The Locomotive Of Today

The Boiler : Combustion. 57 bination with oxygen, the hydrogen being most useful as more easily ignited ; these two, hydrogen and oxygen, will some- times combine in small quantities and form steam, especially at the first ignition of the coal. The nitrogen does not enter into the combustion of the coal, but escapes up the chimney in the form of nitrogen gas, the sulphur escaping as sulphur- ous acid, or sulphuretted hydrogen. The mineral ash is made up of various incombustible matters, principally sand and clay, and is the only really incombustible portion of the coal; it should not exceed 2 J per cent, of the total weight of coal in good samples. Cinders are partially burnt coke, and are still combustible at a high temperature and a sharp draft. The air in the ordinary atmosphere consists of a mechan- ical mixture of approximately one-fifth of oxygen to four-iifths of nitrogen, with small quantities of carbonic acid gas, ammonia, and watery vapour. It is the oxygen alone that is required for purposes of combustion, and it is worth noting- that four-fifths of the air admitted to the fire is useless, it having to be heated to the temperature of the fire and then discharged from the chimney. The products into which the combustible parts of the coal are converted in passing through the firebox and tubes of a boiler, are as follows:—first, steam, which is formed by the combination of the hydrogen from the coal with oxygen from the air in the proportion of 2 to 1 by weight, and is highly rarefied, invisible, and incombustible; second, carbonic acid, formed by carbon from the coal mixing with oxygen from the air in the proportion of 6 to 16 by weight, and is invisible and incombustible ; third, carbonic oxide, formed from that portion of the carbonic acid which, after it is formed in the fire, takes up a further portion of carbon from the burning' fuel on the bars, and changes its nature from a non-combustible to a combustible, this additional weight of carbon so taken up being exactly equal to the carbon in the carbonic acid, requires for its combustion the same quantity of oxygen as went to the formation of the acid, and it is invisible but combustible ; and fourth, smoke, which is made up of such portions of the hydro- gen and carbon of the coal gas as have not been combined with oxygen, and so have not been transformed into either steam or carbonic acid ; this hydrogen, so escaping, is trans- parent and invisible, but the carbon on being separated from it returns to its natural State of a black finely-divided. body, and then becomes visible and gives the colour to the smoke : it is only partly combustible. Ihe former two are the product of perfect, and the latter two of imperfect combustion, generally caused by an insufficient supply of air, which can