than the plate and deposited its dust 
on its cold surface. It may be mentioned that the plates used for 
collecting the dust were small pieces of glass mirror about 10 cm. 
square. Mirrors were used, as the condensed vapour is more easily 
detected on them than on clear glass. 
Having collected the dust in either of these ways, its condensing 
power was tested in the following manner. A small cell was pre- 
pared about 4 cm. square and 1 '5 cm. deep, and open at the top. 
The top edge of the cell was covered with a thickness of india-rubber. 
The dusty plate was placed so as to form a cover to the cell, being 
held down by means of spring catches ; the india-rubber enabled the 
plate to make a water-tight joint with the cell. The cell was pro- 
vided with two pipes, one for taking in water and the other for 
taking away the overflow. A thermometer was fixed with its bulb 
occupying the centre of the cell; and further the cell was provided 
as a Dines’ hygrometer. The dusty plate, before it was put in its 
position, had the dust carefully cleaned off one half of it, so that 
when put in its place one half of the glass covering the cell was 
dusty and the other half clean. Cold water was then run through 
the cell and the temperature of the plate gradually lowered, the 
plate meanwhile being closely watched to see when condensation 
began on the different halves, and the temperature noted when it 
began on the dusty half and when it began on the clean part, and 
the difference, if any, noted. In this way a measure of the con- 
densing power of the dust was obtained, as the difference between 
these two temperatures gives the temperature above the dew-point 
at which the dust condensed vapour. 
It will be noticed that the plates for collecting the dust were 
much larger than the cell. By this means the cooled surface over 
