Teller has directed my attention to the fact that similar behaviour may be shown by nitrous oxide as opposed to the symmetrical carbon dioxide: this has proved to be true. The entropy of vaporization is a state when there is an increase in entropy as liquid changes into a vapour. Helium-4 also has a very slightly negative entropy of fusion below 0.8 K. No such effect is to be expected in the case of nitrogen, which is a symmetrical molecule. Helium-3 has a negative entropy of fusion at temperatures below 0.3 K. The formation of a fully ordered crystal is thus prevented.

Clay ton and Giauque have confirmed this result 3 and explained the discrepancy by the suggestion that the asymmetry of the carbon monoxide molecule gives rise to two positions in the lattice of nearly equal energy. Note that Sex increases with temperature just like the full entropy S does in fact Sex 0 as temperature goes to infinity at fixed density because the system. exhibits such a state in spite of its crystalline structure 2.

Teske and I, for example, have found that solid carbon monoxide near 10°K. A definite zero-point entropy arises from the existence of ‘frozen-in’ phases stable at higher temperatures which do not reach the real thermal equilibrium on cooling to the neighbourhood of the absolute zero. EUCKEN and his collaborators have pointed out that the failure of Nernst's heat theorem-the so-called third law of thermodynamics-which occurs in some cases, is due to the zero-point entropy 1.