Fig. 10.
The paper is left clean at the hollow chamber, but marked with smoke at the luminous part of the flame. (See Fig. 10.)
Now we must find the differences between the non-luminous outer flame and the luminous inner flame. To do this thoroughly we must have a Bunsen burner to afford the best illustration. This is not an expensive item. A cheap and simple form of it can be obtained for 1s. 3d. To understand the nature of the flame we must first understand the principles of the Bunsen. It is a burner in which a mixture of air and gas is consumed. A is a brass tube, mounted on a solid foot K, with a small tube C to admit the gas. There are two holes at the bottom of the brass tube to admit air in the direction of the arrows, and a movable brass collar fits over these holes, so that the air can be admitted or excluded at will. On igniting the gas, with the holes of the Bunsen open, we shall see that it burns with a non-luminous but exceedingly hot flame. On closing the holes we shall notice that the flame becomes luminous, much more languid, and does not give off nearly so much heat. (See Fig. 11.)
Fig. 11.
We must ask ourselves the question, What is the cause of this difference? The answer is a simple but very instructive one. Coal-gas, like the fat of the candle, contains carbon, and in the luminous flame, owing to the limited supply of oxygen, these particles of carbon are made white hot, and so emit light, but are not entirely consumed till they reach the outer edge of the flame, where combustion is more complete, owing to the contact of the flame with the air, and even then many of them escape; and so where gas is burnt the ceilings after a time become blackened.
In the non-luminous flame, owing to the air being admitted and mixed with the gas, the increased supply of oxygen renders combustion more complete, greatly increases the heat of the flame, but renders it incapable of giving light. Now, the reasons for the differences of the two flames are made clear.