198' Mr. Daniell on a New Register-Pt^rbmeter 



giiished philosophers with copper, the only other solid metal 

 to which they extended their inquiries. 



Previously to this, I trust it may not be thought tedious, if 

 I briefly relate the results of some trials for obtaining registers 

 of uniform composition, which might preclude the necessity of 

 determining the rate of expansion in each individual instance. 



Exp. 23. — For this I had recourse to Wedgwood's ware, of 

 which I obtained some bars carefully constructed and highly 

 baked for the purpose. The expansion of these I found pre- 

 cisely equal to that of platinum ; so that when 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 pla- 

 tinum, 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 I 45 



the remainder 1 4 



is sufficiently 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 ex- 

 pansion 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 measures 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 foui'th, 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 pur- 

 poses, therefore, the mean expansion of 1° 42' might have 

 been adopted without any serious error in the final results. 

 In investigations, 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 re- 

 gisters 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. 



