308 SALTS. 



Or it may be produced by exposing cer- 

 tain clays or slates, which contain sul- 

 phuret of iron to the action of the air. 

 Under these circumstances, the sulphur 

 becomes converted into sulphuric acid, 

 which unites with both oxide of iron and alumina. 

 From this mixture the protosulphate of iron is sepa- 

 rated by crystallization, leaving a solution of sulphate 

 of alumina to be used in the preparation of alum. 

 What is burnt ^8. On heating alum in a crucible or 

 alum ? pipe-bowl, it swells up into a light porous 



mass, and is converted into burnt alum. At 

 the same time it loses its water of crystalliza- 

 tion, of which it contains twenty-four molecules 

 to each molecule of the double sulphate. 



779. OTHER ALUMS. The name, alum, 



What is said 



of other is appplied to a number of salts of analo- 



gous composition to the common alum al- 

 ready described. In one of these, sesquioxide of chro- 

 mium, and in another, sesquioxide of iron, takes the 

 place of the alumina or sesquioxide of alumina. In 

 a third kind of alum, oxide of ammonium replaces the 

 potassa. All of these alums contain the same number 

 of molecules of water of crystallization. They have 

 all the same crystalline form, and. if mixed in solu- 

 tion will crystallize together. They are, therefore, 

 isomorphous salts. Their perfect analogy of composi- 

 tion will be best seen by the inspection of their formu' 

 Ise, given in the Appendix. 



What is said 780. OTHER SULPHATES. VlTRIOLS. 



of vitriols? Several of the sulphates have received the 



