274 Mr. Daniell oti a Neiv Register-Pyrometer^ 



register . for the 1 50°, from the greater expansion previously 

 determined by the boiling point of mercury for 



596° : -00831 : : 150° : -00209 

 which only differs Too.Wo^tl^s of an inch from the above 

 mean. 



Exp. 4. It was a principal object to ascertain whether any 

 and what difference existed in the expansion of different spe- 

 cimens of the black-lead earthenware : two or three registers 

 which I had cut out of the same crucible gave me almost 

 identical results by exposure to boiling mercury. I then se- 

 lected another specimen by a different manufacturer. Its 

 grain was very fine, and its texture more close and compact 

 than the former. It was twice exposed with the platinum bar 

 to boiling mercury. The first time it was boiled for a quarter 

 of an hour, and the arc measured was 1° 45'. The second 

 time the boiling was continued for only ten minutes, and the 

 reading was precisely the same. The expansion was there- 

 fore -01526. 



Absolute expansion as before -01947 



Apparent expansion -01526 



Expansion of black-lead -00421 



Exp. 5. The same register of the fine-grained black-lead 

 was exposed for a quarter of an hour with the iron bar to 

 boiling mercury : the arc measured on the scale was 2° 49' 

 = expansion -02457. 



Absolute expansion as before -02914 



Apparent expansion , -02457 



Expansion of black-lead... -00457 



Fine-grained black-lead by platinum -00421 

 by iron -00457 



Mean... -00439 

 the two experiments differing from the mean by less than 

 ^-5-%-5^dths of an inch. This shows that the fine-grained ware 

 expands less than the coarser, and proves the necessity of 

 ascertaining the expansion of each register for itself by boiling 

 in mercury ; at least till some means be taken to insure their 

 uniform composition. Every register should also be marked 

 with a reference to its proper expansion ; and I would recom- 

 mend all those who may use the instrument for delicate re- 

 searches, to verify this point for themselves ; as they may easily 

 do with the apparatus before described, 



Exp. 6. The expansion of the last specimen of black-lead 



ware 



