ON FLUOR 49 



piration of fifteen minutes, neither sponge had any earth 

 adhering to it; but after continuing suspended for a 

 whole night, the result was exactly such as Mr. Scheele 

 described. 



Here, therefore, the siliceous earth alone, on mixture with 

 fluor, had yielded the dry siliceous earth which I observed 

 only when I added glass. Hence it became necessary to 

 attempt a few more experiments. 



SECTION XIII. 



1 drm. of fluor, mixed with a double quantity of oil of 

 vitriol in the iron apparatus, afforded, after a distillation of 

 two hours, a thin film of lead on the surface of the water 

 in the receiver, but no siliceous earth. While I was washing 

 it out, I perceived some few particles, which were like the 

 siliceous crust, but they were too inconsiderable to be 

 weighed. If the mixture had been treated in glass vessels, 

 the siliceous earth would have amounted to 2| grs., which, 

 on account of its rarity, makes a tolerable bulk. In order to 

 be better able to observe it, the same mixture was disposed in 

 the same manner, except that, instead of the leaden receiver, 

 one of glass, with 3J oz. of water, was so applied, that the 

 mouth of the iron retort nearly touched the surface of the 

 water. In the beginning of the distillation, a' small spot 

 appeared under the neck of the retort, and the mouth 

 itself was covered with white powder. But it all soon 

 disappeared, and I saw no more of it, though the process 

 w r as long continued. 



The empty part of the receiver was corroded, yet I 

 procured, after edulcoration, but half a grain of earth. 



This seems further to show that nothing but the glass 

 has any share in the production of the siliceous earth. 

 4 



