PHYSICS: F. G. KEYES 
325 
with experiment and the sign of I comes out positive and not negative, 
as there is good reason to suppose it should, increased faith in the vaHdity 
of the fundamental thought will have been established. 
The term (v — b) of van der Waals has given rise to much discussion. 
It now appears that a rigorous application of van der Waals' mode of 
thought really leads to a different expression for the Volume correc- 
tion.' It is, however, (v — b) which seems to be the corrrect form as 
judged by comparison with experiment. With potential forces existing 
between the particles, it is difficult to see how collisions occur in the 
sense of the classical kinetic theory, at any rate within the mass of the 
gas. Indeed for an assemblage of particles such as here imagined a 
collision, as conceived by the older theory would not occur in general, 
except possibly at the boundary. It would, however, still be necessary 
to take account of the finite size of the atoms or rather more Hkely a 
certain space the center of which was occupied by an atom. In the 
case of a monatomic gas this volume would be fixed and the b of van der 
Waals therefore a constant. The equation of state for a monatomic 
gas at those volumes and temperatures where association is excluded 
would accordingly be 
V — ^ [v — iy 
where jS, a, I are constants and R is the absolute gas constant defined 
hy R = pv/T, the equation of a perfect gas. 
When two of the model atoms are united to form a diatomic molecule 
an effect comes into play which is absent so long as monatomic particles 
are alone considered. A diatomic molecule is assumed to be held to- 
gether by forces due to the positive and negative charges and the mag- 
netic effect of the field due to the rotating electrons. The distance 
apart of these diatomic molecules in general will be large compared to the 
distance separating the two atoms forming the di-atom; but in an assem- 
blage of these di-atoms, the field due to the assemblage must react to 
vary the distance separating the two atoms of a diatomic molecule. 
This would be inferred from the fact that for a magnetic shell in a mag- 
netic field the reaction is always such as to bring about a decrease in the 
mutual potential energy. This decrease in the molecular case under 
consideration would result in a shortening of the distance between the 
two atoms of the di-atom or a decrease in its apparent volume as the 
strength of the field due to the surrounding di-atoms increased. It ap- 
pears at present an impossible task to calculate exactly the apparent 
volume corresponding to van der Waals' b from such a physical picture. 
