It is consequently apparent that it is practically impossible to admit enough air through the grates to effect a constantly perfect combustion of bituminous coal. It is, therefore, necessary to admit a portion of the air above the fire. In doing this, however, in order to effect perfect combustion the air thus admitted must be thoroughly mixed with the gases, and in order to be able to enter into chemical combination, or in other words, to burn, the gases must combine with the air at an igniting temperature. If too much air is admitted, it will reduce the temperature in the fire-box so much that the gases will not ignite; or, if it is admitted in strong currents, the air and the gases will flow side by side like the currents of two streams of water, the one muddy and the other clear, which, as is well known, mingle very slowly. Besides, if a hot stream of gas encounters a strong stream of cold air and comes in contact with it only at its surface, the latter will be cooled down below the igniting temperature; whereas if the two had been intimately mixed in the right proportion, the whole mixture would have been hot enough to burn. It is therefore of the utmost importance that the air which is admitted above the fire should enter the fire-box in many small jets. None of the openings for its admission should exceed ¹⁄₂ inch in diameter. With the violent draft in a locomotive fire-box there is an extremely brief period of time for chemical combination to take place after the gases are expelled from the coal and before they are hurried into the tubes. As the chemical action between the gases and the oxygen can only take place when the two are in intimate contact, too much pains cannot be taken to distribute the currents of admitted air and thus mix them with the combustible gases. In many cases means are adopted to delay the air and the gases in the fire-box so as to give them time for chemical combination or combustion before entering the tubes. QUESTION 389. _Does any combustion take place after the gases enter the tubes?_ _Answer._ Very little; as the flames are extinguished soon after they enter. QUESTION 390. _Why are the flames extinguished in the tubes?_ _Answer._ They are then in contact with large quantities of incombustible gas and beyond the reach of a supply of air; besides, the temperature of the tubes which are surrounded with water is so low that the flame is soon cooled down below an igniting temperature. QUESTION 391. _What temperature is necessary to ignite coal gas or produce flame?_ _Answer._ A temperature considerably hotter than red-hot iron is needed, as can easily be shown by the fact that a gas-light can not be ignited with a red-hot poker. QUESTION 392. _Are there any parts of the fire-box where the temperature is probably below the igniting point?_