182 



THEORY OF COHESION. 



The art of plating bars of iron, copper, &c. 

 with gold, silver, zinc, tin, and other metals, is 

 founded wholly on the cementing power of 

 caloric, which forces them together, by what 

 may be termed chemical cohesion, to distinguish 

 it from the aggregation of simple bodies. Plates 

 of gold are made to unite with bars of iron, and 

 other metals, by being^ pressed together, and 

 placed in a stove or furnace. 



When bars or plates of iron are thoroughly 

 cleaned and polished until quite bright, and 

 immersed in melted tin, they are soon covered 

 over with a thin white coat ; or if a slip of copper 

 be perfectly cleaned and polished, then heated, 

 and rubbed over with a piece of tin, a portion of 

 the latter metal combines with the copper, 

 giving it a silvery coat, which adheres to it for 

 the same reason that the hand adheres to frozen 

 mercury, or other metals, when reduced to very 

 low temperatures. 



The common mode of tinning copper vessels, 

 is to make the surface bright by scraping and 

 washing them with a solution of muriate of am- 

 monia. They are then wanned, when the tin is 

 melted and poured into them, by which it incor- 

 porates with every part of their surfaces, and 

 when cold remains firmly united. In the same 

 way, iron vessels are coated with zinc. It is 

 well known that silex cannot be made to com- 

 bine with potass or soda to form glass without 

 the agency of heat, which is equally essential to 



