.

_Example 1._ Taking K = 0.1423 for carbon dioxide and air (at
temperature 0 deg. C. and pressure 76 cm. of mercury) referred to a
centimetre and a second as units, we may interpret the result as
follows:--Supposing in a mixture of carbon dioxide and air, the
density of the carbon dioxide decreases by, say, 1, 2 or 3% of itself
in a distance of 1 cm., then the corresponding velocities of the
diffusing carbon dioxide will be respectively 0.01, 0.02 and 0.03
times 0.1423, that is, 0.001423, 0.002846 and 0.004269 cm. per second
in the three cases.

_Example 2._ If we wished to take a foot and a second as our units, we
should have to divide the value of the coefficient of diffusion in
Example 1 by the square of the number of centimetres in 1 ft., that
is, roughly speaking, by 900, giving the new value of K = 0.00016
roughly.

7. _Numerical Values of the Coefficient of Diffusion._--The table on p.
258 gives the values of the coefficient of diffusion of several of the
principal pairs of gases at a pressure of 76 cm. of mercury, and also of
a number of other substances. In the gases the centimetre and second are
taken as fundamental units, in other cases the centimetre and day.

8. _Irreversible Changes accompanying Diffusion._--The diffusion of two
gases at constant pressure and temperature is a good example of an
"irreversible process." The gases always tend to mix, never to separate.
In order to separate the gases a change must be effected in the external
conditions to which the mixture is subjected, either by liquefying one
of the gases, or by separating them by diffusion through a membrane, or
by bringing other outside influences to bear on them. In the case of
liquids, electrolysis affords a means of separating the constituents of
a mixture. Every such method involves some change taking place outside
the mixture, and this change may be regarded as a "compensating
transformation." We thus have an instance of the property that every
irreversible change leaves an indelible imprint somewhere or other on
the progress of events in the universe. That the process of diffusion
obeys the laws of irreversible thermodynamics (if these laws are
properly stated) is proved by the fact that the compensating
transformations required to separate mixed gases do not essentially
involve anything but transformation of energy. The process of allowing
gases to mix by diffusion, and then separating them by a com