OF EARTH. 



9 



Now, besides the description and characters we have given of these 

 several moulds and earths, as they reside in their several beds and couches, 

 there are divers other indications, by which we may discover their quali- 

 ties and perfections ; as, amongst others, a most infallible one is, their 

 disposition to melt and crumble into fine morsels, not turn to mud and 

 mortar, upon the descent of gentle showers, how hard soever they seem 

 before, and if in stirring, they rise rather in granules, than massy clods. 



If, upon excavating a pit, the mould you exhaust do more than fill it 

 again, Virgil tells us it is a good augury ; upon which Laurembergius 

 affirms, that at Wittemberg, in Germany, where the mould lies so close, 



together. They do not burn, nor are they malleable ; are easily diffusible, but not soluble, 

 in water. Properly there are but two sorts : 



I. Argillaceous earths, which harden in the fire, and do not dissolve in the mineral acids. 



II. Alkaline, or calcareous earths, which in the fire burn to lime, and dissolve in the mineral 

 acids. 



Argillaceous earths consist either of spongeous, or of smooth, tenacious parts : 

 the former is called vegetable earth, or mould ; the latter clay. 

 Among the clays are reckoned : 



I. Potters' earths, viz. 



1. Loam, which is coarse, irony, and very sandy. 



2. Common potters' clay, which is heavy, without sand, of different substance and 



colour, whence some require a greater degree of heat to flux them than others. 



3. Fine clay, or porcelain clay, which is smooth and greasy to the touch, and of va- 



rious colours. 



II. Medicinal earths, viz. 



1. Boles and terrae sigillatae. 



III. Mechanical earths, viz. 



1. Tripoli. 



2. Fullers' earth, which lathers like soap, and raises a froth in the water. But the 

 true fullers' earth dissolves in acids, and consequently belongs to the marl- 

 earths. 



IV. Painters' earths, viz. 



1. White. 



2. Mineral yellow. 



3. Umber. 



4. Mineral red. 



5. Mineral blue. 



6. Mineral green. 



To the Alkaline or Calcareous earths, belong the following : 



I. Chalk, This is composed of fine particles, adhering closely together, and forming a pretty 

 compact texture. It colours the hand upon being touched, and commonly is white, but 

 sometimes is of different colours and kinds. 



II. Marls. These are of a loose, friable texture, easily reducible into powder, and readily 

 separating and diffusing in water. Whfen recently dug out of the ground, they 



Volume II. 3 G 



