THE SCIENCE OF A SNOW-FLAKE. 
49 
temperature sufi&ciently high to maintain the vapour in its 
transparency, is brought into contact with a cold belt of air — that 
is, below the freezing point of water — the vapour is frozen as it is 
condensed, and each particle forms a spicula of ice, and these 
spiculse combining form snoiu. When, by a reduction of tem- 
perature, a condensation of vapour takes place, a multitude of in- 
finitely fine drops form a cloud, a mist, or a fog. These minute 
particles of water descend very slowly through the air, and if they 
enter into a stratum of air — which, being warmer, has a capacity 
for imbibing vapour — they may be; and often are, rapidly reab- 
sorbed, and the cloud which they produced, may disappear. It 
is exceedingly interesting to watch the formation of the clouds 
in summer. A cloud, like a floating feather, gradually increases 
in size, and ever varying in form floats slowly on, relieved against 
the deep blue of space ; it falls and it rises, and perchance even 
while we are watching its dream-like changes it is gone — the 
vapoury wreath has been absorbed by a warmer belt of air. On 
the other hand, if the condensation goes forward, and the lower 
region of air has its quantity of vapour, the small particles 
meeting one another will coalesce and form drops to fall as rain 
on the Earth’s surface. The rain particles are ever fluid ; those 
which form snow are frozen, ere by coalescing they arrange them- 
selves — in obedience to some mysterious law of crystallisation — 
into the beautiful snoiv-flake. If the capability of the atmosphere 
for absorbing moistures remained the same at all temperatures, 
or were its capability increased in an exact ratio with the increase 
of heat, no change produced by the admixture of two streams of 
air, of different temperatures, could occasion the formation of rain 
or snow. But as was first shown by Hutton, and confirmed by 
Leslie, while the temperature advances uniformly in arithme- 
tical progression, the dissolving power which this communicates 
to the air mounts with the accelerating rapidity of a geometri- 
cal series, and this in such a ratio that the air has its dryness 
doubled at each rise of temperature answering to 27° of Fahren- 
heit. Hence, whatever may be the actual condition of a mass of 
air, there must always exist some temperature at which it would 
become perfectly damp.” Whenever two streams of air, satu- 
rated with moisture, of different temperatures are mixed toge- 
ther, or float in contact with each other, in the form of different 
currents of wind, there must be a quantity of moisture precipi- 
tated, cloud formed, and if the temperature of one of the serial 
currents falls below 32°, snoiu must be the result. We know that 
electricity appears to produce the condensation of cloud-vapour 
into ice — Hail — and the devastating hail-storms of southern 
Europe are evidently of electrical origin. But it must be re- 
membered, that the electricity acts by producing a rapid reduc- 
tion of serial temperature, probably by the expansion of the 
VOL, VII. — NO. XXVI. E 
