FOR MEASURING THE EXPANSION OF SOLIDS. 
259 
to remove certain errors which had prevailed up to that time concerning the 
pyrometer then most in use (Wedgwood’s), and which might possibly prove 
most commodious, and consequently most useful, if once the degree of exacti- 
tude could be determined of which it was susceptible.” The remainder of the 
paper is taken up with an account of the most accurate experiments upon the 
expansion of the metals from the time of Newton. 
The third and last Essay was delayed till the year 1811; and in it no further 
description of the platinum pyrometer is to be found ; but a laborious com- 
parison, 
1st, of the indications of the platinum pyrometer with those of the mercurial 
thermometer ; 
2nd, of the same pyrometer with that of Wedgwood ; and 
3rd, of the degrees determined by these instruments with those previously 
known of the expansion, ebullition and fusion of various substances ; in a range 
of temperature comprising the highest degrees of the thermometric scale and 
the lowest of Wedgwood’s. 
Now it is very remarkable that all M. Guyton’s efforts in this paper are di- 
rected to the valuation of the degrees determined by Mr. Wedgwood’s clay 
pieces ; but that he carries the comparison of the platinum pyrometer by actual 
experiment no higher than the melting point of antimony. He clearly esta- 
blishes a great error in Mr. Wedgwood’s original estimation of his degrees to 
that point ; and, by calculation upon this basis, continues the correction to the 
melting point of iron, “ en admettant toujours une progression uniforme j usque 
dans les plus hautes temperatures.” The experimental comparison was obvi- 
ously stopped by some practical difficulty at higher temperatures ; and it is 
easy to perceive in what this must have consisted. Platinum at a red heat be- 
comes very soft and ductile ; and the lever against which the pyrometric bar 
pressed, being of such very slender dimensions, would obviously be liable to 
bend and thus frustrate the experiment : in addition to which, I can speak from 
my own experience, the platinum spring plate and the centre pin would be 
liable to a change of texture which would impede the motion of the lever and 
it would finally become welded to the index ; for a very moderate pressure at 
a high temperature would produce this effect. 
The conclusion, indeed, of these Essays seems to admit that the author did 
