220 Lunar Caitftic. [Book VI. 



place, but if expofed to the light they loon become 

 black ; fo that if a quantity of this mixture is put into a 

 glafs phial fealed up, any maiks or letters may be made 

 to appear on it as if by magic. To effect this, die 

 phial is covered with thick paper, in which the marks 

 or letters are nicely cur, and it is then expofed to the 

 light, which, acting only through the apertures of the 

 paper, produces the effect. When the (tains are pro- 

 duced, aqua fortis, by re-diffolving the filvcr, will 

 make them difappear again. It therefore feems, that 

 the production of colour is owing to an imperfect re- 

 duction of the iiiver, and that the action of light dif- 

 engages oxygen from this compound in the fame man- 

 ner as it does from the pale nitrous acid, and from 

 vegetables. 



When this folution of filver is evaporated with a 

 gentle heat, it may be made to afford cryftals nearly 

 refernbling thofe of nitre. Thefe, being melted in a 

 crucible, are freed from water, and being poured into 

 moulds of a convenient form, become the filver or 

 lunar * cauftic, or argentum nitratum of the pharma- 

 copoeia. 



Silver is never given internally, and this is the only 

 preparation of it made ufe of externally. The nitrous 

 acid may be entirely feparated from this compound by 

 he.at alone. 



Silver, however, is mod conveniently feparated from 

 its nitrous folution by -immerfing plates of copper in 

 it. The copper is then 'diflfolved, while the filver is 

 deposited in 'its metallic ftate. This metal may be 

 feparated from gold by expofing the mixed metals to 



* A name derived from the whim of the alchemifts, who called 

 the metals by the names of the heavenly bodies : gold, Sol ; 

 filver, Luna; copper, Venus; iron, Mars (whence martial vitriol^ 

 &c.) ; lead, Saturn; tin, Jupiter; quicknlver, Mercury, .xc. 



the 



