of Edinburgh, Session 1880 - 81 . 
123 
been altered and improved. Instead of detecting the dnst by the 
cloudy condensation of steam, the saturated air of the receiver was 
cooled by slightly expanding it. Working in this way, it was 
found that the dust driven off by heat from a piece of iron wire, 
the 2W0 a grain gave such an evident and abundant result, 
that if the yyoIWo °f a g ra in of iron could be manipulated the 
effect would be perfectly definite and decided. This indicates an 
extremely small size of some of the condensation nuclei. Thousands 
of particles driven off the 2 1^0 0 a grain of iron, and the iron 
afterwards not perceptibly lighter, indicates almost molecular 
dimensions. It was pointed out that some of these nuclei may 
be driven off as gases, which afterwards condense and form nuclei 
for the vapour to condense upon. 
In the first paper attention was called to the composition of the 
atmospheric dust. It was pointed out that some kinds of dust 
would have a greater attraction for water vapour than other kinds, 
and that chloride of sodium dust would probably condense water 
vapour before the vapour was cooled to the saturated point. 
It was shown that there are two distinct ways in which dust 
acts as a centre of condensation, and causes vapour to condense 
before it is saturated. The first is the chemical affinity between 
the dust and the water vapour. The second is the condensing 
power possessed by the surfaces of some bodies. This power is 
different in different kinds of matter. 
Some experiments have lately been made to see to what extent 
this attraction of the nuclei for water vapour would cause conden- 
sation to take place in air which was not saturated. A little 
sulphur was lighted, and an open-mouthed glass receiver held over 
it for a few seconds, and then placed on the table. At first scarcely 
any effect was noticed, but after a time a haze or fog appeared. 
The density of this fogging depended on the humidity of the air 
experimented on. The damper the air the thicker the fogging. 
If the inside of the receiver was wetted so as to moisten the air, 
the sulphur products on entering are a little more evident, and on 
placing the receiver on the table a thin haze can be seen. After a 
time, however, this haze gradually grows denser, and at the end of 
fifteen or twenty minutes the receiver becomes full of a very dense 
white fog, which remains for a long time. A similar result is got 
