124 Proceedings of Royal Society of Edinburgh. [mar. 7, 
was in a liquid state. It seems, therefore, quite in keeping with our 
knowledge that these fog particles in frosty weather may he liquid. 
Such being the case, we have water particles floating in the 
atmosphere during frosty and foggy weather, and the pressure of 
the vapour in the air will correspond to that of a liquid surface ; 
it will therefore be greater than that of an ice one at the same 
temperature. Under these conditions the air will rapidly unburden 
itself of part of its vapour when it comes into contact with an ice 
surface. This seems to be the reason why hoar-frost grows in the 
direction from which the air is moving, because the air, being super- 
saturated, unburdens itself on the first ice surface with which it 
comes into contact, and does not, as when dew is forming, require 
to be brought into a condition to cause it to give up its vapour. 
In the foregoing I have taken extreme conditions under which 
dew and hoar-frost are formed, as they are better suited to illustrate 
the point. There are, however, many intermediate conditions in 
which both dew and hoar-frost appear to be deposited in nearly the 
same way. On some nights the sheet of glass is dewed all over and 
up to all the edges, and there are some nights on which no hoar-frost 
is deposited on the windward edges of the plate, the air having to 
pass some distance over the cold surface before its temperature is 
low enough for it to deposit its moisture. The conditions under 
which the plate is dewed to its edges are when there is no wind 
and the air nearly saturated; and the conditions under which no 
hoar-frost is deposited along the windward edges are when there is 
some wind, a clear sky, and the air not saturated. 
So far as my memory and recorded observations go, we never 
have a heavy deposit of hoar-frost when the sky is clear, or in those 
conditions in which we have our heaviest deposits of dew. On all 
those occasions on which trees and every exposed surface become 
clothed in crystal garments, and all nature in a single night be- 
comes changed to a wondrously pure and fairy-like scene, the trans- 
formation seems always to be accomplished in a thick and foggy 
atmosphere, which requires the morning’s sun to dissolve the veil 
and disclose its beauties. The thick and foggy state seems to be 
the general condition of our atmosphere during the growth of these 
heavy deposits of hoar-frost, and it is a necessary one, if the ex- 
planation we have given is correct. 
