A
determination of this quantity of heat has been made experimentally by
several investigators, including Lewes, who has found that the heat
evolved on decomposing 1 gramme of ordinary commercial carbide with water
is 0.406 large calorie. [Footnote: Lewes returns his result as 406
calories, because he employs the "small calorie." The small calorie is
the quantity of heat needed to raise 1 gramme of water 1 deg. C.; but as
there are 1000 grammes in 1 kilogramme, the large calorie is equal to
1000 small calories. In many respects the former unit is to be
preferred.] As the material operated upon contained only 91.3 per cent.
of true calcium carbide, he estimates the heat corresponding with the
decomposition of 1 gramme of pure carbide to be 0.4446 large calorie. As,
however, it is better, and more in accordance with modern practice, to
quote such data in terms of the atomic or molecular weight of the
substance concerned, and as the molecular weight of calcium carbide is
64, it is preferable to multiply these figures by 64, stating that,
according to Lewes' researches, the heat of decomposition of "1 gramme-
molecule" (_i.e._, 64 grammes) of a calcium carbide having a purity
of 91.
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