Dietary restriction (DR) is a key focus in ageing research. Specific conditions and genotypes were recently found to negate lifespan extension by DR, questioning its universal relevance. However, the concept of dietary reaction norms explains why DR's effects might be obscured in some situations. We tested the importance of dietary reaction norms by measuring longevity and fecundity on five diets in five genotypes, with and without water supplementation in female Drosophila melanogaster (N>25,000). We found substantial genetic variation in the response of lifespan to diet. Flies supplemented with water rescued putative desiccation stress at the richest diets, suggesting water availability can be an experimental confound. Fecundity declined at these richest diets, but was unaffected by water, and this reduction is thus most likely caused by nutritional toxicity. Our results demonstrate empirically that a range of diets need to be considered to conclude an absence of the DR longevity effect.

A modified form of Turnbull & Fisher’s equation for the rate of nueleation in pure liquids is derived from the more general solution of Frenkel to the non-equilibrium steady -state kinetic problem . Recent developments in relaxation theory, based on the Zeldovich— Frenkel formulation of the time-dependent flow of embryos over the size variable g , show that under certain conditions the nucleation time lag may be exceedingly sensitive to the initial state of the condensing system. Frisch’s integral for the nucleation time lag is evaluated for two initial distributions of embryo size. The results, which are of general form, are used to derive expressions for the time lag in supercooled liquids. Methods are given for estimating certain parameters required in the computation of time lags in supercooled liquids.

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Current methods of interpreting critical supercooling are discussed. In the method of Turnbull it is assumed that the experimental observation time matches the reciprocal of the steady-state nucleation frequency. This assumption is shown to be incompatible, for certain types of experiment, with the nucleation time lag predicted by the theory of part II. An alternative criterion of freezing which avoids this difficulty is that under the conditions of a threshold experiment the observation time is comparable with the nucleation time lag. This criterion may be used in conjunction with the theory of part II to interpret critical supercoolings without making separate assumptions about nucleation frequencies. Threshold nucleation rates, sizes of crystal nuclei, and liquid-solid interfacial free energies for alkali halides are calculated from the supercooling data of part I. Data for nuclei are examined in relation to the properties of the liquid and solid compounds in bulk form. Certain outstanding problems in the interpretation of results obtained in this and other studies are discussed with respect to present nucleation theory.

The linearized transport theory of space charge polarization in alternating fields is applied to relaxation of the diffuse double layer in melts and solutions of the salt MtfX&.The method reveals factors which limit the validity of empirical a.c. network analogues of the layer. In the approximation to which the calculation is taken, there is a single relaxation time which depends on the cell length and the permittivity and transport coefficients of the electrolyte. Formation of the layer causes d.c. polarization in conductivity cells. Its relaxation with frequency is illustrated by the dispersion curves for conductance and capacitance in a parallel network. The series capacitance, to which the parallel capacitance reduces at low frequencies, depends on the ratios of the transport coefficients and contains separate contributions from each ion. It is converted to the low-potential limit of the Gouy capacitance by substitution of the Nernst-Einstein relations. The relaxation time is numerically equal to the product of the series resistance and the series capacitance and does not resolve into separate ionic contributions. The origin and significance of these results is discussed.