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
liberated on the combustion of 6 times that amount.
A further example of the difference between quantity of heat and sensible
temperature may be seen in the combustion of coal, for (say) one
hundredweight of that fuel might be consumed in a very few minutes in a
furnace fitted with a powerful blast of air, the operation might be
spread over a considerable number of hours in a domestic grate, or the
coal might be allowed to oxidise by exposure to warm air for a year or
more. In the last case the temperature might not attain that of boiling
water, in the second it would be about that of dull redness, and in the
first it would be that of dazzling whiteness; but in all three cases the
total quantity of heat produced by the time the coal was entirely
consumed would be absolutely identical.
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