But since at pressures which do not materially exceed that of
the atmosphere, water changes into vapour at 100 deg. C., above that
temperature there can be no question of a reaction between carbide and
liquid water. Moreover, as has been pointed out, steam or water vapour
will continue to exist as such at temperatures even as low as the
freezing-point so long as the vapour is suspended among the particles of
a permanent gas. Between calcium carbide and water vapour a double
decomposition occurs chemically identical with that between carbide and
liquid water; but the physical effect of the reaction and its practical
bearings are considerably modified. The quantity of heat liberated when
30 parts by weight of steam react with 64 parts of calcium carbide should
be essentially unaltered from that evolved when the reagent is in the
liquid state; but the temperature likely to be attained when the speed of
reaction remains the same as before will be considerably higher for two
conspicuous reasons. In the first place, the specific heat of steam in is
only 0.48, while that of liquid water is 1.0. Hence, the quantity of heat
which is sufficient to raise the temperature of a given weight of liquid
water through _n_ thermometric degrees, will raise the temperature
of the same weight of water vapour through rather more than 2 _n_
degrees.
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