FOR MEASURING THE EXPANSION OF SOLIDS. 
273 
Exp. 4. It was a principal object to ascertain whether any and what differ- 
ence existed in the expansion of different specimens of the black-lead earthen- 
ware : 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 selected 
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 therefore .01526. 
Absolute expansion as before 01947 
Apparent expansion 01526 
Expansion of black-lead 0042 1 
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 jp^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 com- 
position. Every register should also be marked with a reference to its proper 
expansion ; and I would recommend all those who may use the instrument for 
delicate researches, to verify this point for themselves ; as they may easily do 
with the apparatus before described. 
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