252 Chemistry. [Lecture 32. 



ful compound prussian blue, as stated in a former 

 lecture. 



7th. Tin is also a metal that has been long 

 known. The Phoenicians procured it from 

 England, and from Spain. It is very malleable, 

 but in ductility and tenacity it is inferior to any 

 of the metals hitherto described. 



Its combinations with the combustibles and 

 acids are scarcely deserving of notice. Its com- 

 binations with the metals are of more import- 

 ance. Cannons are made of an alloy of copper 

 and tin ; aid bronze, bell-metal, and the reflect- 

 ing mirrors of telescopes are all formed by dif- 

 ferent proportions of the same metals. Copper 

 vessels are often tinned^ that is covered within- . 

 side with a thin coat of tin to prevent their oxi- 

 dizing: all that is necessary is to clean the cop- 

 per well (for tin will not unite with the oxide of 

 copper), and this is generally done by applying 

 sal ammoniac to the surface. The copper is 

 then heated, and a piece of tin gently rubbed 

 over its surface, which at once assumes a silvery 

 appearance. Tin-plate, tlie material of tin 

 saucepans, &c., is made by dipping thin plates 

 of iron, well cleansed, into melted tin. The tin 

 in this case not only covers the iron, but pene- 

 trates it completely. 



8th. Lead is almost as generally in use as iron. 

 It is one of the heaviest and most tenacious of 

 metals. It melts easily, and becomes oxidized 

 on its surface by the air. This oxide, when 



