5 Enne u, Be Ms 1 Les 
(2, = 09) = ey m, + m 1 jn ene 
m, tm, 
u, [An L Melt 2 1 1 [7 2 
 8n wo? m, “1.188 me 3x no? 1.188 Wah mm, 
The symmetry of this expression shows that exactly the same 
value holds for n,=0. The form of D also agrees with STEFAN'’S 
expression: the coefficients are in the relation of 1:1.05; therefore, 
considering the approximate character of the deduction, there is 
practically complete agreement. 
For intermediate compositions the difference between the two 
expressions for D becomes material only when m, and m, are very 
different. This is probably due to the method of calculation which 
compels us to work with averages from the beginning. Moreoyer 
JwanNs’s method of calculating the persistence is not rigorous: it might 
perhaps be found possible by applying more rigorous methods to 
reduce the remaining difference between Mryrr's corrected formula 
and the other one. As a matter of fact the object of this paper was not 
so much to deduce a correct formula, considering that the near 
accuracy of LaNGevIN's method cannot well be doubted, as to remove 
the strong contradiction between the two results. 
In conclusion it may be added, that the method which is indicated 
in this paper can immediately be used to deduce rational formulae 
for the viscosity and the conduction of heat for gas mixtures. 
Mathematics. — “On bilinear null-systems.’ Communicated by 
Prof. JAN DE VRIEs. 
(Communicated in the meeting of January 25, 1913). 
§ 1. In a bilinear null-system any point admits one null-plane, 
any plane one null-point. The lines incident with a point and its 
null-plane are called null-rays. If these lines form a linear complex, 
we have the generally known null-system, which is a special case 
of the correlation of two collocal spaces (null-system of Méstus). 
The null-rays of any other null-system (1,1) fill the entire space of 
rays; with R. Srurm we denote by y the number indicating how 
many times any line is null-ray. 
In the first we suppose y= 1 and we examine the null-systems 
which may be called #rilineur and which can be represented by 
(A) 
