the 'Thermometer for high Degrees of Heat. 40 3 
different clays, and even for different parcels of the fame clay, 
can only be afcertained by repeated trials, adding fucceftive 
quantities of the earth till the defired effect is found to be pro- 
duced. Ten hundred weight of the Cornwall porcelain clay 
which I have now in ufe required all the earth that was- 
afforded by five hundred weight of alum. 
It is material in this place to obferve, that the earth of alum 
is extremely tenacious of water, infomuch that, though appa- 
rently dry, the water and air amount to near as much as the 
pure earth, and are not to be completely driven out without a 
full red heat. When divided by the admixture of other earthy 
bodies, it parts with its water eafier indeed than before ; but a 
mixture containing fo much of it as the thermometric compo- 
fition does, is far more retentive of water than common clay, 
and requires to be kept for fome time in a heat equal to that of 
boiling water, before it is to be confidered as dry, that is, be- 
fore the adjuftment of the pieces in the gage. If they are 
adjufted when only apparently dry, or of fuch a degree of 
drynefs as they can be brought to by a heat that the hand can 
bear, the heat of boiling water will diminifh them two or three 
degrees ; and the greateft part of what they have thus been 
deprived of, they gradually recover again on being expofed 1 6 
the atmofphere, fo that the adjuftment muft be made imme- 
diately after the boiling heat. 
By the fame expedient to which I have thus been obliged to 
have recourfe for procuring to the porcelain clay of Cornwall 
the ftandard degree of diminution, and refiftence to fire, the 
fame qualities may probably be communicated to any other clay 
that is tolerably pure from calcareous earth and iron ; fo that 
the thermometer clay is no longer to be confidered as the pro- 
duce of any particular fpot (which was the principal incon- 
Vol. LXXVI* Ggg venience 
