the Thermometer for high Degrees of Heat . 405 
The changes which take place in all the natural clays that 
have come under my examination are fix. 
1. The firft is, the fr inking of the moift clay in drying, 
from the mere lofs of its water. The purer the clay is, the 
more water it requires to foften it, and the mqre it diminifhes 
in bulk by the lofs of that water. 
2. The dry clay, gradually heated, preferves its bulk un- 
varied up to the approach of ignition. At this period it is 
enlarged a little ; probably, as already obferved, from its com- 
bined air endeavouring to efcape. 
3. When this air has made its efcape, the clay begins to dimi- 
nifh, or to lofe the bulk it had before acquired ; and returns 
back, fooner or later, to the fame dimenfions which it was of 
when dry. It is at this point that the thermometric diminu- 
tion commences. 
l 
4. From this point the day continues to diminijh more and 
more in proportion as the heat is increafed. This I call the 
thermometric fage of diminution : it is of greater or lefs extent, 
terminating at different periods of heat, according to the na- 
ture of the clay : in the ftandard thermometer clay, it com- 
mences with vifible ignition, and continues to (doubtlefs far 
beyond) the extreme heats of our furnaces, an interval con- 
fiding of 160 degrees of the fcale : in others, it begins 4, 6, 
and in fome even 1 5 of thofe degrees later, and terminates alfo 
much fooner : and in fome its whole extent is not above 20 
of the fame degrees. Throughout the greateft part of this 
ftage, the clays are found to retain their property of flicking 
to the tongue and imbibing water : between this bibulous ftate 
and the vitrefcent there is an intermediate one, diftinguifhed by 
the name of porcelain ; and to the higher term of this porcelain 
ftate the ftage of thermometric diminution fee ms to continue. 
Ggg 2 5. When 
