This paper is a continuation of the work already published in three parts in this Journal (1, 2). In it a new theoretical treatment of the effect of the non-adiabatic behaviour of a gas subjected to alternate compressions and rarefactions is developed for both spherical and cylindrical chambers. This theory is then applied to the previous results as well as to some new ones. It is believed now that the results are as accurate as may be obtained by direct experimental methods, under the most favourable conditions.