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The evolution of the central stars of planetary nebulae, interpreted as hot white dwarfs with liquefying cores, towards the cold white dwarf stage is discussed and theoretical (non-computational) evolutionary tracks are built for such central stars as they cool towards the crystallizing region.

The consequences of gas-liquid phase transitions in the core of hot white dwarf stars are discussed. Expressions for the latent heat and the liquefaction curveTl=Tl(Q) are obtained. Then amodel for a hot white dwarf is introduced and the corresponding liquefaction sequences are built on the H-R diagram; relations luminosity-central temperature and effective temperature-central temperature are also given for liquefying white dwarfs.

We consider the gas-liquid first-order phase transitions and prove the existence of the critical point in white dwarf matter. The latent heat released in the liquefaction processes can be used in the interpretation of the spreading of the white dwarf sequence in H-R diagram. Some thermodynamic quantities, e.g., the saturation pressure, the latent heat, etc., are calculated along the gas-liquid coexistence curve, and their behaviour near critical point are studied.

REFERRING to Mr. Don's letter (NATURE, August 17), it may be of interest to note that for many years I have been observing thunderstorms, often from high places on Etna, and from a distance, and I have never seen a lightning flash which was a single one. Lightning flashes from the clouds to the earth are always two, three, or more, either very rapidly repeated on the spot and along the same track, or in different places, sometimes very far apart, and simultaneously.