When the coal is heated up to about 1,200 degrees, the 5 per cent. of hydrogen unites with three times its weight of carbon, and thus 20 per cent. of the coal is converted into the gas described. In this process a large amount of heat is absorbed or becomes latent, as it does when water or any other substance is converted into vapor. It will therefore be seen that the first effect of putting fresh coal on the fire _is to cool the fire_. This fact has an important bearing on the question of combustion and will be referred to hereafter. [90] Usually if more than two or three shovels full are thrown in there will be no immediate formation of flame. [91] A Treatise on Steam Boilers, by Robert Wilson. [92] The quantity of heat required to heat coal is only about one-fifth that needed to heat the same weight of water to the same temperature. QUESTION 377. _How can the process of the combustion of the gas generated from the coal be best explained?_ _Answer_. As this gas is substantially the same as ordinary illuminating gas, the manner in which it burns can perhaps be made clearer by examining the combustion of an ordinary gas-light. As stated before, combustion is a chemical union of the oxygen which forms one of the elements of the air with the hydrogen and carbon of the fuel, which, in this case, form gas. It should be clearly kept in mind that combustion is the result of this union, and that the oxygen is as essential to combustion as coal or gas, and in fact is the fuel of combustion just as much as coal or gas is. If we were to conduct a pipe from the external air into a vessel filled with coal gas we could _light the air_ and it would burn in the gas as the gas burns in the air. It will be noticed, however, that before either the gas or the air will burn, they must be lighted. Air and gas, even if mixed together in the same vessel, will not burn unless they are lighted. This can be done by the flame of any burning material, or with a piece of metal heated to a very high temperature, or by an electric spark. In other words it may be said that the atoms of the two gases must be excited into activity by the application of heat, that is, what is called an _igniting temperature_ must be communicated to them before chemical combination will begin. The chief feature which distinguishes combustion from other chemical union is the circumstance that the heat generated during the combination is sufficient to maintain an igniting temperature, and the necessity of doing so in order to continue the process is of very great importance in the combustion of coal in locomotive boilers, as will be shown hereafter. QUESTION 378. _How does an ordinary gas-light burn after it is lighted?_