212 ALUMINIUM. 



state that it is the white material of the brick, not the red 

 colouring matter, which is concerned in the production of the 

 beautiful metal aluminium. What, then, is clay? I mean, 

 of course, white clay, inasmuch as we have already agreed to 

 consider all colouring materials existing in certain clays as 

 so many impurities. 



Clay may be stated to consist of a mixture of two chemical 

 compounds respectively known as silica and alumina. Various 

 clays hold these materials in varying proportion, but in general 

 terms we may say that clay is made up of equal weights of 

 .silica and alumina. Let us banish silica from the account at 

 once. Though a most extensively diffused and important 

 agent, we will have no concern with it here, because it does 

 -not contain the precious metal of which we are in quest the 

 beautiful metal aluminium ; that is got out of alumina. 



Fifty-three parts and three-tenths exactly of the metal 

 aJumim'mra exist in every hundred parts of alumiwa; say, 

 therefore, almost exactly fifty per cent. 



Now this alumina belongs to the class of bodies called 

 earths : and if you ask me what earths are according to a 

 chemist's notions, I answer in my own homely way, they are 

 rusts of metals. Yes, lime is the rust or oxide of a metal 

 termed calcium ; magnesia is the rust or oxide of a metal 

 termed magnesium; and alumina is the rust or oxide of the 

 metal termed aluminium. 



Leaving the earth alumina for a time, I will say, what 

 occurred when a bright piece of iron was exposed to moist 

 air? The iron under these circumstances lays hold of, or, 

 in chemical language, combines with, the element ( oxygen,' 

 becoming changed into oxide of iron, or, as one generally 

 calls it, iron rust. The point I want to establish is, that the 

 terms rust and oxide, as applied to iron, are one and the 

 same. A person does not usually say rust of copper, rust of 

 lead, and so forth, but a writer may employ these desig- 

 nations. At any rate / take that liberty ; and being taken, 



