FOR MEASURING THE EXPANSION OF SOLIDS. 
285 
The amount of this correction will be as the rate of increase ; and according to 
those gentlemen is 1 1°.6 of the Centigrade thermometer, or 20°.8 Fahr. from 32° 
to 572 °, or the calculated temperature is to the true as .00091827 : .00088420. 
Supposing 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. 
Table XI. 
Melting point of Silver 
Copper 
Gold 
Iron 
Temperature of the maxi- 
mum of expansion of 
} 
Platinum . 
Observed. 
1942 
2070 
2091 
2889 
3401 
Corrected. 
1873* 
1996 
2016 
2786 
3280 
If we reason in the same way from the increase of the dilatation of iron, as 
laid down by the same authors, the discrepancy between the temperature de- 
rived 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 conceive 
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 Fahrenheit’s scale 
at the time of taking its rate of expansion by the boiling of mercury: and a 
* Mr. Prinsep, from a laborious series of experiments upon the expansion of air confined in a bulb 
of gold, determines the melting point of silver to be 1830°. — Phil. Trans. 1828. p. 94. 
