Earths. 181 



effervescing with acids, and from other earthy 

 bodies by being changed by burning into white 

 powder, which, when mixed with a large pro- 

 portion of water, suddenly concretes into a stony 

 mass called plaster of Paris. They are most 

 commonly found in the strata of clay, sometimes 

 in those of sand, of a whitish milky-coloured 

 mass ; but the small pieces are transparent, and 

 sometimes have a red tinge. They are often 

 composed of small shining particles, like the 

 grains of sugar, and when under this appearance 

 they are particularly called gypsum. When 

 hard, so as to be cut into toys and figures, they 

 are then called alabaster. 



The second form under which they are found 

 is that of a fibrous structure of oblique crystal- 

 lizations, which are parallel to themselves, but 

 which cross the mass from the upper to the un- 

 der surface. In this state they are called fibrariae. 

 These differ much in size and regularity of con- 

 cretion. 



The third species is composed of clear trans- 

 parent plates, like glass, in close contact with 

 each other, but which may be separated. They 

 split with a knife into very fine plates, very 

 flexible, though they cannot be bent without 

 producing flaws. They are called glades marhia, 

 and sometimes Muscovy glass, but improperly, 

 as that is a different substance. 



The fourth species is in the form of separate 



