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Book, page 41 / 445 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 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
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