298 Prof. W. McF. Orr on 
work, energy, &c, in the sense in which we adopt them ; 
our distinction between heat and other forms of kinetic 
energy might seem to him specially artificial, and in this 
connexion it may be remarked that, although it is impossible 
to separate a mixture of (perfect) gases into constituents at 
the same pressure as the mixture, if the temperature is kept 
constant, without doing work on them, yet the gases give 
out in the process an equivalent quantity of heat, so that 
their energy remains unaltered. 
The second proposition in all its generality, or the equi- 
valent theorem concerning entropy, does not, indeed, appear 
to be formally deducible from any usually accepted state- 
ment of Thermodynamic laws ; each may form his own 
opinion as to how far experience and experiment warrant 
its acceptance. I imagine, however, that it is admitted 
almost universally that every process occurring in nature 
is irreversible and also that it involves a loss of available 
energy.^ 
But in the particular case of the diffusion of gases, or at 
least of some gases, it is a fact established by direct experi- 
ment, and, I had supposed, beyond all dispute whatever, 
that there is a loss of available energy, a statement which 
Mr. Burbury appears to challenge * (for the case of oxygen 
and nitrogen) . Lord Rayleigh, in the opening paragraph of 
the paper to which Mr. Burbury refers in his second paper, 
called attention to this in 1875 ; and the rest of Lord Ray- 
leigh's paper is devoted, not so much to a proof that there is 
such a loss, but to calculations of its amount. In the simple 
and common experiment there described, a tube containing 
hydrogen and closed at the upper end stands over water : 
the hydrogen escapes through the pores more rapidly than 
the air enters, thus creating a partial vacuum, and the water 
accordingly rises. The available energy is thus actually 
seen to be used, to some extent, in pumping water up a 
glass tube. And, if we were able to separate the hydrogen 
and the atmospheric air, in their original states, from the 
mixture, keeping the temperature constant, without doing 
work on the system — a process the possibility of which 
Mr. Burbury appears to suggest in his second paper f — it 
would be easy to devise a heat engine by which we could 
continue to pump water from a lower to a higher level, and 
in which the working substance would go through a closed 
cycle taking in heat at a constant temperature (atmospheric), 
but requiring no condenser, — thus obtaining a perpetual 
* L. c. p. 123. 
t L. c. p. 423. 
