300 



DOMAIN IV. TALCOUS' 



phosphorets, like the three other alkaline 

 earths, lime, barytes, and strontia. 



In talc it sometimes amounts to one half 

 of the composition; but in the other sub- 

 stances, such as steatites and serpentines, 

 it is only from twenty to forty; but its 

 power is so great as sensibly to alter the 

 appearance and qualities of the stone. The 

 chrysolite or peridot of the French, con- 

 taining about one half magnesia, belongs to 

 this division; and is remarkable as the only 

 magnesian gem. 



The deserts of Siberia are annually co- 

 vered with efflorescences of Epsom salt, so 

 as in the short summer to resemble snow. 

 The talcous rocks in general present a dis- 

 criminating character in their unctuous 

 appearance; they have however, in some 

 cases, been confounded with the argilla- 

 ceous, which occasionally assume the soft- 

 ness and silky lustre of the magnesian. 

 The presence of magnesia is often indi- 

 cated by a green colour. 



