250

FLUIDITY AND PLASTICITY

molecule will be n1/2-f old that of the smaller molecule. But the
number of excursions of the molecules will be in proportion to
rr**, so that the total loss of momentum will be the same as
before, provided only that the number of particles per unit
volume remains the same.

In gases at ordinary pressure, there are considerable differences
in viscosity ranging from 0.0000689 for benzene vapor to
0.0002981 for neon, but they are inconsiderable as compared
with the vast differences we find in the liquid state and these
viscosities are measured at 0° and not under corresponding con-
ditions. Table LXIX shows that the vapors have viscosities
which are smaller than those of the permanent gases except

T*

"lit?

0 100 200
Atomic Weiflht
FIG. 82.—The relation between the viscosity of the elements at their critical
temperature and their atomic weights.
hydrogen. Their viscosities are so nearly identical that it is not
certain whether the viscosity of a given class of chemical com-
pounds such as the ethers differs from that of the esters or
ketones. It is quite impracticable with the data at hand to
assign any effect to an increase in the molecular weight within
a given class of compounds.
Since the viscosities cxf the permanent gases at 0° are not
simply related to each other, it is natural to seek some other
basis of comparison, and Rankine (1911) has achieved success
along this line by comparing the viscosities of the rare gases yc
and their atomic weights M at the critical temperatures. He
finds them related together by the formula
77C2 = 3.93 X 10~10M