452 
MR. DANIELL ON A NEW REGISTER-PYROMETER 
mg' mercury affords a result lower than the true. This might afford some 
grounds for conjecturing that, although the rate of expansion evidently in- 
creases beyond the temperature of boiling water, it does not continue to in- 
crease to the end ; but there is another inference from the fact, which I am 
rather inclined to adopt. 
In calculating the temperature of melting cast iron, from the expansion of 
the platinum bar, I applied a correction, upon the supposition that the same 
rate of increase of expansion which was exhibited by platinum between the 
boiling points of water and mercury continued to the higher degrees ; whereas 
there is great reason to suppose that the rate must be an increasing <one ; and, 
although this might not sensibly affect the final result of the comparatively 
low temperature of melting silver, the calculation of the temperature of melt- 
ing iron, which is more than one third higher, would be sensibly affected by 
it. I think it therefore extremely probable that the true temperature of melt- 
ing cast iron is below 2786°. 
The consistency of these results will, I trust, remove any doubts as to 
the competency of the pyrometer to determine fixed and comparable points 
of very high temperatures, and induce those connected with arts and manu- 
factures to introduce its use, for the purpose of ascertaining many questions of 
the highest interest, both to practical and theoretical science. The experiments 
just detailed upon bars of wrought iron remove even the only trifling objection 
which could be brought against its general use ; namely, the expense of a plati- 
num bar : for it is quite proved that a bar of wrought iron is sufficient for every 
practical purpose, and it affords the important additional advantage of a much 
more open scale. 
I proceed now to remark that zinc, as well as iron, appears by the Tables 
to present an exception to the law of an increasing rate of expansion with 
increasing temperature ; the expansion for the 600° to boiling mercury not 
being so much as four times that for the 150° to boiling water. I cannot, 
however, from some peculiar circumstances attending the experiment, place 
entire confidence in the result. When, after boiling in mercury, the register 
was opened, the vapour was found to have gained admittance, and to have 
acted upon the zinc. It was firmly fixed in the cavity, and was not removed 
without considerable difficulty and piecemeal. At its upper end, the bar was 
