1824.] M. BerzeliuH on Fluoric Acid. 341 



Fluoric acid (including loss) 31 "35 



Soda 44-25 



Alumina 24-40 



100-00 



The two bases, therefore, contain equal quantities of oxygen, 

 and saturate equal quantities of fluoric acid. 



A similar compound may be obtained by treating the acid 

 fluate of potash with alumina; but it seems to depend upon 

 weaker affinities than the preceding, for a dilute solution of the 

 salt dissolves the hydrate without becoming turbid, and if the 

 hydrate be employed in sufficient quantity to supersaturate the 

 acid, the liquid filtered from the mixture is found to contain a 

 large portion of neutral fluate of potash. The insoluble double 

 salt may however be obtained in a state of purity, either by boil- 

 ing the mixture, or by evaporating the liquid in contact with 

 hydrate of alumina. Like the preceding it is semitransparent 

 while moist, but white and pulverulent when in a state of dry- 

 ness. Gay-Lussac and Thenard have stated that a solution of 

 alum is precipitated by fluate of potash, but this precipitate can 

 be made to appear instantaneously only by reversing their expe- 

 riment ; for when a solution of fluate of potash is added gradually 

 to a solution of alum, it does not occasion the slightest turbid- 

 ness until its quantity is sufficient to form with the alumina the 

 insoluble double fluate. They mistook, therefore, this double 

 salt for the simple fluate of alumina. In the analysis of minerals 

 which contain at once alumina and fluoric acid, and whose 

 decomposition has been effected by potash or soda, this combi- 

 nation of the two fluates is always precipitated along with the 

 alumina ; and when this precipitate is violently ignited, the 

 alumina displaces the fluoric acid from its union with potash, 

 and there is obtained the usual sublimate of silica mixed with a 

 little alumina around the lid of the crucible. The silica proceeds 

 from a small quantity which had been precipitated along with 

 the alumina, and both it and that earth are separated from the 

 fluoric acid by the water which is formed by the hydrogen of 

 the combustible, and are deposited around that part of the edge 

 of the crucible, along which the gases make their escape. 



A double filiate of (liiuiw)iia and alumina may also be prepared 

 by digesting hydrate of alumina either in the acid, or in the 

 neutral fluate of ammonia. While still moist it has a semi- 

 transparent appearance, like gelatinous silica, but is converted 

 into a white powder by drying. When ignited, it gives on 1 ' first 

 ammonia, then acid fluate of ammonia, and subtluate of alumina 

 remains. It dissolves to a certain extent in pure water, but not 

 in the liquid from which it is precipitated, nor in ammonia. The 

 double salts of soda and potash are also soluble in water, but to 

 a much smaller extent; indeed the latter may be washed with- 



