the Mixing of Gases. 299 
motion of the second kind, as Ostwald and Planck call it ; 
it seems unnecessary to describe a suitable arrangement in 
detail. There would, of course, be a loophole of escape if 
there were any reason to think that the energy is actually 
drawn from the plug. 
Mr. Burbury makes no allusion to this paragraph of Lord 
Rayleigh's, but proceeds to refer to one of the possible 
processes of separation of gases which Lord Rayleigh 
employed in calculating the amount of work necessary for 
the operation. In this it is supposed that a tall narrow 
vertical tube is mounted on a large reservoir containing a 
mixture of two gases at sensibly constant pressure : the 
composition varies in different parts of the tube, owing to 
the effect of gravity, there being a greater percentage of the 
lighter gas at the top of the tube than at the bottom. 
Lord Rayleigh supposes that a small quantity of gas is 
(1) removed from the top of the tube (an equivalent amount 
being, I take it, automatically supplied from the reservoir so 
that the composition of the gas at every point of the tube 
remains unaltered) ; (2) compressed until it attains the 
pressure of the gas in the reservoir ; (3) allowed to fall to 
the level of the reservoir ; and (4) forced into the reservoir, 
(but, as I understand it, kept separated from the gas in the 
reservoir by an impermeable membrane) ; the temperature 
is supposed to be constant throughout. Thus, from the 
original contents of the reservoir there has been separated a 
small portion of gas which is of a different composition from 
that which remains. Mr. Burbury, however, apparently 
ascribes to Lord Rayleigh a somewhat different process ; he 
supposes each element of the vertical column, in succession, 
brought down to the level of the reservoir (but without being 
replaced by other similar elements and without being forced 
into the reservoir). This he describes as a partial separation 
of the gases, which appears to be a misdescription ; it is 
merely an alteration in the position and density of gases 
without any change in the proportions of their constituents. 
He then regards these operations as the second half of a 
complete cycle of which he proceeds to supply the first 
half : evidently he does not use the phrase " complete cycle 33 
in its usual sense. In this first half he supposes a tube, 
which is a facsimile of the vertical one, but placed hori- 
zontally at the level of the reservoir, to be filled with the 
reservoir gases, interchanges the molecules of the two gases 
in it until each element has the same constitution as the 
corresponding element of the vertical column, expands each 
element until it has the same pressure and density as the 
