A. Because any “al’kali” (such as potash) will arrest the hydrogen (as it escapes from the fuel), and prevent its combination with the oxygen of air.
Q. What is an al’kali?
A. The con’verse of an acid; as bitter is the con’verse of sweet, or insipid the con’verse of pungent.
Q. Why does a jet of flame sometimes burst into the room through the bars of a stove?
A. The iron bars conduct heat to the interior of some lump of coal: and its volatile gas (bursting through the weakest part) is kindled by the glowing coals over which it passes.
Q. Why is this jet sometimes of a greenish yellow colour?
A. When a lump of coals lies over the hot bars, or the coals below it are not red hot, the gas which bursts from the lump escapes unburnt, and is of a greenish colour.
Q. Why does the gas escape unburnt?
A. Because neither the bars nor coals (over which it passes) are red-hot.