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Water, Mr. D. finds, will absorb and decom-pose about 263 times its bulk of silicated fluoricgas by direct experiment. Dr. Priestley foundthat when muriatic gas is admitted to waterthat has absorbed the fluoric gas, the former istotally absorbed, and the latter resumes its ori-ginal gasseous state, taking up with it the silexwhich had been separated by the water. Theliquid that remains is simple muriatic acid.Sulphuric acid has the same effect as the mu-riatic in expelling silicated acid from water bywhich it had been absorbed. In either case,much effervescence takes place in proportionas the silicated gas resumes its gasseous form.

The liquid subsilicated acid does not appearto corrode glass vessels in which it is kept.

Silicated fluoric gas, like carbonic gas, con-denses twice its volume of ammoniacal gaswhen the two are mixed over mercury: andthey unite only in these proportions, whateverexcess of either gas be employed. The resultis a dry salt, containing according to Mr. Davy,24.5 per cent, of ammonia, and saturated withsilex: this salt is decomposed by water as thesilicated fluoric gas is, the same proportion ofsilex being separated, and the liquid residuebeing a subsilicated fluat of ammonia, of whichthe alkali amounts to 28.34 per cent. This salt,obtained by slow evaporation, is in the form ofbrilliant tetrahedral prisms, very soluble inwater but not deliquescent, and subliming un-altered by heat. It just reddens litmus. Mr.Davy observes, that if the concentrated solutionof this salt is evaporated in glass or earthenvessels, it powerfully erodes them if heated tothe boiling point, as is shewn by a deposition ofsilex on dilution with more water.

The sulphuric acid decomposes the subsili-cated fluat of ammonia, expelling silicated fluoricgas. Potash also decomposes it, expelling theammonia, and forming subsilicated fluat ofpotash.

Contrary to the experience of Messrs. GayLussac and Thenard, Mr. J. Davy finds that anexcess of ammonia separates the whole of thesilex from the subsilicated fluat of ammonia,and the remaining liquid furnishes by evapora-tion in metallic vessels a simple fluat of ammo-nia, containing 76.4 of alkali, and 23.6 of purefluoric acid, besides water of crystallization.This fluat thus formed is quite neutral, and de-liquesces in the air. When heated somewhatabove the boiling-water point, a part of the am-monia flies off, leaving a salt with excess of acidbehind. When the heat is further raised, theWhole salt sublimes in very pungent flimes,

highly dangerous to respire. If this be done Ina glass vessel it is strongly corroded, and thesalt is now converted to silicated fluat Indeedthe action of this simple fluat of ammonia is sostrong on glass, that a little of the solution pen-cilled on and moderately warmed, will speedilyproduce an etching.

FLUOBORACIC ACID.

This name has been given by the discoverers,Messrs. Gay Lussac and Thenard, to a singulargasseous acid, a compound of the fluoric andboracic. These chemists finding that the purefluoric acid, obtained from fluor spar and sul-phuric acid in the manner described in the fore-going article, though a highly concentratedliquid, was probably combined with a portionof water (as concentrated sulphuric acid itselfis) endeavoured to expell the fluoric acid pure*dry and gasseous, from fluor spar by the actionof another acid, which could not be suspectedto contain any water in its composition. Thevitrified boracic acid was selected for that pur-pose, and on distilling it in a strong heat withfluor spar, it was found that the fluoric acid wasindeed expelled in a perfectly dry state, but thatin the process it became united chemically witha portion of boracic acid, which it thus volati-lized, and retained as a permanently elastic gas,in the same manner as silex is gassified withfluoric acid when fluor spar, glass and sulphuricacid are distilled together.

Therefore, though these ingenious chemistsfailed in their object of producing a pure gas-seous fluoric acid, they have discovered a newgasseous compound of fluoric and boracic acids,which has several remarkable properties.

The fluoboracic acid is thus prepared. Takethe lower end of a gun-barrel with the touch-hole plugged up, coat it with good fire lute, putinto it a quantity of very pure fluor spar mixedwith half its weight of pure vitrified boracicacid, both reduced to powder; lute on the endof the barrel a bent glass tube ; put the barrelinto a furnace so that the open end projects, andlet the extremity of the glass tube dip under ajar full of mercury. When the heat is raisedto full redness, the acid gas is expelled in densevapours, and as soon as they are totally absorb-able by water, the gas is pure and may be col-lected under mercury.

This compound acid gas is a colourless per-manently elastic fluid; its smell is pungent; itextinguishes light instantly, and powerfully red-dens blue vegetable colours. When this gascomes in contact with any air containing hygro-metric moisture, it condenses in thick white