427 
that of the experiments, and the explanation proposed seems 
sufficient to account for all the results. 
If the heating is due to friction, the amount was shown 
to be about -0096 Ibs. per square feet, and that this would 
produce an alteration in the length of the day of not less 
than 006” in a century. 
(2) On the Equilibriwm of Heterogeneous Substances. 
By Prof. Cumrk Maxwett. 
The thermodynamical problem of the equilibrium of hetero- 
geneous substances was first attacked by Kirchhoff in 1855, who 
studied the properties of mixtures of sulphuric acid with water, 
and the density of the vapour in equilibrium with the mixture. 
His method has recently been adopted by C. Neumann in his 
Vorlesungen tiber die mechamsche Theorie der Wirme (Leipzig, 
1875). Neither of these writers, however, make use of two of 
the most valuable concepts in Thermodynamics, namely, the 
‘intrinsic energy and the eutropy of the substance. 
It is probably for this reason that their methods do not 
readily give an explanation of those states of equilibrium which 
are stable in themselves, but which the contact of certain sub- 
stances may render unstable. 
I therefore wish to point out to the Society the methods 
adopted by Professor J. Willard Gibbs of Yale College, published 
sn the Transactions of the Academy of Sciences of Connecticut, 
which seem to me to throw a new light on Thermodynamics. 
He considers the intrinsic energy (¢) of a homogeneous mass 
consisting of m kinds of component matter to be a function of 
n+ Z variables, namely, the volume of the mass v, its eutropy 7, 
and the n masses, ™,, 1,---M,, of its component substances. 
Each of these variables represents a physical quantity, the 
