unless heat is supplied to it, or work is done upon it; whereas,
according to its degree of endothermicity, an endothermic substance is
more or less unstable, for it is always ready to emit heat, or to do
work, as soon as an opportunity is given to it to decompose. The
theoretical and practical results of this circumstance will be elaborated
in Chapter VI., when the endothermic nature of acetylene is more fully
discussed.

A very simple experiment will show that a notable quantity of heat is set
free when calcium carbide is brought into contact with water, and by
arranging the details of the apparatus in a suitable manner, the quantity
of heat manifested may be measured with considerable accuracy. A lengthy
description of the method of performing this operation, however, scarcely
comes within the province of the present book, and it must be sufficient
to say that the heat is estimated by decomposing a known weight of
carbide by means of water in a small vessel surrounded on all sides by a
carefully jacketed receptacle full of water and provided with a sensitive
thermometer. The quantity of water contained in the outer vessel being
known, and its temperature having been noted before the reaction
commences, an observation of the thermometer after the decomposition is
finished, and when the mercury has reached its highest point, gives data
which show that the reaction between water and a known weight of calcium
carbide produces heat sufficient in amount to raise a known weight of
water through a known thermometric distance; and from these figures the
corresponding number of large calories may easily be calculated. 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