50 
POPULAR SCIENCE REVIEW. 
gaseous fluid, and that therefore the hail-storm is directly pro- 
duced by the abstraction of heat alone. 
The snow-flake must now engage our attention. In the 
colder regions of the earth, when the external air is allowed to 
enter into a heated apartment, which is consequently charged 
with water- vapour, a very fine snow is at once formed. As the 
warm air of a first-class railway carriage, full of passengers, 
moves towards the ice-covered windows, it is not unusual to 
witness this phenomenon — the production of snow in fine 
powder. This may be regarded as the elementary state of 
snow. These particles, when viewed under a microscope, 
although they are transparent particles of ice, have not the 
appearance of any regularly crystallised form ; but they possess 
the power of arranging themselves into compound crystalline 
forms of exquisite beauty and of almost infinite variety. There 
is little doubt but careful examination under favourable circum- 
stances would lead to the discovery of a primary form, constant 
to the snow^ crystal, in this snow powder. 
M. Quetelet has endeavoured to show that there is a relation 
between the density of the snow particles and the forms which by 
coalescing they assume. The density of well-formed small stars 
being about Jg — water, from a constant mass of snow being 
regarded as unity — the temperature varying from 29° 7' to 18° 5'. 
Unformed flakes at a temperature of 33° had a density of about 
-^}q, and fine snow, the temperature varying from 32° to 30° 2', 
was found with a density of and yly. 
Dr. Nettis, of Middleburgh, was the first to describe snow 
crystals. In the severe winter of 1740 the cold was most 
intense. Dr. Nettis collected the snow on plain surfaces of 
glass. The crystals were hard and pellucid ; by means of a 
pencil they were removed to the microscope and examined. 
Eighty different figures were obtained, the size of which varied 
from ^ to -J- of an inch. To the late Dr. Scoresby we are, 
however, especially indebted for an extensive examination of 
those exquisite productions of nature (^Account of the Arctic 
Regions, &c., 1820). He tells us that nine days out of ten 
during the months of April, May, and June, snow falls in the 
Arctic regions. With southerly winds, near the borders of the 
frozen sea, or in situations where humid air blowing from the sea 
assimilates with a gelid breeze from the ice, the heaviest falls of 
snow occur. When the temperature of the air is within a 
degree or two of the freezing point, the snow is usually in large 
irregular flakes, such as fall in this country. Sometimes it 
exhibits small granular or large rough white concretions ; at 
others it consists of white spiculse, or flakes composed of coarse 
spiculaB,' or rude stellated crystals, formed of visible grains. 
But in severe frosts, though the sky appears perfectly clear, 
