524 Prot. W. McF. Orr on Clausius’ 
cycle any part of which is actually possible it must inter- 
change heat with external bodies; and by Lord Kelvin’s 
version of the Second Law it is impossible that in all such 
interchanges the system should absorb heat ; it must there- 
fore at some stage give out heat. 
It is, however, contended that with a view to a proper 
logical development of the subject, it is absolutely necessary 
either to state clearly some such additional premiss, or 
definitely to abandon all attempts to give a deductive proof 
of the theorem under discussion. 
Of course, the necessary emendation might be made to 
Clausius’ version of the Second Law instead of to Lord 
Kelvin’s. 
The statement is frequently made that all operations in 
Nature are frictional*. In the endeavour to attach any 
precise meaning to this statement or to explain how its truth 
or falsehood could be tested, we should, I think, fall back 
on some such statement as that suggested above. 
An Irreversible Process in which effects of Inertia and of 
Irreversibility may be confounded. 
22. Attention may here also be drawn toa point in the 
description of a certain class of irreversible processes in which 
the student might be misled by some writers. In discussing 
the expansion of gas in a cylinder it is frequently said that 
the external pressure is less than the internal. The phraseology 
in some cases might even suggest to the reader that ‘ action” 
and “ reaction” are unequal. In such processes the effects of 
inertia and of irreversibility are necessarily blended together, 
but some care should be taken to endeavour to distinguish 
between them, otherwise the student is likely to attribute to 
the former results which are really due to the latter. To 
illustrate the point, consider, for example, the definite state- 
ment that if steam expands with more or less suddenness in 
a cylinder whose walls are impermeable to heat, the work 
done by the steam on the piston is less than that which it 
would perform in an indefinitely slow adiabatic expansion 
starting with the same initial pressure, volume, and tem- 
perature, and ending with the same final volume. In so far 
as this statement is based not on direct experience but on 
any general theoretical considerations, it cannot, I conceive, 
* E. g. Planck, 7. c. Art. 115 :—“ Since there exists in nature no process 
entirely free from friction or heat conduction....” Parker, ‘ Elementary 
Thermodynamics,’ 1891, p. 142 :—“As no process in nature can exactly be 
, 9 
a non-frictional process... 
