1824.] M. Berzeli us on Fluoric Acid. 331 



are filled with liquid. If a saturated solution be abandoned to 

 spontaneous evaporation, the salt gradually crystallizes in rect- 

 angular four-sided tables, with truncated lateral edges ; resem- 

 bling the form which we would produce by truncating two oppo- 

 site apexes of an octahedron so deeply, as to convert it into a 

 table. Sometimes also it crystallizes in cubes. It is very soluble 

 in water, but its solubility is diminished in a remarkable degree 

 by the presence of an excess of acid. When heated, it melts, 

 gives off its excess of acid, and again becomes solid. The resi- 

 due weighs 74-9 per cent, and consists of the neutral salt. When 

 the salt is incorporated with six times its weight of oxide of lead, 

 and ignited, there is expelled 11*6 per cent, of pure water, which, 

 according as we consider fluoric acid to be an oxide or a 

 hydracid, may be supposed either to have constituted a basis for 

 the excess of acid, or to have been generated by the union of 

 the hydrogen of the acid with the oxygen of the oxide of lead. 

 The acid salt is composed, therefore, of an atom of fluate of 

 potash and an atom of hydrous fluoric acid. 



b. The neutral fluate of potash is most easily prepared by 

 supersaturating bicarbonate of potash with fluoric acid, evapo- 

 rating the solution to dryness, and expelling the excess of acid 

 from the residue by ignition. It has a sharp saline taste, reacts 

 strongly as an alkali, and is excessively deliquescent. It is very 

 difficultly crystallizable ; but if a solution be allowed to evapo- 

 rate in a' temperature between 95° and 104°, the salt may be 

 obtained in crystals, which are sometimes cubes, and sometimes 

 rectangular four-sided prisms. If a concentrated solution of this 

 salt be neutralized with acetic acid, it may be evaporated to 

 dryness without any of the acid separating ; nor can the acetic 

 acid be completely expelled, or the original salt regenerated, 

 except by subjecting the residue to ignition. The solution of 

 this compound salt is strictly neutral while in a state of concen- 

 tration ; but if it be largely diluted with water, it acquires a 

 strongly acid reaction, and the acetic acid becomes at the same 

 time disengaged. I consider this property to be very remark- 

 able. A solution of this salt, even when cold, slowly attacks 

 glass, and destroys its polish. This property, for which I can 

 conceive no satisfactory explanation, at first appeared to me to 

 be occasioned by a tendency in the neutral fluate to combine 

 with an excess of base. To determine, therefore, if it be possible 

 to produce a sub-fluate, I mixed a concentrated solution of the 

 neutral salt with an alcoholic solution of potash ; but I could not 

 perceive that any alteration was produced on its properties by 

 this addition. I then fused a mixture of the neutral salt and 

 subcarbonate of potash ; but no carbonic acid was expelled, nor 

 did the mixture sustain any diminution of weight. Hence it 

 appears that a subsalt cannot be easily formed, so long at least 



