CHAPTER IX

THE VISCOSITY OF GASES

In 1846, the same year in which Poiseuille published his
principal paper on the laws of viscous flow in liquids, Thomas
Graham published the first of a series of papers on the "trans-
piration" of gases through tubes of small diameter, which
have great historic interest. Graham sharply differentiated the
flow of gases through an aperture (effusion) and flow through a
long narrow tube (transpiration); he noted that the resistance of
a tube of a given diameter was directly proportional to its
length. Also " dense cold air is transpired most rapidly/7 and his
experiments led him to a relation between the time of transpira-
tion and the density of the gas. Graham studied the effect of
different pressures and concluded that "for equal volumes of air
of different densities, the times of transpiration are inversely
as the densities," as exemplified in the following table:

TABLE LXIV.—THE EFFECT OF PRESSURE UPON THE TRANSPIRATION
OF AIR (FROM GRAHAM)

Observed time of trans-

Pressure, atmospheres
piration for equal vol-
Calculated time

umes (relative)

1.0
1.0
1.0

1.25
0.795
0.800

1.5
0.673
0.667

1.75
0.589
0.571

2.0
0.524
0.5

When Clausius proposed the kinetic theory in 1857, all of
the properties of gases took on increased interest, and Maxwell
in 1861 published a paper in which he discussed the three kinds
of diffusion: (1) Diffusion of heat or conductivity, (2) Diffusion of
matter, and (3) Diffusion of motion or viscosity. The third or
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