mth his New Register-Pyrometer. 203 



from the expansion supposed equal, by 75°. This again in- 

 dicates an expansion increasing at nearly the same rate (75° 

 in 560°), as in the preceding instances of tin and lead. I pass 

 over at present the result obtained by calculating from the 

 expansion to the boiling point of mercury, as it presents an 

 anomaly upon which I shall presently make some observa- 

 tions. 



Srdly. The melting point of silver, determined in the same 

 way by immersion, differs from that calculated from expan- 

 sion in the same direction ; and the difference (286° in 1660°) 

 is nearly in the same proportion. The calculation from the 

 rate of expansion to the boiling point of mercury comes much 

 nearer to the melting point directly determined, and only dif- 

 fers from it 1 76° : proving that the rate of expansion increases 

 with the increasing temperature. 



4thly. A similar comparison instituted with copper presents 

 us with a rate of expansion increasing much more rapidly 

 than in the preceding instances ; so that the melting point, 

 calculated from the expansion to boiling water, differs from 

 the true melting point no less than 1266°. Taking the rate 

 of expansion to boiling mercury, the difference is reduced to 

 370°. And here again I may refer to the experiments of 

 MM. Dulong and Petit in confirmation of the result; for they 

 found that the temperature indicated by the expansion of a 

 rod of copper was 50° Fahr. higher than the ti'ue temperature 

 at 572° Fahr. 



5ih\y. The interesting nature of the results which I ob- 

 tained with iron, and the peculiar difficulties in arranging the 

 experiments from which they were derived, will, I trust, ex- 

 cuse my entering more into their details than I have thought 

 necessary in the preceding instances. I have already given 

 the expansion of wrought-iron to the temperatures of boiling 

 water and boiling mercury, and shown that the measures ob- 

 tained with the pyrometer agree essentially with those deter- 

 mined by very different means by MM. Dulong and Petit. 

 I have also proved that the melting points of gold and silver, 

 determined by the expansion of the same bar of iron, agreed 

 very closely with the same points determined by the expan- 

 sion of platinum. I was extremely anxious to com))lete this 

 series of experiments by measuring the expansion of iron 

 to its melting point. For this purpose I had a small bar of 

 ircm cast from the best gray iron, and afterwards cleaned of 

 all oxide and reduced to the size of the other bars employed 

 by filing. Upon measuring its expansion to the temperatures 

 of boiling water and boiling mercury, 1 found the arcs upon 

 llie scale respectively ' 2;/ and 2-' 25' ; and this being ton- 

 2D2 



