EMILY. And what is the cause of the regular shape of the flame of a candle? MRS. B. The regular stream of hydrogen gas which exhales from its combustible matter. CAROLINE. But the hydrogen gas must, from its great levity, ascend into the upper regions of the atmosphere; why therefore does not the flame continue to accompany it? MRS. B. The combustion of the hydrogen gas is completed at the point where the flame terminates; it then ceases to be hydrogen gas, as it is converted by its combination with oxygen into watery vapour; but in a state of such minute division as to be invisible. CAROLINE. I do not understand what is the use of the wick of a candle, since the hydrogen gas burns so well without it? MRS. B. The combustible matter of the candle must be decomposed in order to emit the hydrogen gas, and the wick is instrumental in effecting this decomposition. Its combustion first melts the combustible matter, and . . . . CAROLINE. But in lamps the combustible matter is already fluid, and yet they also require wicks? MRS. B. I am going to add that, afterwards, the burning wick (by the power of capillary attraction) gradually draws up the fluid to the point where combustion takes place; for you must have observed that the wick does not burn quite to the bottom. CAROLINE. Yes; but I do not understand why it does not. MRS. B. Because the air has not so free an access to that part of the wick which is immediately in contact with the candle, as to the part just above, so that the heat there is not sufficient to produce its decomposition; the combustion therefore begins a little above this point. CAROLINE. But, Mrs. B., in those beautiful lights, called _gas-lights_, which are now seen in many streets, and will, I hope, be soon adopted every where, I can perceive no wick at all. How are these lights managed? MRS. B. I am glad you have put me in mind of saying a few words on this very useful and interesting improvement. In this mode of lighting, the gas is conveyed to the extremity of a tube, where it is kindled, and burns as long as the supply continues. There is, therefore, no occasion for a wick, or any other fuel whatever. EMILY. But how is all this gas procured in such large quantities? MRS. B. It is obtained from coal, by distillation. --Coal, when exposed to heat in a close vessel, is decomposed; and hydrogen, which is one of its constituents, rises in the state of gas, combined with another of its component parts, carbon, forming a compound gas, called _Hydrocarbonat_, the nature of which we shall again have an opportunity of noticing when we treat of carbon. This gas, like hydrogen, is perfectly transparent, invisible, and highly inflammable; and in burning it emits that vivid light which you have so often observed. CAROLINE. And does the process for procuring it require nothing but heating the coals, and conveying the gas through tubes? MRS. B.