92 CLAY. YTTRIA. 



It rarely occurs in a pure unmixed state. But it 

 has been found native, in small masses, at New- 

 haven, in Sussex, and also in Hall, in Saxony. 



Clay consists of this earth, joined to silex. Por- 

 celain clay proceeds from the decomposition of 

 felspar ; it consists of silica, alumina, and some- 

 times a little lime and potash. Pipe-clay, and 

 potters'-clay are pure clays, but of variable com- 

 position. 



Alumina has no smell nor taste ; is insoluble in 

 water, but forms with it a ductile paste, and shrinks 

 much when exposed to heat. It is dissolved by 

 the liquid fixed alkalies, and unites chemically with 

 barytes, strontia, lime, and magnesia. It is dis- 

 solved by most of the acids. 



The salt called alum, which gives its name to 

 this earth, is a sulphate of alumina and potash. 

 Sulphate of alumina alone will not crystallize ; but 

 when sulphate of potash is added, octahedral crystals 

 of alum are produced. When alum is exposed to 

 heat, it loses part of the acid and water of crystal- 

 lization, becomes light and spongy, and is called 

 burnt alum. Alum is extensively employed in the 

 arts of dyeing and calico printing, in consequence 

 of the attraction which alumina has for colouring 

 matter. Alumina also forms the basis upon which 

 are precipitated certain colours used as pigments. 



YTTRIA. 



This rare earth, so called from Ytterby, in Swe- 

 den, where it was discovered, is found only in a 

 stone called gadolinite, so named from Professor 

 Gadolin. It is insipid, and insoluble in water, but 

 dissolves in carbonate of ammonia. It forms salts 



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