3 ° 6 An Attempt to make a 'thermometer 
on many ©ccafions ; particularly when he attempts to follow, or 
apply to ufe, the curious experiments of Mr. pqtt, related in 
his Lithogeognofia, and other modern writers upon fimilar 
fubjedts. When we are told, for inftance, that fuch and fuch 
materials were changed by fire into a fine white, yellow, green, 
or other coloured glafs : and find, that thefe effects do 
not happen, unlefs a particular degree of fire has fortunately 
been hit upon, which degree we cannot be fure of fucceeding 
in again : — when we are difappointed, by having the refult at 
lome times an un vitrified mafs, and at others an over- vitrified 
fcoria, from a little deficiency or excels of heat : — when we fee 
colours altered, not only in fhade but in kind, and in 
many cafes deflroyed, by a fmall augmentation of the heat 
which had produced them ; infomuch, that in the gradual in- 
creafe of the fire, a precife moment of time muff be happily 
ieized, in order to catch them in perfection : — and when incon- 
veniences, fimilar to thefe, arife in operations by fire upon 
metals and other fubftances : — how much is it to be wifhed, 
that the authors had been able to convey to us a meafure of the 
heat made ufe of in their valuable proceffes ! 
In a long courfe of experiments, for the improvement of the 
manufacture I am engaged in, fome of my greateft difficulties 
and perplexities have arifen from not being able to afeertain the 
heat to which the experiment-pieces had been expofed. A red, 
bright red, and white heat, are indeterminate expreffions ; and 
even though the three ftages were fufficiently diftinCt from 
each other, they are of too great latitude ; as the brightnefs or 
Uiminoufnefs of fire increafes, with its force, through nume- 
rous gradations, which can neither be exprefled in words, nor 
di (criminated by the eye. Having no other refource, I have 
been 
