FLUORIC acid, in chemistry, is obtain ed from fluor spar, or, as it is technically called, fluate of lime. It has not yet been decomposed, unless it be among the grand discoveries of Mr. Davy, not yet announced to the world. We have at tended the lectures of this professor, and think, in one of them, he said he had de composed the fluoric acid : for want, however, of any written document on the subject, we must content ourselves with a summary account of the properties of this acid, which were investigated with accuracy and precision by Scheele and Priestley. The spar was not distinguish ed from others of a similar appearance till about the year 1768, when Margraff at tempted to decompose it by means of the sulphuric acid. He found that it consisted of a white sublimate, and a peculiar acid; the sublimate proved afterwards to be lime, and the acid being denominated fluoric acid, it is now called the fluate of lime. Margraff found, to his astonish ment, that the glass retort in which the experiment had been made was corrod ed, and even pierced with holes.
Fluoric acid may be obtained by put ting a quantity of the spar in powder into a retort, pouring over it an equal quan tity 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 dis solved in water, by luting to the retort a receiver containing that fluid. The dis tillation is to be conducted with a very moderate heat, to allow the gas to con dense, and to prevent the fluor itself from subliming.
Soon after the diScovery of this acid, it was doubted whether it possessed those properties that rendered it different from all other acids. Scheele, however, who had already investigated the subject, in stituted another set of experiments, which completely established the fact.
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 com mon 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 ob tained again from the compound by means of heat. aced 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 fluate of lime, consists of Lime 57 Fluoric acid - - - - 16 Water 27 100 The most remarkable property is that already alluded to, viz. the facility with which it corrodes glass and siliceous bo dies, especially when hot, and the ease , 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. The order of its affinities is, Alumina Potash Ammonia Silex Barytes Soda Lime Strontian.
Magnesia As fluoric acid produces an insoluble compound with lime, it may be employed to detect the presence of that earth when held in solution. Two or three drops only of the acid will cause a milky cloud or pre cipitate to appear, if any lime is present.
Fluoric acid has been applied to engra ving or etching on glass, and was used, ac cording to Beckman, nearly a century and a half ago for that purpose, by an artist at Nuremberg, who obtained it from digest ing fluor spar in nitric acid. Since, how ever, the discoveries of Scheele and Priestley,it has been more generally used, and the art is performed by covering the glass with wax, and then that part where the figures are to appear is laid bare, and the whole is exposed for some time to the hot vapour of fluoric acid. This simple process is employed with great advantage in writing labels on glass vessels, and in graduating thermometers, &c. See Thom. son's Chemistry.