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. The former experiment with water and a gas-burner, too, might easily be modified to throw light upon another problem in acetylene generation, for it would be found that if almost any other liquid than water were taken, less gas (_i.e._, a smaller quantity of heat) would be required to raise a given weight of it from a certain low to a certain high temperature than in the case of water itself; while if it were possible similarly to treat the same weight of iron (of which acetylene generators are constructed), or of calcium carbide, the quantity of heat used to raise it through a given number of thermometric degrees would hardly exceed one-tenth or one- quarter of that needed by water itself. In technical language this difference is due to the different specific heats of the substances mentioned; the specific heat of a body being the relative quantity of heat consumed in raising a certain weight of it a certain number of degrees when the quantity of heat needed to produce the same effect on the same weight of water is called unity. Thus, the specific heat of water being termed 1.0, that of iron or steel is 0.1138, and that of calcium carbide 0.247, [Footnote: This is Carlson's figure. Morel has taken the value 0.103 in certain calculations.] both measured at temperatures where water is a liquid. Putting the foregoing facts in another shape, for a given rise in temperature that substance will absorb the most heat which has the highest specific heat, and therefore, in this respect, 1 part by weight of water will do the work of roughly 9 parts by weight of iron, and of about 4 parts by weight of calcium carbide.