59 
Mr Blackadder on the Formation of Dew. 
time when the quantity of moisture, united with the air, is very 
considerable. The soil under the grass is also more than usu- 
ally heated, and hence, after sunset, the air immediately in con- 
tact with it, and the vapour generated at its surface, are elevated 
to some distance above the ground, previous to their being so 
cooled as to produce a condensation in the form of a mist. It 
may be considered as a strong proof that this mist is condensed 
vapour rising immediately from the ground, that the space in- 
termediate between its lower surface and the earth is always ob- 
scured, more or less, by a haze ; and that while its upper sur- 
face is always marked by a well-defined line, its under surface 
is much less distinct, so that it is difficult to determine its exact 
boundary ; or rather we should say that it gradually dies away 
into a dilute haze, scarcely perceptible as it approaches the 
surface of the ground. As the night advances this mist sub- 
sides to the surface, and is ultimately deposited on the grass, 
constituting part of what has been termed the Secondary Forma- 
tion of Dew. Those who account for the origin of such mists, 
by supposing them to be produced by the subsiding of aqueous 
particles floating in the air, whether in the form of clouds or 
otherwise, will find some difficulty in satisfactorily explaining 
how they usually make their appearance over the tracks of rivers, 
canals and ditches, over collections of stagnant water, and over 
meadows, or parts of meadows, where draining has either been 
neglected, or has proved only partially successful. But, though 
the opinion referred to be incorrect, it is at the same time cer- 
tain, that on many occasions the aqueous particles suspended in 
the air during the day, are observed to subside towards even- 
ing, and in the course of the night ; and, at the same period, 
the particles congregated in the form of clouds, are often ob- 
served to separate and disperse themselves through the air, in 
the form of a haze, which renders the azure sky obviously less 
brilliant, though the air still retains a considerable degree of 
transparency. 
As the air becomes more and more expanded as it increases 
in temperature during the day, so in like manner, after sunset, 
it becomes more and more contracted as it becomes colder, from 
the supply of heat being cut off, and from the refrigerating influ- 
ence of evaporation at the surface of the earth. The contrac- 
