240 Chemistry. [Lecture 32. 



tallic parts remain behind. This operation is 

 called washing the ore. 



When the matrix is not divisible by water, a 

 stamping-mill is employed, which consists of 

 an axis turned by a water-wheel. On the axis 

 there are a number of cogs, which lift up a per- 

 pendicular pillar of wood, plated at bottom with 

 iron ; this, falling down, bruises the matrix to 

 powder. It often happens that the matrix is 

 harder than the ore, and in this case the ore will 

 be reduced to a much finer pow r der than it. 

 Though the ore is a much heavier substance ; 

 yet its surface may be so much increased, that it 

 may be carried off by the water before the matrix. 

 This may be obviated by subjecting the mass to 

 a brisk heat, and throwing water upon it when 

 red-hot, which renders the matrix more easy to 

 be pow r dered. There are many ores of this kind 

 which undergo a fusion by heat ; hence the small 

 particles of the matrix which are angular and ir- 

 regular, contract themselves into little spheres, 

 by which means, losing part of their surface, 

 they become specifically heavier, and fall more 

 readily to the bottom of the water : the ore too 

 generally loses part of the sulphur it contains, 

 and, on this account, becomes specifically 

 heavier : the stone becomes softer, and is some- 

 times disposed to fall into powder, merely by the 

 application of water, especially if composed of 

 calcareous or gypseous spar. Quartz is not in- 

 deed so easily heated in this way, but it becomes 



