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But not only for the latent heat it is of importance to keep apart 
the two cases, viz. space between the molecules greater or smaller 
than the dimensions of the molecule itself — it is also of import- 
ance for other phenomena. For example, friction, diffusion ete. It 
is not my purpose to demonstrate this myself in detail -— the more 
so because it has already been pointed out by different observers 
that these phenomena follow different laws for liquids and gases. 
But I will particularly call attention to a paper by BartscHinsky, 
who already according to a communication in the Journal de 
Physique 1914 tried to explain the diffusion for liquids not by the 
thermal movement, but by the attraction of the molecules, hence 
by the quantity a of the equation of state, and who has given a 
view which is most probably at bottom essentially analogous to the 
above '). 
That in the title I have put the transition of the twin cases at 
the critical density, holds of course only for normal substances, so 
without association. But in general the transition case is that for 
which the distance of the molecules is equal to the dimension of 
the molecule. Before we have got perfect certainty about this, a 
thorough investigation also above the critical temperature would be 
necessary. This would probably be much more difficult, and I hope 
that also other investigators will show an interest in this research. 
Astronomy. — “Planetary motion and the motion of the moon 
according to Einstein's theory.” By Dr. W. pr Sitter. 
(Communicated in the meeting of June 24, 1916). 
1. The gravitational field of the sun. 
In Einstein's new theory gravitation is determined by 10 quantities 
Yar, Which are given by the differential equations *) 
a) Gah. Doper ee ae a 
These equations are invariant for any arbitrary transformation of 
the “coordinates” «v,...2,, by means of which the phenomena are 
described. It is an essential feature of HiNsrein's theory that the 
1) [ have only read a short review in the Journal de Physique, but | think that 
I may conclude from it that BATscHinsKy finds back the quantity 5 of the equation 
of state as characteristic quantity. This would be a corroboration of my opinion 
that the attraction of the molecules is only exerted in case of perfect contact. 
2) Einstein: Die Feldgleichungen der Gravitation, Sitzungsber. Berlin, Nov. 
1915 page 845, formula (2u). It is easily found that G= «7. 
