FOR MEASURING THE EXPANSION OF SOLIDS. 
449 
Now by these results, the accuracy of the pyrometer may, again, be placed 
beyond doubt, in a manner which was perfectly unforeseen at the time of in- 
stituting the experiments. 
In the first place we have two metals, tin and lead, whose melting points 
being within the temperature of boiling mercury, have been accurately deter- 
mined by the common thermometer. Upon calculating the same points from 
their several expansions to boiling water, measured by the pyrometer, upon 
the supposition that they maintain the same rate to their points of fusion, the 
temperature of the first comes out 29°, and of the second 58°, higher : that is to 
say, the rate of expansion of these two metals increases with the increase of 
temperature, as has been found to be the case with platinum, iron and copper, 
by the experiments of MM. Dulong and Petit. It is worthy of remark, that 
this increased rate in tin is equivalent to 29° in about 200°, and in lead to 58° 
in about 400°, above the boiling point of water. These results therefore indi- 
cate a very close agreement between the thermometer and pyrometer. 
2ndly. The melting point of the next metal, zinc, is one of those which has 
been determined by immersion of the pyrometer into it, when it was in the act 
of fusion. Its temperature, so determined, falls short of the same point, cal- 
culated from the expansion supposed equal, by 75°. This again indicates an 
expansion increasing at nearly the same rate (7 5° in 560°), as in the preceding 
instances of tin and lead. I pass over at present the result obtained by calcu- 
lating from the expansion to the boiling point of mercury, as it presents an 
anomaly upon which I shall presently make some observations. 
3rdly. The melting point of silver, determined in the same way by immer- 
sion, differs from that calculated from expansion in the same direction ; and 
the difference (286° in 1660°) is nearly in the same proportion. The calcula- 
tion from the rate of expansion to the boiling point of mercury comes much 
nearer to the melting point directly determined, and only differs from it 176°: 
proving that the rate of expansion increases with the increasing temperature. 
4thly. A similar comparison instituted with copper presents us with a rate 
of expansion increasing much more rapidly than in the preceding instances ; so 
that the melting point, calculated from the expansion to boiling water, differs 
from the true melting point no less than 1266°. Taking the rate of expansion 
to boiling mercury, the difference is reduced to 370°. And here again I may 
3 m 2 
