The name given to an undecomposed substance, which, combined with lime, constitutes the fluor spar. Fluoric acid may be obtained by putting a quantity of the spar in powder into a leaden retort, pouring over it an equal quantity of sulphuric acid, and then applying a gentle heat; a gas ensues, which may be received in the usual manner in jars, standing over mercury. This gas is the fluoric acid, which may be obtained, dissolved in water, by luting to the retort a receiver containing that fluid. The distillation is to be conducted with a very moderate heat, to allow the gas to condense, and to prevent the fluor itself from subliming. The properties of this acid are, that, as a gas, it is invisible and elastic like air; but it will not maintain combustion, nor can animals breathe it without death. In smell it is pungent, something similar to muriatic acid. It is heavier than common air, and corrodes the skin. When water is admitted in contact with this gas, it absorbs it rapidly; and if the gas be obtained by means of glass vessels, it deposits, at the same time, a quantity of silica.

Water absorbs a large portion of this gas; and in that state it is usually called fluoric acid by chemists.

It is then heavier than water, has an acid taste, reddens vegetable blues, and has the property of not congealing till cooled down to 23°. The pure acid may be obtained again from the compound by means of heat. Fluoric acid gas does not act upon any of the metals; but liquid fluoric acid is capable of oxyding iron, zinc, copper, and arsenic. It does not act upon the precious metals, nor upon platina, mercury, lead, tin, antimony, cobalt. It combines with alkalies, earths, and metallic oxides, and, with them, forms salts denominated fluates, of which the true fluor, Derbyshire spar, or filiate of lime, consists of -

Lime . . . . . .. . . . . . .


Fluoric acid . . . . . ... . . .


Water . . . .. .. . .. .. . .. .



The most remarkable property is that already alluded to, viz. the facility with which it corrodes glass and siliceous bodies, especially when hot, and the care with which it holds silica in solution, even when in a state of gas. This affinity for silica is so great that the thickest glass vessels can withstand its action only a short time. With respect to the nature of the substance usually termed fluoric acid, it has not yet been determined experimentally whether it be really a compound of some unknown base with either oxygen or hydrogen, or whether it be a simple substance like chlorine. Dr. Thompson inclines to the opinion that it is a compound of an unknown radical with hydrogen, and not with oxygen; but Sir H. Davy made several attempts to separate its hydrogen, but without success, although he applied the power of the great voltaic batteries of the Royal Institution to the liquid fluoric acid; neither could he decompose it by passing it with chlorine through a platina tube heated red hot, nor by distilling it from salts containing abundance of chlorine or of oxygen.

Dr. Ure therefore observes, that by the strict rules of chemical logic, fluoric acid ought to be regarded as a simple body, for we have no evidence of its having ever been decomposed; and nothing but its analogy with the other acid bodies has given rise to the assumption of its being a compound.


The imaginary radical of the above acid.