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 91.3 per cent. is just under 26 calories, or that of 1 gramme-molecule
of pure carbide 28.454 calories. It is customary now to omit the phrase
"one gramme-molecule" in giving similar figures, physicists saying simply
that the heat of decomposition of calcium carbide by water when calcium
hydroxide is the by-product, is 28.454 large calories.

Assuming all the necessary data known, as happens to be the case in the
present instance, it is also possible to calculate theoretically the heat
which should be evolved on decomposing calcium carbide by means of water.
Equation (2), given on page 24, shows that of the substances taking part
in the reaction 1 molecular weight of calcium carbide is decomposed, and
1 molecular weight of acetylene is formed. Of the two molecules of water,
only one is decomposed, the other passing to the calcium hydroxide
unchanged; and the 1 molecule of calcium hydroxide is formed by the
combination of 1 atom of free calcium, 1 atom of free oxygen, and 1
molecule of water already existing as such. Calcium hydroxide and water
are both exothermic substances, absorbing heat when they are decomposed,
liberating it when they are formed. Acetylene is endothermic, liberating