perfectly definite quantity of heat--a quantity of heat which cannot be
reduced or increased by any artifice whatever. The result of a production
of heat is usually to raise the temperature of the material in which it
is produced; but this is not always the case, and indeed there is no
necessary connexion or ratio between the quantity of heat liberated in
any form of chemical reaction--of which ordinary combustion is the
commonest type--and the temperature attained by the substances concerned.
This matter has so weighty a bearing upon acetylene generation, and
appears to be so frequently misunderstood, that a couple of illustrations
may with advantage be studied. If a vessel full of cold water, and
containing also a thermometer, is placed over a lighted gas-burner, at
first the temperature of the liquid rises steadily, and there is clearly
a ratio between the size of the flame and the speed at which the mercury
mounts up the scale. Finally, however, the thermometer indicates a
certain point, viz., 100 deg. C, and the water begins to boil; yet although
the burner is untouched, and consequently, although heat must be passing
into the vessel at the same rate as before, the mercury refuses to move
as long as any liquid water is left. By the use of a gas meter it might
be shown that the same volume of gas is always consumed (_a_) in
raising the temperature of a given quantity of cold water to the boiling-
point, and another equally constant volume of gas is always consumed
(_b_) in causing the boiling water to disappear as steam. Hence, as
coal-gas is assumed for the present purpose to possess invariably the
same heating power, it appears that the same quantity of heat is always
needed to convert a given amount of cold water at a certain temperature
into steam; but inasmuch as reference to the meter would show that about
5 times the volume of gas is consumed in changing the boiling water into
steam as is used in heating the cold water to the boiling-point, it will
be evident that the temperature of the mass is raised as high by the heat
evolved during the combustion of one part of gas as it is by that