MINERALOGY^ 
10 1 
life, as the fealed earths, &c. but alfo that it is 
iio eafy matter diftindly to defcribe fonle little 
circumftances that occur to the eye, both in their 
natural ftate, and during the experiments. Be- 
fides, they cannot but remember, that the pro- 
grellicnal degrees both of hardnefs, and of the 
quantity of mixed heterogeneous bodies, efpe- 
^ially iron, produce a number of imperceptible 
differences between them, in regard to colour and 
effebls ; fo that they cannot with due precifion be 
feparated and divided into their true genera, fpe- 
Cies, and varieties, before fome more evident 
differences between them may, by repeated ex- 
periments, and perhaps by procelfes yet un- 
known, be difcovered. In examining the clays, 
one ought carefully, to obferve the different de*^ 
grees of fire due to each kind : for v/ithout this 
knowledge they can never be employed to any 
real ufe in common life. Next to this, there is 
another point equally neceffary to be taken, no-^ 
tice of, that is, the manner of v/orking the clays, 
which is often different in different kinds, and 
which, not lefs than the different degrees of fire, 
is produbtive of different effebts , and therefore^ 
if both thefe circumftances are not at the fame 
time exablly defcribed, it is as wrong to afTert 
with fome authors, that arefradory clay does never 
crack in the fire, as it is deceiving to pretend that 
the fame clay does never imbibe the water, when 
it has been baked. Hence comes that great dif- 
ference in regard both to appearances and qua- 
lities, between a tobacco-pipe, which is very little 
baked, and a jar from Waldenburg, between a 
common brick and the other fort called a water 
ciinkert. 
The 
