Metals. 239 



branching masses, wandering irregularly through 

 it, and is often rudely mixed with the matrix in 

 veins of different thicknesses. These are called 

 br angled ores. 



The veins or fissures of the rocky strata are 

 sometimes only a few inches wide, and some- 

 times many yards. In rich mines there are im- 

 mense masses of ore many feet broad. Where 

 the veins happen not to be filled up, we find the 

 ores crystallized round the cavity. 



The ore, when separated from the matrix, ge- 

 nerally contains some other matter ; as sulphur, 

 arsenic, or both, and sometimes an earthy sub- 

 stance, the whole being united into a compound 

 which at first appears homogeneous. 



The first operation on metals is to separate 

 the ore from the matrix. When the ore is found 

 in large masses, most of it may be dug up free 

 from the matrix, and those pieces to which it 

 adheres may be freed by a hammer. But as the 

 ore is often intimately mixed with the matrix, it 

 is necessary to try other methods. 



Sometimes the whole is reduced to powder, 

 and thrown into water : the water is then put in 

 motion, and the earthy matter floats above the 

 ore, on account of their different specific gravity. 

 It is still better to place the powder on a board, 

 over which water may be made to run; being 

 stirred while the water runs over it, the earthy 

 parts float, and are carried off, whilst the me- 



