212 PHYSICS. 
Of Frost. 
Frost is, in most cases, nothing more than dew which has been frozen in 
the form of minute crystals, after being deposited on the surface of bodies 
cooled below the freezing point. The formation of frost, then, follows the 
same laws as that of dew. Frost is distinguished from ice in not forming a 
smooth covering, but consisting of crystals of ice which hang sometimes 
loosely, sometimes fast, to the different objects, and resemble snow erystals 
in form. That frost is somewhat different in its origin which arises in 
winter, when, after a long period of cold, a warm wind follows, depositing 
watery vapor on all bodies without exception. This kind of frost also 
consists of ice crystals, with which projecting thin bodies, especially the 
stalks of plants, &c., are coated. It generally arises in clear nights, 
although sometimes when the sky is cevered with clouds, and is especially 
frequent in polar regions, where the cordage of vessels is frequently covered 
with brilliantly white fringes. Fig. 59 exhibits a remarkable frost formation 
on the chiselled star of a tomb-stone. Figs. 60 and 61 represent the same 
on fallen leaves. Pl. 24, fig. 2, presents two trees trained along a wall, in 
which only those branches outside of the line, AB, are exposed to frost. 
To the second kind of frost belong the fine crystals which coat the thick 
walls of houses, when a returning warmth succeeds a period of continued 
great cold. An allied phenomenon, very interesting in its character, is 
exhibited in the formation of “ice flowers,” or crystallizations of ice on the 
window panes of chambers. Some of these are shown in pl. 24, figs. 3-6. 
Fig. 1 presents a peculiar crystallization on the windows of a drug store, 
where the jars and bottles standing near the window were depicted on it in 
frost. 
Of Fog. 
Fog may be produced in two ways: first, by the cooling of air filled with 
moisture to the dew point, the condensation taking place in the air itself, 
and impairing its transparency ; secondly, by the diffusion of vapor arising 
from seas, lakes, rivers, or moist ground, in a cold atmosphere already near 
the point of saturation. It consists of minute vesicles of water filled with 
air. Fogs are very frequently seen over the surface of water in winter 
during calm and cold weather ; also early in the morning during spring and 
autumn. Of a similar character is the cloud which forms over the surface 
of meadow or other moist lands. In all these cases the temperature of the 
air at the surface is less than that of the water or soil, the air itself being 
nearly saturated. Water cools much more slowly than air, owing to sinking 
of the superficial strata when cooled, their place being supplied by warmer 
portions. If, then, considerable bodies uf water be of nearly the same 
temperature as the air, it will happen that during clear nights their 
temperature will be higher than that of the surrounding land. The air 
over the land will thus be cooler than that over the water. If the two 
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