Carbon combines with oxygen in two proportions: first, 6 parts of the former will unite with 8 of the latter, forming what is called _carbonic oxide_; or 6 parts of carbon will combine with 16 parts of oxygen, forming _carbonic acid gas_ or _carbonic dioxide_, as it is called in some of the new books on chemistry. It is probable that the former compound, that is carbonic oxide, is never or very rarely formed in the flame of coal gas; but, as will be seen hereafter, is a very common and wasteful product of the combustion of the solid portion of the coal which is left after the gas is expelled from it. When there is not enough oxygen for the perfect combustion of the carbon in the flame, it smokes, and the carbon escapes in the form of soot. This, as will be shown, may in a locomotive fire-box help to form carbonic oxide after it leaves the flame. QUESTION 383. _What remains in the coal after all the gas is expelled by heat?_ _Answer._ What remains is ordinarily called coke, which, with the exception of some incombustible substances, such as sand, ashes and cinders, which the coal contains, is nearly pure carbon. QUESTION 384. _What is the chemical process of the combustion of coke?_ _Answer._ The solid carbon of the coke when raised to an igniting temperature; or, in other words, on being lighted, unites with the oxygen in one of the two proportions already given; that is, if the supply of oxygen is sufficient, 6 parts of the carbon of the coke unite with 16 parts of oxygen, forming carbonic acid gas, or carbonic dioxide. If, however, the layer of fuel on the grates is thick, or the supply of air is comparatively small, there will not be enough oxygen to supply 16 parts of the latter to each 6 parts of the carbon, so that when that occurs, instead of combining in that proportion, and thus forming carbonic dioxide, 8 parts of oxygen will unite with 6 parts of carbon and form carbonic oxide. Now it should be carefully kept in mind that the heat of combustion is due to the union, or, as it is sometimes expressed, it is the clashing together of the molecules of the two elements which unite. If, therefore, only half the quantity of oxygen unites with 6 parts of carbon, evidently there will be less heat evolved than there would be if twice that amount of oxygen combined with the carbon. From carefully made experiments it was found that the total heat of the combustion of one pound of carbon when converted into _carbonic oxide_ was 4,400 units, whereas when it was converted into _carbonic dioxide_ 14,500 units were given out. It will thus be seen that it is extremely wasteful to burn coal without a sufficient supply of air to produce carbonic dioxide. The danger of waste from this cause is also increased by the fact that carbonic oxide is colorless and odorless, and therefore its production is not apparent, especially as most persons have the impression that when there is no smoke from a fire combustion is then complete.