224 Proceedings of Royal Society of Edinburgh. [sess. 
has become visible to the observer. It may, however, be mentioned 
that dust collected in a smoking-room showed a decidedly greater 
condensing power than that from the outer air. Of ten tests made 
with dust from the smoking-room, the dust that had the least con- 
densing power showed condensation at a temperature of 2° 2 above 
the dew-point, whilst the most powerfully condensing dust con- 
densed at 4° *5 above the dew-point, while the mean of the ten gave 
3° above the dew-point. Of the ten tests of the dust from the 
outer air, the lowest showed a condensing point of l°-8 above the 
dew-point, and the highest of 3 0, 2, and a mean of 2° ’3. It should 
he mentioned that the depth of the deposit did not seem to have 
any influence on the condensing power of the dust, as the amount 
of dust that collected in one day gave about the same effect as that 
collected in thirteen days. 
This condensing power of dust would seem to explain why it is 
that the glass in picture frames, and other places, frequently looks 
damp when the air is not saturated. The same damp deposit may 
he easily seen on windows during cold weather, particularly if they 
have not been cleaned for some days. The damp- looking deposit 
can be easily detected by cleaning a small part of the pane. The 
cleaned part will remain undewed, while the surface surrounding it 
will be damp, and be greasy when rubbed. I need not say that a 
certain degree of humidity in the air is necessary for seeing this 
clearly, as the glass must be cooled by the outer air to near the 
dew-point of the air in the room. Again, this condensing power of 
dust may in part explain the reason why it is so necessary to keep 
electrical apparatus free from dust, if we wish the insulation to 
he good. The damp collected hy the dust will decrease the insulat- 
ing power of the glass or other insulator. 
Dust and Condensation. 
We have seen that these observations all point to the conclusion 
that moisture is deposited on the particles of atmospheric dust in 
air which is not saturated. This condensation seems to take place 
while the air is comparatively dry, and the amount deposited in- 
creases with the humidity. The deposit which takes place under 
these conditions will probably be caused by the affinity — chemical 
or surface — of the particles for vapour, while true cloud condensation 
