  This short article was submitted to Nature Physics as a Correspondence. The
intention was to provide a brief albeit significant criticism of the work of J.
Dunkel and S. Hilbert, \textit{Consistent Thermostatistics Forbids Negative
Absolute Temperatures}, Nature Physics \textbf{10}, (2014). The respected
editor decided not to publish the Correspondence. We have therefore decided to
submit the paper to arXiv. Comments/criticisms are welcomed, particularly from
the authors of the mentioned paper.
