The peculiarity in the c vs. p graph (Fig. 2) exhibiting the tendency for c to decrease and finally become negative as the density increases is believed to be related to the similar tendency of 4> with hydrogen, and both are evidence for the predominance of repulsive forces between molecules at high densities.

It is shown that d(θΔH)/dT=0 is a necessary condition for the attainment of Carnot cycle efficiency, (T1–T2)/T1, by a thermally regenerated fuel cell system in which ideal gases are reacted in a fuel cell to convert chemical to electrical energy, and are then regenerated for recycling by thermal dissociation of the product of the reaction, also assumed to be an ideal gas. In d(θΔH)/dT, T is the temperature, ΔH is the change in enthalpy for the reaction in the fuel cell, and θ is the degree of reaction, or the fraction of the road from zero to 100% product which is covered by the reaction when chemical equilibrium is reached. Formulas are given for calculating the theoretical efficiency. Curves show calculated efficiencies for hypothetical thermodynamic data chosen to illustrate the effect of thermodynamic properties on efficiency. Temperature-entropy diagrams are used to explain why the efficiency reaches a maximum and then decreases when T2 is held constant and T1 is increased. The efficiency is pressure independent when the reaction does not involve change in the number of molecules, pressure dependent when it does.