52 
POPULAR SCIENCE REVIEW. 
menon of the snow-flake ; and to do this we must observe the 
conditions of freezing water in all its aspects. A little experi- 
ment will assist us. If a saturated hot solution of sal-ammoniac 
is allowed to cool in a tall glass, it will be seen that, as the 
surface, which is the first part to cool — being exposed to the 
currents of air passing over it — solidifies, feathery crystals, 
like those of snow, fall through the fluid. Here, with a little 
care, we may watch the accretion of particle to particle, to 
form eventually, ere it reaches the bottoDi, the resulting crystal, 
which is not unlike a snow-flake, and thus learn how nature 
builds up her crystalline forms. 
If we freeze water in a transparent vessel, we shall at first 
see needle-like points of ice shooting out from the sides : that 
is, from hair-like crystals just visible, we may observe them 
enlarging into needles, or small blades of ice, as shown in fig. 38. 
By-and-by, the process of refrigeration going forward, it will 
be perceived that these needles or blades combine, as shown in 
fig. 39. Here, as in the snow crystals, we may detect the same 
law of combination ; every two of the spiculse are separated by 
an angle of 60°. If we watch the freezing of moisture on the 
window-pane we shall seethe same process; resulting eventually 
in the formation of fern-like figures, mimicking the graces of the 
most exquisite productions of the vegetable world. Sometimes 
a sheet of frost is formed, by rapid congelation, over one portion 
of a window-pane, and the arborescent forms are generated 
above it, by a slower process of solidification (fig. 35); and at 
other times, the full sheet is covered with a fairy vegetation, 
the temperature determining the one or the other state (fig. 
34). The same condition is observed under other circumstances, 
especially on the surface of a pavement (figs. 36, 37). Hessell 
and Luke Howard have both observed the formation of hex- 
agonal crystals on the window-pane ; and Howard remarks that 
the air next the earth is sometimes loaded with particles of 
freezing water, such as in the higher regions would produce 
snow. These attach themselves to all objects, crystallising in 
the most regular and beautiful manner. Shrubs, covered with 
spreading tufts of crystals, look as if they were in blossom, while 
others, more firmly encrusted, appear like gigantic specimens 
of white coral. The leaves of evergreens are found with a 
transparent varnish of ice, and a delicate white fringe around. 
On such an occasion the whole face of nature seems dressed out 
in frost-work.” Thus we have evidence that the same law, 
which regulates the formation of snow in the higher regions of 
the atmosphere, is in operation in the formation of frost upon the 
surface of the Earth. We also now know, from the experiments 
of Hr. Tyndall, that “ every atom of the solid ice which sheets 
the frozen lakes of the North has been fixed according to this 
law” — the law which determines the structure of the snow- 
