Entropy as an adiabatic invariant

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.