202 Binning' of Copper. [Book VI, 



fcum, which contains the copper. Silver is feparatcd 

 from copper by melting the latter with three times its 

 weight of lead j the lead is then melted out, and carfies 

 with it the filver. The filver is afterwards feparated 

 from the lead by the calcination of the latter metal. 



From the pernicious effects which have been ex- 

 perienced from impregnations of copper in aliments 

 drefTed in copper veffels) it has become a very general 

 cuftom to cover thefe veflels with an internal coat of 

 fome other metal. The method of tinning iron has 

 been defcribed, and that of tinning copper is very fimilar, 

 except that the compofition for covering the furface of 

 copper confifts of about one part of lead to two of tin, 

 whereas iron is covered with tin alone. Zinc has been 

 recommended as a fubftitute for tin in thefe operations, 

 and has the advantage of fuperior hardnefs. The me- 

 thod of tinning copper veffels is by making their inter- 

 nal furface perfectly bright, and then wafhing them 

 with a folution of fal ammoniac. The veffel is next 

 heated, and the tin or metallic mixture is melted and 

 poured into it, and being made to flow over every part 

 of the furface of the veffel, it incorporates with the, 

 copper, and when cold remains united with it. Rofin, 

 or pitch, are fometimes ufed to prevent the tin from 

 being calcined, and the copper from being fcaled, 

 cither of which circumftances would prevent the ad- 

 hefion of the tin. Bifhop Watfon eftimated the quan- 

 tity of pure tin which is ufed in tinning a definite fur- 

 face of copper, and found that half an ounce of tin was 

 fpread over two hundred and fifty-four fquare inches, 

 or fomewhat lefs than a grain of tin on each fquare 

 inch i but the fame author fufpecls, that not a quarter 

 of a grain of tin is fpread over a fquare inch in the 

 ordinary mode of tinning, and therefore recommends 

 it as a neceffary -caution againft the coat being rubbed 



