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in some degree afford a scale for our appreciation of the
influence of the foreign substances in the ice. If we consider:
that an amount of chlorine of 0.27.3 per cent causes the ice
[see table V and curve V on plate 21] to contract already at
— 14° 0
that a sample of ice, which contains O.015 p. c. of chlorine
begins to contract at — 4° C (see table IV and curve IV)
and that ice from ordinary distilled water, which contains
scarcely perceptible traces of chlorine, begins to shrink in
volume at about — 0°.25 C, we will hardly feel inclined to
underrate the influence of foreign substances on the entire
physical behavior of the ice and especially on the
thawing-process. Many facts, familiar to most of my readers, will find
their interpretation from this circumstance.
Walking upon a snowy plain on a winter day, when the
thermometer shows about -— 7° or — 8° C in the air, we will
perceive, that every footstep causes a keen, crunching sound
from the ice-particles crushed under our feet. Suppose the
temperature of the atmosphere to rise a few centigrades f. ex.
to —5° or —4° C, then we will tread quite noiselessly upon
the snow. The ice particles still retain their solid form, but
the thawing has already begun; instead of a network of hard
and brittle crystal needles, we tread upon a soft mass, which
shrinks beneath our feet to a plastic mould and retains the
impression of the footsteps.
Our pleasure of skating is very much dependent upon
the temperature, but also in a certain degree upon the
purity of the ice. If the temperature of the air, and
consequently also of the upper layer of the ice, is next to zero, the
steel of the skates will draw deeper furrows in the ice, and
the friction will be considerably increased. The same will be
the case, if we practise upon a frozen fiord, where the ice
has formed from salt. water, instead of on the ice of an
inland lake.
Ice, which arises by the freezing of salt water, retains a
part of its saltness, the greater, the more suddenly the freezing
has begun. We are told by eye witnesses, such as
Wey-precht, Nordenskiöld a. 0. that the new ice, which arises
by sudden freezing of the calm surface of the arctic sea, is a
tough substance, which can be wrinkled and folded by external
pressure without breaking. Although it may be thick enough
to bear the weight of a man, it is so plastic, that a footstep
makes a deep impression as in mouldable clay. »If you
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