JET, a black, inflammable substance of the bituminous kind, harder than asphaltum, and susceptible of a good polish. It becomes electrical by rubbing, attracting light bodies like yellow amber. It swims on water, so that its specific gravity must be less than 1000; notwithstanding which it has been frequently confounded with the lapis obsidianus, the specific gravity of which, according to Kirwan, is no less than 1744. It also resembles cannel coal in its hardness, receiving a polish, and not soiling the fingers, &c. so that it has also been confounded with this substance. The distinction, however, is easily made between the two; for cannel coal wants the electrical properties of jet, and is likewise so heavy as to sink in water, its specific gravity being no less than 1278, whereas that of jet, as has already been said, is less than 1000.
JER D'EAU, a French term, used to signify a fountain which casts up water to a considerable height in the air.