1400 
perature with the empirical correction of Rerneanum’s C which 
RAPPENECKER bas suggested. 
We might for instance with helium take C= C! 7’ log T, to come 
to agreement with the observations. But then Remeanum’s formula 
would simply be converted into our interpolation formula. 
Krrsom, in Suppl. N°. 25 and 26 of these communications has 
shown, that the second virial coefficient in the equation of state for 
hydrogen, at temperatures at which this gas may be regarded as 
di-atomical, can be very statisfactorily explained by the supposition 
that hydrogen molecules are hard spheres with electric doublets in 
the centra. His formula for the virial of the collisional forces under 
these circumstances gives a change of the radius of the molecule 
with the temperature, which for higher temperatures agrees fairly 
mV T 
well with that deduced from the viscosity by ——-—. At lower tem- 
1] 
0 
peratures at which hydrogen behaves like a monatomic gas, the 
formula for hard spheres with a central force according to the law 
r—Y becomes applicable, and Kerrsom finds this again confirmed by 
the change of the viscosity with the temperature. 
But when we go down to —198° C. deviations appear, in accord- 
ance with what we said above about SUTHERLAND's formula, and 
at lower temperatures the value of the viscosity becomes much 
too small. 
None of the formulas deduced from theoretical suppositions can 
represent the observations for helium ; for the present we can only 
use our empirical representation for this substance, which for hydrogen 
also holds good for lower temperatures than the theoretical formulas, 
viz. as far as the temperature of reduced oxygen. As regards the 
formula for helium, it is not impossible, that the straight line in 
the logarithmic diagram must be replaced by one that at low tem- 
peratures, and perhaps at higher ones also, curves somewhat towards 
greater values of the viscosity. 
§ 2. Application of the principle of mechanical similarity upon 
the comparison of the viscosities in corresponding conditions. 
If two substances may be taken as mechanically similar systems 
of molecules, it follows *) that the viseosities for both in corresponding 
conditions must be in a constant ratio which may be calculated 
from the ratios of the units of length, time, and mass in both 
systems. On the other hand from the values of 
1) H. Kamertincu Onnes. Verh. Kon. Akad. Amsterdam 21, p. 22. 1881 Beibl. 
5. p. 718. 1881, 
