CHEMISTRY; WASHBURN AND READ 
193 
ice taken from an air bath at — 2° where it was suspended on a silk thread. 
As an additional check on the water-equivalent of the calorimeter the 
first operation was then repeated, using however test-tubes containing 
naphthalene instead of diphenyl. The heats of fusion of ice and of 
naphthalene being known, that of diphenyl could be readily calculated 
from these experiments. It was thus found to be 4020 calories per mole. 
The freezing-points of the pure substances and the eutectic points of 
the three solutions were determined by the equilibrium method. The 
apparatus consisted of a Baudin thermometer standardized by the 
National Bureau of Standards and a small silvered cylindrical vacuum- 
tube which was heated (or cooled) to the required temperature previous 
to beginning the experiment. For temperatures higher than that of the 
room, the crystals (pure substance or eutectic mixture) were melted in 
a small casserole, and the Hquid (about 25 cc.) was poured into the 
vacuum-tube and the thermometer inserted. When the temperature of 
crystallization had been reached more of the crystals were added, after 
which the thermometer reading soon became constant and remained so 
(within less than 0.1°) for ten or fifteen minutes, with constant stirring 
and tapping of the thermometer. For temperatures lower than room- 
temperature the soKd crystals (previously cooled, in the case of the eu- 
tectic mixture) were placed in the vacuum-tube, and equilibrium be- 
tween the crystals and liquid in contact with the thermometer bulb 
was obtained as before. In this way the three melting-points and the 
three eutectic temperatures could be easily measured with an accuracy 
of at least 0.1°. The constancy of the three melting-points within this 
limit is sufficient criterion of the purity of the materials employed. 
In order to calculate the eutectic temperatures from the freezing- 
point equations given above, these equations were first integrated on 
the assumption that L is independent of T for the ranges involved. 
This assumption is justified in the case of benzene and naphthalene by 
the small differences between the specific heats in the solid and Hquid 
states. In the case of diphenyl specific-heat data are not available, but 
the assumption seemed justifiable by analogy. The two integrals are: 
where Tqa and Tob are the absolute melting-points of the pure sub- 
stances. By solving these two equations simultaneously (with the aid 
of the axiom, + — 1) values of the eutectic temperature T were 
obtained. 
The melting-points of the pure substances were found to be: Benzene, 
2.303 logioO^A = — 
RTo^T 
; 2.303 logio = — 
L,{To,-T) _ 
RT^T ' 
