1131 
: 
The observations were made in the second order spectrum of a 
large ROWLAND grating. 
The red of the second order is superposed on the blue of the 
third order so that the line 6708 is seen in the. absorption spectrum 
as a blue line. With small vapour density the line resolved into 
two components; this proves that the conclusion drawn from the 
analogy of the spectrum series of the alkali metals is true. That 
component of the double line which has the smaller wavelength 
seemed to be the most intense. The distance between the components 
could only be measured in a roundabout manner by means of a 
divided scale in the eye piece of the spectroscope. This measurement 
gave for the distance between the components about one fourth of 
an Angstrém unit. From the empirical rule that in the case of the 
elements of the same family the frequency differences of the pairs 
are nearly proportional to the square of the atomic weights, it would 
follow that for lithium this distance ought to be 6 54 
Angstrom units. The observed distance is much smaller. 
Physics. — “Some remarks on the course of the variability of the 
quantity b of the equation of state.” By Prof. J. D. vaN DER 
Waals. 
(Communicated in the meeting of January 25, 1913). 
In my preceding communications I came to the conclusion 
that the differences which occur in the normal, not really associating, 
substances are to be ascribed to the different value of the quantity 
b ED B 
ae As this quantity is greater, both f and s are greater, viz. 
lim 
1 b 8 b en za 
meet and s = — "The deviation exhibited by the law 
3 blim 3 Diim 
of corresponding states, is also a consequence of the different course 
of the quantity 6. Thus it becomes more and more clear that every- 
thing that can contribute to elucidate the cause of the difference in 
this course must be considered of the highest importance. 
If the course of & is traced as function of v, a line is obtained 
which runs almost parallel to the v-axis with great value of r, and 
approaches asymptotically to a line parallel to the v-axis at a distance 
6, from the latter. Not before v= 2b, does an appreciable difference 
begin to appear, and has the value of 6 descended to e.g. about 
