14 ESSAY I. 



SECTION XXVI. 



The fluor acid, obtained in the manner described in Sec. 

 vi. , was filtered, and rectified in a glass retort, by means of 

 a gentle fire. The whole liquor which ascended at first was 

 of a sourish taste ; at the end of the process the vapours 

 were concentrated fluor acid; they formed a white crust 

 upon the more diluted acid which came over first. After 

 those vapours had done rising, I found the internal surface 

 of the retort much corroded by the concentrated acid, and, 

 at the bottom of the retort, some siliceous earth was left 

 behind, which had been before dissolved in fluor acid, and 

 may be separated from it besides by another method, of 

 which I shall speak hereafter. 



The siliceous earth thus produced towards the end of the 

 process was separated by filtration from the rectified acid, 

 with which I then repeated the same process. At the end 

 of it I again found siliceous earth in the retort, as well as 

 in the receiver. After a third rectification, I obtained the 

 same. Thence it seems to follow with certainty, that the 

 acid also, by itself, might be changed entirely, with vapours 

 of water, into siliceous earth, if the distillation was repeated 

 with water always in the receiver. 



SECTION XXVII. EFFECTS PRODUCED BY THIS ACID ON 

 ALKALIES. 



The solution of alkali of tartar in fluor acid immediately 

 formed a gelatinous mass, almost without taste. I filtered 

 and evaporated it, but was not able to bring it to crystallisa- 

 tion. I evaporated it then to dryness, and obtained a saline 



