8 ESSAY I. 



so that, after the last essay, the water had hardly any mark 

 of acidity. 



Thence I conclude that all the fluor acid united itself 

 by degrees with the vapours of water, and thus formed the 

 siliceous earth. It may be objected that the fluor acid is 

 perhaps already united, by nature, with a fine siliceous 

 powder, which it volatilises and carries over in distillation, 

 but leaves it as soon as it finds water to unite with, just as 

 muriatic acid parts from the regulus of antimony when 

 butter of antimony is dropped into water. But if this was 

 the case, the fluor acid would leave the whole quantity of 

 siliceous earth thus combined with it in the first distillation, 

 and therefore show no mark of its presence in the following 

 processes. 



(c) When I put spirit of wine into the receiver instead 

 of water, no siliceous earth was produced ; but the alcohol 

 became sour. 



(d) When I put an unctuous oil into the receiver, all 

 the fluor acid penetrated through the crevices of the lute, 

 and did neither unite with the oil, nor produce a siliceous 

 earth. 



(e) This also happened when acid of vitriol was put into 

 the receiver. If, therefore, the siliceous earth was not a 

 product of each distillation, but, being previously contained 

 in the acid, was only deposited from it, in consequence of 

 the union of the acid with a third substance, I think the 

 siliceous earth ought equally to appear when alcohol was put 

 into the receiver, with which it unites, as well as with 

 water ; but as this does not happen, I conclude that not 

 all the siliceous earth, which is deposited upon the surface 

 of water during the distillation of the fluor acid, was previ- 

 ously dissolved in this acid. 



