the Thermometer for high Degrees of Beat. 40 y 
degree ; that while the natural clays have their whole mafs 
diftended by the efforts of the air in forcing its paflage, the 
compoftien is only reftrained in its diminution, or prevented 
from diminifhing fo faft as it otherwife would do, and as it is 
found to do in the fubfequent part of its courfe, after the air 
has efcaped from it. 
As the compofition of day and alum earth is far more tena- 
cious of water than the clay itfelf, and was found, after being 
dried by the heat of boiling water, to yield, by diftillation in a 
retort, above three times as much aqueous fluid as the original 
thermometric clay did ; it feems probable, that a part of this 
water, retained to the approach of ignition, and in a ftate of 
chemical combination, may facilitate the paflage of the air, 
ferving as a vehicle to convey it off through interftices not 
permeable to air alone, and confequently enabling it to efcape 
without doing that violence to the mafs, which the natural 
clays fuftain from the expuliion of their air after the water has 
been detached from it ; for the experiments of Dr. Priestley 
have (hewn, that veflels even of burnt clay are permeable to 
air when they have imbibed water into their ftibftance,, though 
not at all fo in a dry ftate,. 
I have now communicated the refult of a feries of experi- 
ments which have taken confiderable time, attention, and 
labour to> complete. Whether the importance of the objeCf 
will juftify me in troubling this illuftrious Society with fo 
minute a detail of the moft material operations, and their 
refults, is not for me to determine. If the thermometer 
Ihould not yet be brought to the perfection that may be wifhed, 
1 flatter myfelf that fome abler hand may now take up the fub- 
je£t to more advantage ; and that philofophers and artifts will 
