for Measuring the Expansions of Solids, S^c. 193 



inventor had at that time only begun "a series of experiments 

 to determine its march, to compare it with the pyrometer 

 pieces of Wedgwood, and to ascertain the degree of confidence 

 which might be placed in the indications of the latter." The 

 second Essay did not appear till the year 1808, and in it M. 

 Guyton observes that "many persons had expressed a wish 

 to be made acquainted with the improvements which he had 

 made in the instrument since its first construction ; and that 

 he had determined in consequence to give a fresh description 

 of it accompanied by drawings, which might enable artists 

 who undertook its construction to render it comparable. He, 

 however, thought it right to give a previous account of the 

 labours of others in this branch of science, and to remove 

 certain errors which had prevailed up to that time concerning 

 the pyrometer then most in use (Wedgwood's), and which 

 mio-ht possibly prove most commodious, and consequently 

 most useful, if once the degree of exactitude could be deter- 

 mined of which it was susceptible." The remainder of the 

 paper is taken up with an account of the most accurate expe- 

 riments upon the expansion of the metals from the time ot 

 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 comparison, 



1st, of the indications of the platinum pyrometer with those 

 of the mercurial themometer ; 



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 m 

 this paper are directed to the valuation of the degrees deter- 

 mined by Mr. Wedgwood's clay pieces; but that he carries 

 the comparison of the platinum pyrometer by actual experi- 

 ment no higher than the melting point of antimony. He 

 clearly establishes 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 

 jusquc dans les plus hautes temperatures." The experimental 

 comparison was obviously 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 becomes very 

 soft and ductile ; and the lever against which the pyrometric 

 N. S. Vol. 10. No. 57. Sejd. 1831. 2 C bar 



