444 
MR. DANIELL ON A NEW REGISTER PYROMETER 
the register was immersed in boiling mercury, the index was found not to 
have moved. When a bar of iron was substituted for that of platinum, the 
arc measured was 1° 7 '> 
With black-lead the same expansion gave a measure of 2° 49', from which 
if we deduct the expansion of platinum in black-lead . 1 45 
the remainder .14 is suffici- 
ently near to confirm the result. 
Exp. 24. My next trial was with registers of black-lead of various and 
known mixtures of plumbago and Stourbridge clay. Four fifths proportion 
of the former to one fifth of the latter produced a composition which was too 
tender for the purpose ; but a mixture in the proportion of three fourths to one 
fourth formed a ware of a fine, even texture ; whose expansion was very equal, 
and not exceeding the least of those which I had formerly tried. 
Three different registers of this composition afforded me the following mea- 
sures of the expansion of a platinum bar to the boiling point of mercury. 
1° 45' 
1 42 
1 38 
To which I may add a fourth, which gave for the expansion of an iron bar to 
the same point an arc of 2° 42', which is equivalent to 1° 40' for a platinum 
bar. For all common purposes, therefore, the mean expansion of 1° 42' might 
have been adopted without any serious error in the final results. In investiga- 
tions, however, which require the utmost precision, I still think it advisable 
to fix the expansion of each register by experiment. 
Exp. 25. A bar of copper was adjusted in one of the registers and exposed, 
in the manner formerly described, to boiling mercury ; the arc measured on 
the scale was 4° 10', equivalent to an expansion of .03633. 
Let us now compare this result with the determination of MM. Dulong 
and Petit, as we formerly did the expansions of platinum and iron. 
