FOR MEASURING THE EXPANSION OF SOLIDS. 
275 
The agreement of this second series with each other is quite as close as that 
of the first ; and it is worthy of remark, that the greatest variation from the 
mean is in both cases with the iron in deficiency, and nearly to the same 
amount of one half. It is not unlikely, therefore, that there may be some 
error in estimating the absolute dilatation of this metal, which is probably 
something greater than we have assumed. 
If we estimate the expansion for these 150° to the boiling point of water 
from the result obtained by the boiling of mercury, we shall have the following 
proportion:— 596° : .00439 :: 150° : .00110 
which does not differ quite ^Igodths of an inch from the foregoing mean. 
Having thus, I trust satisfactorily, established the accuracy of the pyrometer, 
and the degree to which confidence may be placed in its indications, I shall 
conclude this part of my subject with the details of some experiments upon the 
fusing points of different metals. I shall designate the registers of coarse and 
fine-grained black-lead respectively by the letters A and B. 
Exp. 7. About 30lbs. of the clippings of thin sheet copper were very gradually 
melted in a crucible in the blast furnace of the Royal Institution. The platinum 
bar was adjusted in the register B, and when the metal was about half run 
down, it was placed perpendicularly with the index upwards in the crucible, and 
held down with a pair of tongs. The crucible was then gradually fed with the 
clippings till the melted metal covered about two-thirds of the register. In this 
situation it was kept ten minutes, and when it was lifted out some of the metal 
remained unmelted. A crust of oxide, mixed with metal, had also affixed itself 
to the upper part of the black-lead. This was partially dissolved away and 
loosened by immersing the register with great care, when cold, in a diluted 
mixture of sulphuric and nitric acids. The whole was thus easily removed, 
and the black-lead exhibited a perfectly clean surface. The arc measured 
upon the scale was 5° 49', denoting an expansion of .0508. The temperature 
of the laboratory was about 65°. 
I am indebted to the kindness of Mr. Mathison for unexceptionable oppor- 
tunities of taking the melting points of gold and silver at the Royal Mint, who 
also most obligingly assisted me in the operations. Two new registers were 
prepared, which I shall designate as II and III : their rates of expansion were 
not determined till after the experiments. 
Exp. 8. The register II was carefully adjusted with the platinum bar. 
