PLASTIC EARTHS. 
GENUS II. 
PLASTIC EARTHS. 
E ſhall now proceed to ſtudy the second order of the earths, 
V 
the PLASTIC, which are commonly called in our language 
CLAYS, or CLAVEY EARTHS. 
The natural earths which are assembled under this division, all con 
tain more or less of a particular kind of earth in their composition, 
which gives them their plastic qualities, or gives them, in different de 
grees, the qualities which belong especially to itself. 
The earth I now mean is at present considered by the chemists as 
another of the pure elementary earths, and is called the argillaceous. 
It is distinguiſhed from the earths hitherto described by these proper 
ties: 
1 mo, It does not effervesce with acids when they are simply mixed 
with it. 
2do, It is composed of exceedingly fine impalpable particles : It 
therefore feels fat or fmooth between the fingers, like marrow; and is 
not in the least gritty between the teeth. 
3tio, When a dry maſs of it is applied to the tongue, it imbibes 
the superficial humidity of that organ so strongly that it adheres to it; 
and it gives a peculiar odour on all these occasions when humidity is 
applied to it. 
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