Acids. 161 



lime*). This acid possesses the singular pro- 

 perty of dissolving silicious earth, and in con- 

 sequence corrodes glass with such ease that it 

 soon penetrates through a thick bottle. Hence 

 it is applied to the elegant ornamental art of 

 etching on glass, in the following manner. A 

 wax ground being laid, and the design traced 

 with a needle, the acid (kept in a leaden bottle) 

 is poured on in a thin stratum, and remains a few 

 hours, more or less, as strength of impression is 

 required. Being then washed off, and the ground 

 cleared away by heating, Sec. the design is found 

 as well executed as on copper. It is smooth and 

 transparent: when opake strokes are required, 

 the piece, after preparation, is placed over the 

 vapour arising from a mixture of pounded fluor 

 and sulphuric acid. 



^The carbonic acid, formerly called fixed or fix- 

 able air, may be said to belong either to the vege- 

 table or mineral class, since it exists in vast abun- 

 dance, combined with chalk, marble, and lime- 

 stone, from which it may be expelled either by heat 

 or the stronger acids, and is also the invariable and 

 copious product of the vinous fermentation. When 

 set free by an acid, a strong effervescence takes 

 place, and it is separated in the form of an invisible 



* It is extracted or expelled fiom the substance with 

 which it is connected (lime in this case), as all the weaker 

 acidsare, by pouringonitsnlphuricacid, which byastronser 

 attraction expels ihe fluoric acid, and seizes the lime. 



