48 ESSAY IV. 



attacked in the cold, yet DO crust appeared ; but heat was 

 scarce applied, when the whole surface of the water was 

 covered, and all the phenomena which the natural exhibits 

 were shown by this regenerated fluor. 



This shows that Mr. Scheele's theory of fluor acid is true, 

 and that such an acid actually exists. 



SECTION XT. 



My first experiments were communicated to Mr. Scheele, 

 with a request that he would repeat them ; by which means 

 I hope to guard against any new mistake. That worthy 

 philosopher informed me, that he had long observed that a 

 mixture of fluor, as transparent as mountain crystal, and 

 of oil of vitriol in a metallic cylinder, produced no appear- 

 ance of siliceous earth on a wet sponge suspended in the 

 inside. 



At my request he made a new experiment, which con- 

 sisted in adding oil of vitriol to portions of fluor of this trans- 

 parent kind that were placed in two tin cylinders ; some 

 siliceous earth was put into one, a wet sponge suspended 

 in both, and a cover fixed on them. The next morning the 

 sponge that was suspended over the mixture to which 

 siliceous earth had been added was covered with that earth, 

 while the other exhibited no such appearance. 



However contrary this may appear to my experiment 

 (Sec. VITI.), I have no doubt of the fact, having often ex- 

 perienced my friend's accuracy ; I might have committed a 

 mistake before. 



SECTION XIT. 



The experiment was therefore repeated, but no heat was 

 applied to the mixtures in the tin cylinders. After the ex- 



