359 
Deviation of the Middle thread. 
KAM HENNEKELER 
Pericd | — 15 — 42 
I] al — 38 
Va 54 — '§4 
VI A ates 
For Kam the value is distinctly smaller in the first period, whereas 
in the case of HENNEKuLER there is no distinct evidence of a change. 
Physics. — “The Increase of the Quantity a of the Equation of 
State for Densities Greater than the Critical Density’. By 
Prof. J. D. van per Waals. 
(Communicated in the meeting of June 24, 1916). 
Already in 1873 when drawing up the equation of state I realised 
that it must follow from the derivation from the kinetic theory 
that the quantity 4 would have to decrease with diminishing volume. 
Accordingly I stated explicitly already then that 5, which represents 
4-times the molecular. volume at infinite volume, would have to 
decrease. Afterwards I came to the opinion that it would have to 
diminish to twice that volume, or even to a still smaller value. 
And that 6 decreases is pretty generally accepted at present. The 
cause of this decrease is, however, in my opinion, often sought in 
a wrong direction, namely in the real diminution of the molecule. 
I will not return to this point at present. Not until 1910 did 1 
express my doubt of the invariability of the quantity a (These Proc. 
XIII p. 107). I aseribed the variation of this quantity to what [| 
called then: “quasi association or molecule complexes”. That I looked 
for the cause for this increase of « in what acts as an enlargement 
of the molecule, as is the case for real association, I still consider 
correct. But I treated these possible complexes approximately as if 
we had to do with real association, this [ should certainly not 
do now. In subsequent calculations in the course taken then I 
became more and more eonvinced that the result could not be valid 
even as an approximation. And in the following pages | will unfold 
the idea, which has more and more forced itself upon me, accord- 
ing to which the cause for this increase of a must be sought with- 
out molecule complexes being necessary, but as a consequence of 
the ordinary regular molecular movement of molecules which have 
