44 ESSAY IV. 



A scruple of this salt, mixed with an equal quantity of 

 white sand in fine powder, and distilled in the iron apparatus 

 with 1J drm. of oil of vitriol, 1 oz. of water being put into 

 the leaden receiver, showed no vestige of a crust. The 

 water had a putrid smell, and left on the filter 2| grs. of 

 grey earth, which ran under the blowpipe into 1 gr. of lead ; 

 and, by volatile alkali, 5 grs. of grey earth were precipitated, 

 which melted on the addition of a little salt of tartar into a 

 black globule, though the blowpipe alone produced no change 

 on it. This probably arose from some dissolved lead ; but 

 as there was a more copious precipitate here than in the 

 preparation of the ammoniacal salt without siliceous earth, 

 it must unquestionably have arisen from a portion of that 

 earth being dissolved and carried over by the fluor 

 acid. 



My ammoniacal salt being expended, I prepared more 

 from a weaker acid, which I had remaining from another 

 experiment, in a leaden vessel, but procured only 13 grs. 



To this quantity I added 1 drm. of oil of vitriol in the 

 above-described apparatus, putting in moreover 2 scruples 

 of green glass in small pieces. Scarce had the iron tube 

 grown warm, when I perceived on the water in the leaden 

 receiver a great spot of siliceous crust ; and I saw the same 

 appearance on the moist sides of the vessel. I continued 

 the distillation for two hours ; but there did not seem 

 to be any increase of the siliceous crust ; 1 J gr. remained 

 on the filter, partly consisting of grey earth, partly of 

 white films, which ran under the blowpipe into a greenish 

 glass. 



It seems therefore certain, that the earth which passed 

 over in these experiments comes not from the stone, as any 

 constituent part of it, but that it is siliceous earth dissolved 

 by the acid. Did the stone contain .an earth which so 



