FLU′ORIDE OF HYDROGEN. HF. Syn. Fluohydric acid; Hydrofluoric acid; A. hydrofluoricum, L. An acid composed of hydrogen and fluorine. It was discovered by Scheele, but was first obtained in a pure state by Gay-Lussac and Thénard, in 1810.
Prep. Pour concentrated sulphuric acid on half its weight of fluor spar, carefully separated from siliceous earth, and reduced to fine powder. The mixture must be made in a capacious leaden retort, and a gentle heat only applied, and the evolved gas must be collected in a leaden receiver, surrounded by ice.
Prop., &c. A colourless fluid below 59° Fahr., which speedily evaporates in dense white fumes when exposed to the air. Its affinity for water exceeds that of sulphuric acid, and its combination with that fluid is accompanied with a hissing noise, and a considerable increase of its sp. gr. up to a certain point. It attacks glass and silica, for which reason it cannot be preserved in glass vessels. Bottles of lead, silver, platinum, or pure gutta percha, are used to keep it in. It is highly corrosive, instantaneously destroying the skin on contact, and producing deep and serious ulcerations; its vapour is pungent, irritating, irrespirable, and poisonous. With the bases it unites to form FLUORIDES.
In the arts, hydrofluoric acid is used for etching on glass.
FLU′ORIDES. Compounds of fluorine with metals and other basic radicals. The fluorides of the metals are, with the exception of those of the alkaline metals, insoluble in water, while the fluorides of hydrogen, boron, and silicon, are gaseous, condensing at a low temperature to volatile liquids.
FLU′ORINE. F. Syn. Fluorinium, L. An element that has not yet been isolated, owing to its attacking and combining with every element or compound that at present has been exposed to it, except oxygen. It is presumably gaseous, and of a pale greenish-yellow colour.
FLU′OSILICIC ACID. Syn. Fluoride of silicon and hydrogen; Hydrofluosilic acid. Prep. From powdered fluor spar, and siliceous sand or powdered glass, of each 1 part; concentrated sulphuric acid, 2 parts: mix in a glass retort, apply a gentle heat, and pass the evolved gas into water through a layer of mercury. Decomposition ensues, silica being deposited in a gelatinous state, and hydrofluosilicic acid or fluosilic acid remains in solution. The acid liquor is used as a test for potassium and barium, with whose salts it yields nearly insoluble precipitates.
FLUX. Syn. Fluxus, Fluor, L. In medicine, a term formerly applied to several diseases attended with a copious discharge, as diarrhœa (FLUX), dysentery (BLOODY FLUX), English cholera (BILIOUS FLUX), fluor albus (WHITE FLUX), &c. These terms are still current among the vulgar.
Flux. In metallurgy, &c., a term applied to various substances of easy fusibility, which are added to others which are more refractory, to promote their fusion.
Prep. 1. (Black flux.) Nitre, 1 part; crude tartar or cream of tartar, 2 parts; mix, and deflagrate, by small quantities at a time, in a crucible, heated to dull redness. The product consists of carbonate of potassa, mixed with charcoal in a finely divided state. Used for smelting metallic ores. It exercises a reducing action, as well as promotes the fusion. It must be kept in a dry corked bottle.