In the second place, that relatively large quantity of heat
which in the case of liquid water merely changes the liquid into a
vapour, becoming "latent" or otherwise unrecognisable, and which, as
already shown, forms roughly five-sixths of the total heat needed to
convert cold water into steam, has no analogue if the water has
previously been vaporised by other means; and therefore the whole of the
heat supplied to water vapour raises its sensible temperature, as
indicated by the thermometer. Thus it appears that, except for the
sufficient amount of cooling that can be applied to a large vessel
containing carbide by surrounding it with a water jacket, there is no way
of governing its temperature satisfactorily if water vapour is allowed to
act upon a mass of carbide--assuming, of course, that the reaction
proceeds at any moderate speed, _e.g._, at a rate much above that
required to supply one or two burners with gas.
The decomposition which with perfect chemical accuracy has been stated to
occur quantitatively between 36 parts by weight, of water and 64 parts of
calcium carbide scarcely ever takes place in so simple a fashion in an
actual generator.
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