29 
daily to the examination of his aquarium, giving promptly any 
attention it may require, his collection will never get into that 
hopeless condition of disorder and decay, which has disheartened so 
many. On the contrary, he will have increasing satisfaction in the 
continual improvement of the aquarium, and the lessening demands 
on his time and attention as it becomes more and more self- 
supporting. Then it will be a source of pride as well as pleasure, 
and the interest it excites will not be confined within the walls of 
the tank, but will stimulate enquiry into other departments of 
marine zoology. It will give additional zest to every visit to the 
seaside, and the humblest animals and plants inhabiting the shore 
pools will become objects of greater interest and closer study. 
VII. 
A SNOW FLAKE, 
BY 
MR. A. S. REID, M.A., F.G.S. 
Read APRIL 2, 1884. 
Snow is probably most generally formed by a process akin to 
‘“‘ Sublimation” that is by the direct passage of the water-vapour 
to its solid form without passing through the intervening liquid 
stage. Many of the delicate white cloudlets which sail far above 
our heads in summer time are such aggregations of minute ice- 
erystals, and phenomena of certain halos and parhelia are to be 
explained by the refraction of such ice-crystals present in the air. 
When snow falls, if there is much wind it falls in shapeless masses, 
or if the flakes have passed through a warmer layer of air during 
their descent they are partially melted and fall as sleet ; or if rain- 
drops pass from a warm stratum through a colder stratum they are 
at once frozen into pellets of ice, and fall as hail, which form of 
frozen rain would seem to be connected with electrical changes in 
the air, for hailstorms are a frequent accompaniment of thunder- 
storms. But the snowflakes that fall on a calm cold winter’s day 
have a beautifully symmetrical structure. They are built up of 
minute ice crystals, grouped for the most part into six-rayed stars, 
each ray feathered with tinier crystals still. All the six rays are 
set at an angle of 60° to one another. Whatever may be the form 
of the rain-drop, it must in becoming solid obey the laws which 
bid its molecules group themselves in hexagonal symmetry. 
A transparent. block of ice does not appear to be made up of these 
same feathered stars, but it is so. If a sunbeam be passed through 
it and the image be received on a screen the heat of the. beam 
