e hydrogen will diffuse out, leaving a vacuum.

The details of the experiment may be quoted here:--"A glass tube of
about 5 mm. internal diameter, blown out to a bulb at the middle, was
provided with a stop-cock at one end. To the other a platinum tube 10
cm. long was fastened, and closed at the end. The whole tube was
exhausted by a mercury pump, filled with hydrogen at ordinary
atmospheric pressure, and then closed. The closed end of the platinum
portion was then heated in a horizontal position by a Bunsen burner.
The connexion between the glass and platinum tubes, having been made
by means of sealing-wax, had to be kept cool by a continuous current
of water to prevent the softening of the wax. After four hours the
tube was taken from the flame, cooled to the temperature of the room,
and the stop-cock opened under mercury. The mercury rose rapidly,
almost completely filling the tube, proving that the tube had been
very nearly exhausted."

[Illustration]

In order that diffusion through a membrane may be reversible so far as a
particular gas is concerned, the process must take place so slowly that
equilibrium is set up at every stage (see S 9 above). In order to
separate one gas from another consistently with this condition it is
necessary that no diffusion of the latter gas should accompany the
process. The name "semi-permeable" is applied to an ideal membrane or
partition through which one gas can pass, and which offers an
insuperable barrier to any diffusion whatever of a second gas. By means
of two semi-permeable partitions acting oppositely with respect to two
different gases A and B these gases could be mixed or separated by
reversible methods. The annexed figure shows a diagrammatic
representation of the process.

We suppose the gases contained in a cylindrical tube; P, Q, R, S are
four pistons, of which P and R are joined to one connecting rod, Q and
S to another. P, S are impermeable to both gases; Q is semi-permeable,
allowing the gas A to pass through but not B, similarly R allows the
gas B to pass through but not A. The distance PR is equal to the
distance QS, so that if the rods are pushed towards each other as far
as they will go, P and Q will be in contact, as also R and S. Imagine
the space RQ filled with a mixture of the two gases under these
conditions. Then by slowly drawing the connecting rods apart until R,
Q touch, the gas A will pass into the sp