356 Mr. Daiiiell 07* a New Register-Pyrometer. 



for ten minutes in mercury. The arc measured was 1° 19^ = 

 expansion -01 148 : the difference of \' may safelv be ascribed 

 to the uncertainty of the reading. 



The temperatures thus determined will require correction, 

 if we adopt the conclusion derived from the experiments of 

 ]MM. Dulong and Petit,— that the dilatability of solids, re- 

 ferred to an air-thermometer, increases with the heat. The 

 amount of this correction will be as the rate of increase; and 

 according to those gentlemen is ll°-6 of the centigrade'ther- 

 mometer, or 20°-8 Fahr. from 32^ to 572^ or th^calculated 

 temperature is to the true as -00091827 : •00088420. Sup- 

 posing the increase of dilatability to continue the same for 

 equal intervals of temperature, which however has not yet 

 been proved, the following Table will exhibit the corrected 

 temperatures derived from the preceding experiments with 

 the platinum bar. ^^ _. . 



1 ABLE XI. r>u 1 r. 1 



Observed. Corrected. 



Melting point of silver 1942 ... 1873* 



copper 2070 ... 1996 



"old 2091 ... 2016 



iron 2889 ... 2786 



Temperature of the maxi- > , . 



mumofexpansionof...5Pl''^^^""'"- ^^^l ... 3280 



If we reason in the same way from the increase of the dila- 

 tation of iron, as laid down by the same authors, the discre- 

 pancy between the temperature derived from the platinum and 

 iron is very considerable ; the melting point of silver coming 

 out 1682°," and that of gold 1815° by the latter: but I con- 

 ceive that the determination of this point in the iron is open 

 to objections which do not apply to the platinum, and my 

 suspicion is confirmed by the anomalous expansion of the iron 

 exhibited in Tables V. and IX., and to which I shall recur 

 upon a future occasion. 



The general utility of the pyrometer, however, will in no 

 way be affected by any uncertainty in these corrections. The 

 indications which it is capable of affording will always be 

 positive determinations, which it will be easy to modify by 

 calculation, as our theories may improve. For all common 

 purposes (and I must own that I look forward with hope that 

 this instrument will prove eminently useful in many of the 

 common processes of the arts) it will not even be necessary to 

 note the expansion indicated by the arc measured ; but each 

 minute of the degree may at once be valued in degrees of 



* Mr. Prinsep, from a laborious series of experiments upon the expan- 

 sion of air confined in a bulb of gold, determines the meltins point of silver 

 to be ]830'.-l'hil. Trans. 1828. p. 94. 



Fahrenheit's 



