CHEMISTRY: W, D. HARKINS 
541 
15% less than the thermodynamic entropy calculated for the area oc- 
cupied by one molecule. The entropy which corresponds to the value 
given above is that which is calculated for the area of surface occupied 
by one molecule on the supposition that the arrangement of the molecules 
is such as to give approximately the same number of molecules in the 
surface as if it were built up on the plan of a cubic lattice, and the area 
for which the entropy is calculated is always that occupied by one 
molecule no matter what the temperature may be. The difference 
between these two entropies will be discussed later, but it is not essential 
since either of them shows the constancy required hy the relation under dis- 
cussion. When the extent of the molecular orientation in the surface 
is known it should be taken into account. 
Relation of the entropy law to molecular association. — Although the 
entropy principle presented in this paper was discovered by the writer 
not more than a few years ago, a number of empirical relations which 
involve its validity without its recognition by their discoverers, have 
been well known for many years. The empirical relation directly related 
to the form of the entropy principle already given, is known as the law 
of Eotvos,^ Ramsay, and Shields.^ While a part of this relation was 
developed by Eotvos from a special form of the theorem of corresponding 
states, in its final form, as given by Ramsay and Shields, it may be con- 
sidered as purely empirical, and it is this latter form which is given by 
the entropy principle. Indeed, though it does not seem to have been 
recognized by them, Ramsay and Shields relation gives the entropy of a 
surface, though in very peculiar units, and also the entropy expressed 
by it is not the ordinary thermodynamic entropy. 
The thermodynamic entropy may be expressed in ergs per degree per 
square centimeter, or in other units. The Ramsay and Shields relation 
may be expressed in terms of entropy, if a somewhat unusual unit is used 
for the measurement of the area, in the following terms : The entropy of 
the surface of a liquid is 2.12 ergs per degree for an area which is equal to 
the area of one face of a cube which contains one gram molecule of the liquid. 
Since the volume of such a cube varies with the temperature, the area 
to which the entropy is referred also varies with the temperature. 
The value in the use of such a pecuKar system does not lie in the use 
of the same num_ber of molecules in the box in every case, but in the use 
of the area of one face of the box, the inherent idea being that in this 
way the same number of molecules in the surface are obtained in every 
case when the molecules in the surface are counted by considering that 
their complexity in the surface to be the same as that inside the liquid. 
