971 
tinuously varying current of incoherent material points or for a more 
general case in which there are acting certain molecular forces 
between the points. 
For an ideal gas V—g R will be the sum of the elements of the 
world-lines described per unit of time by the molecules present in 
a unit of volume, each element multiplied by — m, if m is the 
mass of the molecule that describes the element. 
Now it is known that for a molecule with the mass m the 
momentum is given by 
di 
ta = — m & (b) gas bass 
ds 
for a=1, 2,3, and that the energy is —z,. For an ideal gas the 
expressions for the stresses, the momentum and the energy per unit 
of volume and the energy-current can be written down directly. 
Without entering into details by introducing a distribution function 
I only give the table of notations 
pet, Vet WI Vogt XX, Xe 
VIT Vaal! Vor! Var Fe PF, 
dn ed WIT Vg Tt (=) Zz Zij Zels 
Cee IV el At Sx Sy Sz E. 
Here the coordinates w, y, z and ¢') are supposed to be used. 77 
is a mixed tensor. It may be called the dynamucal tensor. It is not 
symmetrical. The covariant tensor 
I ee = (m) YImb ep 
on the contrary is symmetrical. 
It may be remarked that the sum of the diagonal components is 
equal to 
(2) 
B) WV —9 Ts = — WIR. 
5. The contribution to L of the electric current and the electro- 
magnetic field may be divided into two parts, 2V—-g SandAV—g M, 
4 being the same constant as in $ 1. 
For V—gS de,de,de,de, we take the extension of the element 
= (eg dx that occurred in the variation law for a single charged 
particle. If the extension is effected in such a way that we pass to 
a continuous electric convection-current, we find 
1) Xr, Yx, Zz are the forces, exerted in the direction of X, Y, or Z by the sur 
roundings of a unit cube, on a face for which the outwardly directed normal has 
the direction of the axis indicated by the index. 
