A very clever modification of this principle has been utilised in what is known as the Argand burner, in which the gas and air are not mixed as in a Bunsen, but the burner is made circular, and the air is made to pass up the centre of the flame, so that it gets its supply of oxygen, burns steadily, and presents a very large surface of luminous flame. (See Fig. 12.)
Fig. 12.
A Pretty Experiment.—Let us now go back to our candle flame. We see that it gives light, emits smoke, and does not yield a very large amount of heat. We have learnt that it gives light because the particles of carbon are heated to a white heat, but not entirely consumed. These particles in the flame are held very closely together, and so present a continuous surface. If we could get inside the flame and scatter them we should have a pretty shower of glowing sparks.
We can illustrate this by the following experiment. Take as much gunpowder as will rest on a sixpence, and a like quantity of iron filings, mix them together on a small tin dish. (See Fig. 13.)
Fig. 13.
This must be done carefully and without friction. Then ignite with a taper. The gunpowder burns, makes the particles of iron red hot, and scatters them in a beautiful shower of glowing sparks. This is a fair representation of pulling a candle flame to pieces, the only difference is that the glowing particles are of iron instead of carbon.
Artificial Lightning.—This may be further illustrated by putting a flame together. We may accomplish this by passing any very fine particles of carbonaceous matter through a non-luminous flame, and we shall see that whilst these particles pass through the flame it will give light owing to their presence.
We require a little lycopodium, a piece of glass tubing one foot long, and about a quarter-inch bore, and the non-luminous flame of the Bunsen burner or a spirit lamp. Insert into one end of the tube a little of the lycopodium powder, and then, pea-shooter fashion, apply the mouth to the other end of the tube, and blow the contents into the flame. There will be a great flash of light whilst these infinitely small particles are passing through the flame, thus establishing the fact that luminosity is due to the presence of unconsumed solid matter in the flame. This experiment is sometimes called “making artificial lightning,” and in a dark room it is very effective. (See Fig. 14.)