When we proceed to light our lamps to read or to write by, we find some difficulty in making the wick burn at first. We present to it a lighted taper, and it has no immediate effect. Here we have oil and cotton, two things which would speedily set a warehouse in flames from top to bottom, but we cannot even ignite them, try all we can. Why?—Because we must first obtain a gas, oil will not burn liquid; it must be heated to a gaseous point before it will burn, as all combustion depends upon that,—so flames mount high in air. Now in a candle-flame, as will be seen in the diagram (fig. 331.), there are three portions,—the inner dark core, which consists of unburnt gas; the outer flame, which gives light; and the outside rim of perfect combustion non-luminous. In the centre, A, there is no heat. If we place a piece of gauze wire over the flame at a little distance the flame will not penetrate it. It will remain underneath, because the wire, being of metal, quickly absorbs the heat, and consequently there is no flame. This idea led to the invention of the “safety” lamp by Sir Humphrey Davy, which, although it is not infallible, is the only lamp in general use in mines (figs. 329, 330).
Fig. 331.—Construction of a candle flame.
Mines must have light, but there is a gas in mines, a “marsh” gas, which becomes very explosive when it mixes with oxygen. Of course the gas will be harmless till it meets oxygen, but, in its efforts to meet, it explodes the moment the union takes place; instead of burning slowly like a candle it goes off all at once. This gas, called “fire damp,” is carburetted hydrogen, and when it explodes it develops into carbonic acid gas, which suffocates the miners.
Fig. 332.—Pouring carbonic acid on a lighted taper.
CHAPTER XXVII.
NON-METALLIC ELEMENTS.
OXYGEN—SYMBOL O; ATOMIC WEIGHT 16.