42 PHYSICS. 
- water, or from everything wet; hence it is that anything wet by and by 
becomes dry, and even water in an open vessel will dry up. There is 
always more or less of this vapour in the air, even when the sky is 
clearest. It is only when the vapour, from being cooled in colder air, 
becomes partially liquefied, that it appears as fog, mist, or cloud. Dew, 
-however, is not fog or mist deposited on the ground. After the sun has 
set on the evening of a hot summer day, the heat of the ground radiates 
into the air, or the grass, say, becomes cool, while the heat from the 
interior is not conducted quickly enough to keep up the temperature. 
The vapour in the air, coming in contact with this cooled surface, is now 
condensed into the watery particles of dew. One of the most remarkable 
things about dew is, that it is not deposited, at least to the same extent, 
on a cloudy night as under a clear, cloudless sky. This, at first sight, 
seems a contradiction, but only on the supposition that the moisture falls 
from the clouds, not when we remember how it is really formed. For the 
clouds radiate back to the earth the heat which has been radiated from it ; 
so that the surface of the earth does not become colder than the air above 
it, and therefore the vapour is not condensed. Heat is always transmitted 
from one body to another which is colder. As was seen above, a certain 
amount of heat or of motion communicated to ice expands it into water, and | 
a further amount expands the water into vapour. When the surface of 
the earth is colder than the air containing vapour, the heat of that vapour 
is transmitted into the ground, and the vapour becomes water or dew ; 
and if the ground is extremely cold, the heat in the water keeping it in 
the liquid state, is further transmitted into the ground, the watery 
particles become solid and receive the name of hoar-frost. 
