_Answer._ The gas which escapes from the mouth _d_ of the pipe _b_, fig. 212, is mixed with air, and therefore contains within itself the elements which only need to combine to produce combustion; whereas with an ordinary gas-burner or candle the air comes in contact with the flame only from the outside, or on its surface. This is shown better perhaps in the flame of an ordinary candle. The heat of such a flame distils a gas from the melted tallow, which is similar in nature to that which escapes from coal at a high temperature. Now by observing the candle very closely it will be seen that at the bottom close to the wick there is very little combustion, as the gas there first escapes from the wick and is not heated to a sufficiently high temperature to burn freely. A little above the lowermost part the flame is of a pale bluish color, which is due to the combustion of the hydrogen. Above that, where the carbon is set free, its particles glow with heat imparted by the burning hydrogen and are then consumed by uniting with the oxygen of the air. The combustion occurs only at the surface of the flame, the inside being a mass of combustible gas which cannot burn until it in turn comes in contact with the oxygen of the air. This can be proved by inserting one end of a small tube, fig. 213 (a pipe stem will do), which is open at both ends, into the flame. The combustible gas will then escape at the other end and can easily be lighted with a match. It will be found that the flame from the Bunsen burner is much more intense than that of an ordinary candle or gas-burner. The reason of this is that combustion, as already stated, takes place through the whole mass of its flame, whereas an ordinary flame burns only at its surface. Common gas-jets are therefore arranged so that the flames will be flat, thus exposing as much surface to the air as possible, and, as explained in answer to Question 329, in describing the lamps for head-lights, their burners are usually made with a circular wick, through the centre of which a current of air circulates. This arrangement exposes a larger surface of the flame to the air, and also with the aid of a chimney furnishes an abundant supply for combustion. In stationary boilers with long flues of a large sectional area the flame will often extend for thirty feet, showing that while combustion is going on only at the surface of the flame, it takes a long time to complete the process. The same thing is shown if a gas-burner is made with a single round hole. The flame will then be very long and liable to smoke at the top. QUESTION 381. _From the preceding considerations what may we infer to be necessary in order to consume coal gas perfectly?_