MINERALOGY. 101 



ui?, as the fealed earths, &c. but alfo that it is 

 rio eafy matter diftinctly to defcribe fome little 

 circumftances that occur to the eye, both in their 

 natural Hate, and during the experiments. Be- 

 fides, they cannot but remember, that the pro- 

 greifional degrees both of hardnefs, and of the 

 quantity of mixed heterogeneous bodies, efpe- 

 cially iron, produce a number of imperceptible 

 differences between them, in regard to colour and 

 effects *, ib that they cannot with due precifion be 

 feparated and divided into their true genera, fpe- 

 cies, and varieties, before lome more evident 

 differences between them may, by repeated ex- 

 periments, and perhaps by proceffes yet tin- 

 known, be difcovered. In examining the clays* 

 one ought carefully to obferve the different de- 

 grees of fire due to each kind : for without 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 working the clays, 

 which is often different in different kinds, and 

 which, not lefs than the different degrees of fire, 

 is productive of different effects ; and therefore, 

 if both thefe circumftances are not at the fame 

 time exactly defcribed, it is as wrong to afierc 

 with fome authors, that a refractory 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, betv/een a tobacco-pipe, which is very little 

 baked, and a jar from Waldenburg, betv/een a 

 common brick and the other fort called a water 

 clinker:. 



H 3 The 



