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If the temperature of the inferior strata of the atmosphere be 
sufficiently low, the snow-crystals will reach the earth unaltered ; 
but if, in their descent, they happen to pass through strata whose 
temperature is above the freezing point, they will — if they be not 
actually converted into rain — lose the sharpness and beauty of their 
outline, while partial thawing, followed, at the surfaces of contact, 
by the singular phenomenon of regelation, which, as has been shown 
by Faraday, may take place in an atmosphere considerably above 
the freezing point, will probably contribute to their irregular con- 
glomeration into an ordinary snow-flake, contrasting as this does 
so strongly with the light, open, down-like flake produced under 
circumstances favourable to the perfect development and persistence 
of the crystals. 
On the 27th, the snow which had fallen during the preceding 
day was still lying on the ground, and, with the view of continuing 
my observations, I again placed some of it under the microscope, 
I now, however, found that the crystals had lost all their beauty, 
and no longer presented the sharpness of outline and symmetry of 
form which had so forcibly struck me the day before. And yet, 
during the interval, the thermometer had never risen to the freezing 
point, nor had the snow been exposed to the direct action of the sun. 
The change of form thus undergone by the crystals appears to me to 
admit of but one explanation, and is evidently due to the partial 
dissipation of the crystal by evaporation, thus affording an interest- 
ing example of the evaporability of ice at temperatures considerably 
below the freezing point. 
While on this subject, I may as well mention another fact illus- 
trative of the same phenomenon. 
It will be remembered that on more than one occasion during the 
severe frost a dense fog settled over the city, and was afterwards 
condensed and frozen on the surrounding objects, covering everything 
on which it lay, but especially the naked branches of the trees, with 
the most exquisite frostwork. This beautiful phenomenon, however, 
was but of short duration ; for in less than twenty-four hours, though 
the temperature continued all the while below the freezing point, and 
the air free from wind, which might have shaken the frozen particles 
from the trees, yet not a trace of the frostwork remained. 
To form, then, a true conception of the constitution of a snow- 
flake produced under the conditions which prevailed during the late 
3 H 
VOL. IV. 
