450 
MR. DANJELL ON A NEW REGISTER-PYROMETER 
refer to the experiments of MM. Dulqng 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 true temperature at 572° Fahr. 
5thly. The interesting nature of the results which I obtained with iron, and 
the peculiar difficulties in arranging the experiments from which they were 
derived, will, I trust, excuse my entering more into their details than I have 
thought necessary in the preceding instances. I have already given the expan- 
sion of wrought iron to the temperatures of boiling water and boiling mercury, 
and shown that the measures obtained with the pyrometer agree essentially 
with those determined 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 expansion of platinum. I was extremely anxious to com- 
plete this series of experiments by measuring the expansion of iron to its melt- 
ing point. For this purpose I had a small bar of iron 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, I found the arcs upon the scale respec- 
tively 0° 29' and 2° 25' ; and this being considerably less than what I had 
obtained with the bar of wrought iron, I repeated the experiment with the latter 
in the same register that I had employed for the former, and obtained the 
measures of 0° 35' and 2° 44' — nearly agreeing with the previous determination: 
so that there can be no doubt that cast iron expands less than wrought iron, 
though the rate of increase for the higher temperature appears to be the same 
in both. 
I now arranged the two bars in two registers ; and having strongly heated 
the furnace and filled the air-chamber itself with coke, I cleared out a space 
in which they could be placed, without coming in contact with the fuel on 
each side of them. Their two ends rested on pieces of fire-brick ; the wrought 
iron was placed lowest, and, the thickness of the register, in advance of the 
cast iron; which was placed about two inches higher. The apertures were now 
all closed, and the draught increased to the utmost. At the expiration of a 
quarter of an hour the register with the cast iron was removed with a pair of 
tongs ; and the metal upon lifting it, immediately flowed out at the two end 
