56 
Mr Black adder on the Formation of Dew. 
stracted by the contiguous air; and this, by having its levity 
increased by an elevation of temperature, would necessarily re- 
cede upwards, thereby leaving the grass still in contact with air 
of a low temperature. Hence it follows, that, partly by the pro- 
cess of evaporation, and partly by the recession of the contigu- 
ous air, the depressed temperature of the grass may be main- 
tained, at a time when moisture in the form of visible particles 
is condensed on its surface. 
In considering the causes of that condensation of aqueous 
vapour which takes place in certain states of the atmosphere 
after sunset, our attention has hitherto been confined to what 
may be termed, for the sake of distinction, the Primary Forma- 
tion of Dew ; that is, a condensation of vapour produced by 
moist air, or vapour coming into contact with a solid body of a 
lower temperature, and at a time when the contiguous air is not 
otherwise in a state of saturation. And, in order to simplify 
the investigation as much as possible, the phenomena which ac- 
companv the formation of dew on a grassy surface, have hitherto 
almost exclusively occupied our attention. That which may be 
termed the Secondary Formation of Dew, and which is usually 
by far the most copious, remains to be investigated. By the 
secondary formation of dew is intended a deposition of aqueous 
particles from the air, from its capacity for moisture being dimi- 
nished by a depression of its temperature, and that not imme- 
diately produced by its coming into contact with a solid body of 
a lower temperature. 
On a serene evening, favourable to the formation of dew, the 
atmosphere is not only calm, but often clear, and free, or almost 
free of clouds ; and as long as this state of the air continues, we 
may conclude that it is not, at least in ordinary cases, in a state 
of complete saturation. But we have frequent opportunities of 
remarking, that though the whole atmosphere be obscured by a 
fog, so that objects cannot be distinctly seen at a short distance, 
and though it remain for days in succession thus loaded with 
moisture, still it is not in a state of saturation ; on the contrary, 
evaporation is even then so active as to produce a very consider- 
able depression of temperature in bodies having a wet surface. 
It is also certain, from observations made in almost all climates, 
that wherever the surface of the- earth is capable of supporting 
