his Thermometer with the common mercurial ones. 363 
was, that in confequence of the remarkable porofity which it 
produces in the clay, it would probably diminifh the expanfi- 
bility, by occafioning the mafs to contain, under an equal fur- 
face, a much lefs quantity of folid or expanfible matter. It 
may be objected to this idea, that the expanfions of metals, 
in Mr. Exticotr’s * and Mr. SMEaton’s + experiments, do 
not appear to have any conneCtion at all. with their denfities : 
but the cafes are by no means parallel; for there the compari- 
. fon lies between different fpecies of matter; but here, between 
one and the fame matter in different ftates of compactnefs. If 
a metal could be treated as clay is in this inftance, that 1s, if a 
~ large bulk of any foreign matter could be blended with it, and 
this matter afterwards burnt out, fo as to leave the metallic par- 
ticles at the fame diftances to which they had been feparated by 
the mixture of it, we may prefume that the metal thus enlarged 
would not expand fo much as an equal volume of the folid 
metal. Such at leaft were the ideas which determined my 
choice to a compofition of clay and charcoal powder; and be-. 
ing afterwards defirous of fatisfying myfelf whether they had. 
any foundation in fa&, I have, fince the experiments were 
made, prepared fome pieces of clay with and without charcoal, 
and having burnt them in the fame fire, I ground them at the 
fides, to make them both fit exatly to the fame divifion near 
the narrow end of the gage; then, examining their expanfions 
by equal heats, I found the piece with charcoal to expand enly 
one-third part fo much as that without; and thus was fully 
fatisfied with the compofition of the gage. , 
To afcertain a fixed point on the fcale for the divifions to be 
counted from; the filver piece and gage were laid together for 
* Phil. TranfaG. vol. XLVI. p. 485. . 
+ Ibid. vol. XLVIII. p. 612. : 
Bbb& fome 
