46 ESSAY IV. 



longer clean, nor was this at all necessary ; for, after I had 

 held it a few seconds over the orifice, it was covered with a 

 white powder. 



Mr. Scheele, in his first Essay, says that he observed the 

 white powder on a piece of charcoal that had been moistened 

 and suspended over fluor, to which vitriolic acid was added. 

 As this experiment was made in metallic vessels, I conjecture 

 that the mortar used for reducing the fluor to powder was 

 of soft glass, and that some particles being abraded by the 

 trituration had occasioned the phenomenon. 



The glass was therefore the chief cause of the production 

 of the siliceous pellicle on the surface of the water in the 

 receiver. 



SECTION IX. 



In order to ascertain whether, when a quantity of glass 

 sufficient for the saturation of the acid is added to it, it can 

 carry over much more along with it, to half an ounce of 

 fluor, 1J oz. of white oil of vitriol was added in a retort of 

 glass, and 3 oz. of water put into the receiver. The retort 

 was corroded through in an hour's time, and the crust on the 

 water weighed 10 grs. The water was filtered and divided 

 into two equal parts, of which the one being precipitated 

 with caustic volatile alkali afforded 25 grs. of siliceous earth, 

 and the other with aerated vegetable alkali yielded 58 grs. of 

 a precipitate which flowed under the blowpipe, ran into the 

 pores of the charcoal, and gave out strong vapours of fluor 

 acid. 



In this case, therefore, the siliceous earth was precipitated 

 in a state of purity by the volatile alkali ; but the precipitate 

 by fixed alkali was a mixture of siliceous earth, fluor acid, 

 and alkali, as Professor Bergmann has already observed. 



