This substance has the property of becoming electric by rubbing. On exposure to a gentle heat, it melts; but if the heat be increased, it is entirely consumed, and passes off in vapour. When ignited, and the combustion is slow, it burns with a suffocating and acid fume, and blue flame; but when the combustion is quick it burns with a white and vivid flame. If exposed to a sudden, though gentle heat, by holding it, for instance, in a hand when that is warm, it will sometimes break in pieces with a crackling noise. It is a remarkable circumstance, that, if a bar of iron be heated to perfect whiteness, and then touched with a roll of sulphur, the two bodies combine, and drop down together, in a fluid state, forming what is called _sulphuret of iron_, a compound of the same nature as iron pyrites (236). A piece of iron rolled out very thin may be apparently melted in the hand, by putting it, when heated to whiteness, upon a thick piece of solid sulphur. It is, however, necessary, that this experiment be performed with great care; and under a chimney, or in a place where there is a current of air, to carry off the suffocating vapour. Useful as sulphur is, in various ways, its most important application is supposed to be for the production of _sulphuric acid_, or _spirit of vitriol_ (24). One mode in which this acid is obtained for the purposes of commerce, is by burning a mixture of sulphur and nitre (206) in large chambers lined with lead. In this process the nitre supplies a considerable portion of oxygen (21) to the sulphur, and the air of the atmosphere furnishes the rest. Thus a substance which, in a natural state, is one of the mildest that we are acquainted with, is by this operation converted into a corrosive and dangerous, though useful fluid. Its taste is strongly acid: and, when applied to animal or vegetable substances, it soon corrodes, and destroys their texture. The properties of sulphuric acid have rendered it extremely valuable for numerous purposes, both in the arts and in the laboratory. It has been long employed by chemists, as one of their most useful and frequent agents. The fluid that is put into the bottles for procuring _instantaneous light_ is no other than sulphuric acid; and it is poured among filaments of asbestos (which it will not corrode), for the same purpose as ink is sometimes poured upon cotton. The matches are slips of wood dipped in a mixture of equal weights of sugar or charcoal powder, and what the chemists call hyperoxy─muriat of potash. These are to be rubbed together in a mortar, but with great care, as by strong friction the mixture is apt to explode. To obtain a light, nothing farther is requisite than to dip a match, thus formed, into a bottle containing the acid. BITUMEN FAMILY. 212. _NAPHTHA, or ROCK OIL, is a yellow or brownish bituminous fluid, of strong penetrating odour, somewhat greasy to the touch, and so light as to float even on spirit of wine._