1405 
cient might be ascribed to the same cause; in this case the receding 
of the attraction sphere (or the greater receding than in other sub- 
stances) might be the cause of a slighter decrease of the viscosity 
at the reduction to lower temperatures. There might, however, also 
be an expansion of the molecule (in this case the atom) with tem- 
perature, and finally both phenomena might be dependent upon one 
cause, and go together. The possible small curvature for helium of 
the line in the logarithmic diagram |that we mentioned in § 1] in 
the opposite sense to that of the other lines which expresses the 
difference between helium and other substances, could be ascribed 
to this change in the attraction. 
That which might explain the deviation for helium of the slope 
of the line from that which holds for a large range of temperatures 
for other substances, may also possibly help to explain the deviation 
from Reincanum’s formula at low temperatures, by the quantity 
which determines the attraction becoming smaller. 
With hydrogen at the temperature of liquid air there is a distinct 
change in the slope of the curve. It is remarkable that the same 
is found with nitrogen, and perhaps also with oxygen and carbon 
monoxide, and that the point at which it occurs seems to lie at 
the same reduced temperature for hydrogen and nitrogen and perhaps 
also for oxygen and carbon monoxide. If this is the case, then the 
change which in the hydrogen molecules may according to Krersom 
be taken as a change from hard spheres with electric doublets into 
hard spheres with a central force r—q as far as the vis cosity, is 
concerned would be a similar process for all these different substances, 
determined by the same units of length, time, and mass as hold for 
the critical quantities, while this point only coincides with the point 
of transition in the specific heat of diatomic substances in the special 
case of hydrogen. 
We must further notice the systematic differences between the 
different substances which appear from the non coincidence of the 
curves. It is remarkable that most of them (except a part of argon) 
can be removed by shifting the curves. The mean value of the 
molecular radius which comes into consideration for the viscosity 
seems thus to differ from the mean value which comes into consi- 
deration for the equation of state at the critical temperature, but 
both are in a fixed relation for the various materials over the whole 
field of temperature. This might be ascribed for instance, to a more 
elongated shape of the molecules in substances which give the 
smallest viscosity. PES As 
