236 CHEMISTRY. 



328. Other Alums. Although there is but one substance 

 which is commonly called alum, the chemist recognizes sev- 

 eral different salts as alums. The common alum he calls 

 potassium alum. Then there is a sodium alum, in which 

 sodium has taken the place of potassium, and the salt is 

 therefore sulphate of aluminium and sodium, Al 2 Na 2 (SO 4 ) 4 

 -f 24II 2 O. We have also an ammonium alum, in which am- 

 monium takes the place of the potassium, making a sulphate 

 of aluminium and ammonium. There are others which we 

 will not mention. Now in all these different alums the 

 water of crystallization is exactly the same. And what is 

 more remarkable still, the crystals are alike in all these salts. 

 They are therefore termed isomorphous salts, this name com- 

 ing from two Greek words, isos, equal, and morphe, form. 



329. Magnesium. This is a white malleable and ductile 

 metal, somewhat resembling aluminium, but far lighter and 

 more readily oxidizable. A wire or tape of this metal 

 burns with a magnificent white light, which is sometimes 

 used for lighting up the interior of buildings for the pur- 

 pose of photographing. 



Magnesium oxide, hydrate, carbonate, sulphate, chloride, 

 and iodide are all used in the arts. Magnesium oxide, ob- 

 tained by heating the carbonate to redness, is often called 

 calcined magnesia. This carbonate occurs native, but is 

 generally prepared artificially. Mixed with magnesium hy- 

 drate, it forms the magnesia alba of pharmacy. The sul- 

 phate of magnesia was originally called Epsom-salt, be- 

 cause the waters of Epsom Spa, in England, contained so 

 much of it. In its crystalline state, MgSO 4 -fVH 2 O, more 

 than one half of it is water, and it is efflorescent. 



330. Other Earths. There are several other earths, but they are all 

 rare, some exceedingly so. One of them, glucina, is one of the constitu- 

 ents of the precious stones called emerald, beryl, and chrysoberyl. Crys- 

 tals of another, zirconia, are in common use in jewelry. 



