252 FLUIDITY AND PLASTICITY

small amounts with other gases, as carbon dioxide or methane, the
viscosity of the mixture is much greater than would be calculated
by the simple formula of additive viscosities. In these cases
Puluj (1879) and Breitenbach (1899) have found that the more
complicated formula (103) gives good agreement.

VISCOSITY OF GASES AND DIFFUSION AND HEAT CONDUCTIVITY

We note that the diffusion coefficient D in a mixture of gases is

D = 1 irCVaLiQi + NJkQ*)/N (104)

Ni, Li, and Oi being the number of molecules of the first kind
of gas per unit volume, the length of the mean free path, and
the mean speed respectively, etc. Also N = Ni + N%. Since
the length of the mean free path can most easily be calculated
from the viscosity, it becomes possible to calculate the diffusion
coefficient from the viscosity.

In the conduction of heat the two kinds of gas become identical,
hence the above equation becomes

D = TrOL (105)
o
If we neglect the small difference between Oi and ti due to
temperature difference the conductivity of heat k becomes
k = l<n-QLpCv (106)
o
Cv being the specific heat of the gas at constant volume, and
combining this equation with the viscosity Eq. (97) we obtain
k = C*,C9 (107)
C being a constant (cf. Eucken (1913)).
DETERMINATION OP THE ULTIMATE ELECTRICAL CHARGE
It .is well known that Sir J. J. Thomson (1898) devised a
method for measuring the charge on the particle of a rarefied
gas e by observing the rate of fall under gravity of the particles of
an ionized fog which had been produced by sudden expansion and
then observing the rate of fall of a similar cloud when it is sub-
jected to the action of a vertical electrical field of known intensity
superimposed upon gravity.