280 
MR. DANIELL ON A NEW REGISTER-PYROMETER 
point of gold 35°, and in that of silver only 29°; and this excess is in accord- 
ance with the conclusion of MM. Dulong and Petit, exhibited in Table II., 
that the expansion of iron increases in the higher degrees in a greater propor- 
tion than that of platinum. 
The discrepancy between the temperatures derived from the observations 
with my first pyrometer and the present are considerable, but may be suffi- 
ciently accounted for by the differences in the circumstances of the experi- 
ments, without imputing inaccuracy to the instrument. In the paper to which 
I have before alluded, I stated that “ I did not offer the results as positive and 
accurate determinations of the different degrees, but only as nearer approxi- 
mations than any that had yet been furnished from actual observation. The 
only method which I had it in my power to adopt for the purpose, I do not 
consider to be susceptible of absolute accuracy. The arrangement made con- 
sisted of a muffle of black-lead placed in an excellent draught-furnace. This 
muffle was furnished with a door, through a round hole in which the stem of the 
pyrometer w r as passed up to the shoulder. Another door, which could be 
stopped at pleasure, admitted a full view of the interior. The metal to be 
tried was placed in a small black-lead receptacle, of the same thickness as the 
pyrometer tube, in the middle of the muffle. Now it is evident that the 
pyrometer so situated would indicate the mean heat of the wdiole of the muffle; 
wdiich heat might, and did, vary in different parts. Of tw r o pieces of silver 
of the same size placed within an inch of each other, one fused some time 
before the other.” I also suggested that “ means might be contrived to sur- 
round the instrument with the metal in a state of fusion ; but that it required 
particular opportunities, which it w r as to be hoped that those w r ould avail 
themselves of who had them in their power.” 
That the latter method is the only one which can admit of accuracy will be 
evident from a few reflections. Setting aside the inequality of the heat of 
different parts of the same heated muffle, which however is a consideration of 
the utmost importance, it is obvious that its temperature must considerably 
exceed the true melting point of the metal exposed to its influence. Just as a 
piece of ice would never melt in a chamber of the temperature of 32°, but 
would require a considerably higher heat in proportion to its mass to supply 
the caloric which becomes latent during the process, — a mass of iron would 
