346 DR S. CHAPMAN ON THE LAW OF DISTRIBUTION OF MOLECULAR VELOCITIES, 
The Thermal Conductivity of Polyatomic Gases at Low Temperatures. 
§ 12 (E) The formula 3- —fu C„ is true for polyatomic as well as for monatomic gases, 
f being independent of pressure and temperature over a considerable range. Under 
normal conditions, however, its value is 2‘0 or less, being greatest for diatomic gases 
and diminishing down to about 1'5 for complex molecules. Eucken # has made the 
interesting and important discovery, however, that diatomic gases show an increase 
in f at low temperatures, the conductivity varying with temperature in the sense 
opposite to that observed in the case of helium. This is apparent, to a slight extent, 
in nitrogen, but is most striking in the case of hydrogen. It is found that, simul¬ 
taneously with the rise in f the specific heat C„ progressively falls in value until at 
21 ° C. absolute its amount is that appropriate to a monatomic gas of the same molecular 
weight. At these low temperatures the rotatory motion of the molecules seems to 
fail, for some reason as yet undiscovered, so that the gas behaves in certain respects 
as if its molecules were of the spherically symmetrical type discussed in this paper. 
It is highly interesting and significant that this approach to monatomicity is 
accompanied by an upward tendency of f towards the value (2‘5 approximately) 
which is appropriate to monatomic gases. The same phenomenon may be expected in 
the case of the other diatomic gases, at lower temperatures corresponding to their 
lower boiling points. In the following tabled the results for hydrogen alone are 
given ; the number n in the third column represents the number of “ degrees of 
freedom ” of the molecule, as calculated from the observed values of C„. 
Values for f for Hydrogen. 
Absolute 
temperature. 
/• 
n. 
° C. 
273 
1-96 
4-80 
195 
2-09 
4-41 
81 
2-25 
3-16 
21 
2-37 
2-98 
The Diameter of the Molecide. 
§ 12 (F) In my former paper tables were given showing the values of the molecular 
diameters for several gases, calculated on the hypothesis that the molecules are rigid 
spheres, with or without attractive force. These require a small correction to be 
strictly accurate, on account of the factor (2y,.) 1/2 there omitted from the formula for 
* Cf. Eucken, ‘ Phys. Zeit.,’ 14, p. 329, 1913, Table 6. 
