50 ESSAY IV. 



SECTION XIV. 



It follows from these experiments, that uncombined fluor 

 acid dissolves siliceous earth, and carries it up into the water 

 in the receiver. If no water be present, it ascends in the 

 form of a dry vapour; a remarkable fact, considering the 

 fixity of this earth. May not this serve to illustrate the 

 volatility of the diamond ? I think it probable that fluor 

 acid is a constituent part of this gem. 



The origin of the siliceous crust on the surface of the 

 Water may be explained, by supposing either that the acid 

 dissolves an excess of the earth which is present in the glass, 

 and already half prepared for solution, and that, being over- 

 saturated, it lets fall a part which it cannot retain when 

 mixed with water, or that the alkali in the glass comes into 

 action. 



In Mr Scheele's experiment, the siliceous earth was 

 deposited on the sponge, without any other circumstance 

 to promote the volatilisation. But it is at the same time to 

 be noticed how slowly this happened ; for it doubtless did 

 not begin to take place till the small quantity of water 

 contained in the sponge was saturated with the siliceous 

 earth dissolved in the acid; Fluor acid> which I have kept 

 above a year in a phial, has corroded the glass in many 

 points, which are surrounded with concentric circles, and 

 deposited a powder that adheres to the bottom. 



This remarkable fossil may still furnish a subject for 

 many experiments. Golden vessels would, in my opinion, 

 be the most convenient in such an undertaking, as also for 

 keeping the acid. 1 



1 These experiments so clearly point out the origin of the siliceous 

 crust, that Bergmann, who had adopted Mr. Scheele's opinion concerning 

 its formation during the process, candidly deserted it. (Opusc* vol. iii. 



