the Thermometer for high Degrees of Heat, 401 
ment begins, may be confidered as a criterion of the degree of 
vitrefcibiiity of the compofition ; which points out a new ufe 
of this thermometer, enabling us to afcertain the degree of vi~ 
trefcibility of bodies that cannot actually be vitrified by any 
fires which our furnaces are capable of producing. 
All my refearches among the natural clays proving fruitlefs, 
and the experiments having fhewn that all thofe, which could 
fufficiently" refill vitrification, ditniniflred too little in the fire, 
I endeavoured to find a body poffefied of the oppofite property, 
that is, diminifhing too much , and, by a mixture of thefe two, 
to produce the medium diminution required. As I cou,ld not 
find any natural fubfrance poffefied of that property, which 
would not at the fame time render the compound too vitrefcible, 
I was obliged to have recou rfe to fome artificial preparation ; 
and as the earth of alum is the pure argillaceous earth, to 
which all clays owe their property of diminution in the fire, 
poffefling that property in a greater or lefs degree according to 
the quantity of alum earth in their compofition, I mixed fome 
of this earth with the clay, and found it to anfwer my wifhes 
completely, both in procuring the necefifary degree of dimi- 
nution, and increafing its unvitrefcibility. So little is this 
compound difpofed to vitrification, that the greateft heat I 
could give it, that of 160 0 , did not even bring it to a porcelain 
texture, but left it ftill bibulous ; and as it does not arrive at 
the porcelain ftate in this fire, there can be no danger of its ap- 
proaching too neai to the vitrefcent in any heat that we can 
produce in a furnace. 
In order to obtain the exa£t medium required, I took one of 
the beft of the clays I had procured from Cornwall, and mixed it 
with different proportions of the alum earth, till the compofi- 
tion was found, on repeated trials^ to agree with the original 
