FLUO-SILIC1C ACID. 503 



and if not protected by the mixture of glass, it is usually eaten through 

 and through. 



(c.) Even with the addition of glass, I have never failed to find the 

 vessels covered by an opake white crust of silica,* less remarkable 

 however than when the fluor and sulphuric acid alone are mingled, 



2. PROPERTIES. 



(a.) Received over mercury, it is a gas, colorless and invisible ; 

 it extinguishes a burning candle, but shows a blue border surround- 

 ing the red flame ; it smokes in the air, producing a dense fog like 

 muriatic acid gas, which, in odor, it strongly resembles ; the cloud is 

 produced by the combination of the acid gas with the atmospheric wa- 

 ter, silica being at the same time deposited, in a state of minute di- 

 vision. 



(b.) It is fatal to animals confined in it, and is suffocating to the 

 experimenter; but its properties are so repressed by combination 

 with the silica, that it is not particularly dangerous to inhale a little of 

 it mixed with the air of the room. 



(c.) Sp.gr. 3.6111, air being 1, and 100 cubic inches at 60 Fahr. 

 and 30 inches barometer, weigh 110.138 grains. Thomson. 



(d.) Dr. John Davy, by decomposing it by liquid ammonia, found 

 that 61.4 of the weight of the gas is silica. Dr. Thomson, from 40 

 cubic inches of the gas (=44.05 grains,) obtained 27.14 silica, which 

 is at the rate of 61.60 per cent. It is indeed most singular, that a 

 very volatile vapor, by corroding siliceous bodies and becoming charg- 

 ed with more than 60 per cent, of a naturally very Jixed and almost 

 unalterable earth, should become a gas, which, when dry, is perma- 

 nent. 



(e.) Water. This fluid absorbs about 263 times its volume of 'this 

 gas, and the solution does not corrode glass vessels. During the so- 

 lution, one third of the silica is deposited, and the remainder with the 

 fluoric acid is retained in the water, and was called by Dr. Davy, 

 sub-silicated fluoric acid. It is sour and reddens litmus. 



(/.) The precipitate is a gelatinous hydrate of silica, and after be- 

 ing washed and ignited, it is regarded by Berzelius as pure. It af- 

 fords perhaps the easiest method of obtaining that earth. 



(<") Silicated fluoric acid gas, when passing into the receiver, 

 often becomes cloudy from the precipitation of the silica by the moisture 

 of the air. 



(h.) If distilled into a receiver containing water, it becomes cover- 

 ed with a siliceous crust, which eventually covers the water, and then 



* This may be presented by covering them with a coat of bees wax, or probably 

 copal varnish, but this last I have not tried. It is said, however, that dry glass is not 

 attacked by silicattd fluoric acid gas. 



