FLUOSILICIC ACID. 



FIG. 43. 



Water absorbs fluoboric acid gas with the greatest avidity, 

 taking up, according to J. Davy, 700 times its volume, which in- 

 creases its bulk considerably and raises its density to 1.77- The 

 most ready mode of preparing the solution of this acid, is to dis- 

 solve crystallized boracic acid in hydrofluoric acid. The acid is 

 extremely caustic and corrosive, charring and destroying wood 

 and organic matters, when concentrated, like sulphuric acid, 

 probably from its avidity for moisture. 



A dilute solution of fluoride of boron, undergoes spontaneous 

 decomposition, according to Berzelius, depositing one fourth of 

 its boron in the form of boracic acid, which crystallizes at a low 

 temperature ; while a compound of hydrofluoric acid and fluo- 

 ride of boron remains in solution, which he terms hydro fluoboric 

 add. The fluoride of boron has a great disposition to form 

 double fluorides, and acts upon basic metallic oxides like the 

 following compound. 



Fluoride of silicon, fluosilicic acid, Si F 3 . This gas is ob- 

 tained in the following manner : equal parts of fluor spar and 

 broken glass or quartzy sand, in fine powder, are mixed in a 



glass flask a (figure 43), to be used as 

 a retort, with six parts of concen- 

 trated sulphuric acid, and stirred well 

 together. A disengagement of gas 

 immediately takes place, and the mass 

 swells up considerably, so that the 

 flask must be capacious. After a time, 

 a gentle heat is required to aid the 

 operation. Fluosilicic gas is collected 

 over mercury. In its physical cha- 

 racters it resembles fluoboric gas. Its 

 density is 3574 according to J. Davy, and 3600 according to 

 Dumas ; it contains twice its volume of fluorine. In transmit- 

 ting this gas into water, the tube must not dip in the fluid, for 

 it would speedily be choked by the deposition of silica, produced 

 by the action of water upon the gas. In the arrangement figured, 

 the extremity of the exit tube is covered by a small column of 

 mercury m, in the lower part of the jar, through which the gas 

 passes before it reaches the water w. Every bubble of gas ex- 

 hibits a remarkable phenomenon, as it enters the water, be- 

 coming invested with a white bag of silica, which rises to the 

 surface. It often happens, in the course of the operation, that 



