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General conditions for adiabatic instability of weak axisymmetric toroidal magnetic fields are obtained. In particular all fields which haveH?=0 and ?H?2/?d>0 simultaneously are unstable. This includes all fields which exist near to the axis of symmetry and, indeed, any without an infinite current density anywhere. A detailed discussion is given of fields with an angular dependenceH?2~|Pl (cos ?)|. These fields are dynamically unstable with respect to both axisymmetric and non-axisymmetric perturbations for all values of the azimuthal wave number |m|. The maximum growth rates are independent ofm and they are largest for the fields that are defined in the regions closest to the polar axis.

Vegetables and soft fruit provide the best financial return from irrigation. Supplementary watering is today almost essential for successful market gardening. Among farm crops, potatoes give the most profitable response. Irrigation of grass, which is widely practised and accounts for about 30 per cent of the total watered land in Britain, is seen as a late stage in intensification—for profitable use when other factors are fully exploited.

It is shown that care must be taken in the application of the principle of interchange of stabilities, particularly to problems in which the wave number-growth rate relation contains transcendental functions. Particular application is made to recently studied problems of hydromagnetic stability.

THERE is at least one lepton in addition to the electron and muon and this might have an associated neutrino. If such a neutrino is stable, its existence implies that the amount of helium produced in the early stages of the expanding Universe is higher than is generally believed. If there are several heavy leptons with stable neutrinos, the helium production may become so high that it is in clear contradiction with observations. It is pointed out here that, even if these neutrinos exist, they may not be stable and that there is no clear evidence that even the muon neutrino is stable. If the additional neutrinos are unstable with a short enough half life, they do not affect the cosmological helium production and it is reduced if the muon neutrino is unstable.

By R. N. Thomas, NASA publication, NASA Sp-471. Price FF 100 (about £9), 1983.

Edited by Icko Iben Jr. and Alvio Renzini. D. Reidel Publishing Company. Dordrecht, Holland, xv +488 pp. (ISBN 90-277-1284-0) 1981 Price Dfl. 125.00/US$66.00

THE interpretation of measurements of the uptake by plants of tracer isotopes from the soil is complicated by at least three factors. First, exchange takes place between the added labelled ions and the isotope ions present in the soil; the rate of this exchange has been shown to be affected by many factors1,2. Secondly, continuous exchange occurs between the nutrient ions in the roots and those in the soil3–6. The factors controlling this exchange are unknown. Thirdly, there is the possibility of damage to plant tissues, resulting from accumulation of the tracer. Although a number of experiments have shown no appreciable effects of radiation in terms of yield of dry matter9–12and uptake9,12, the question of radiation damage to plant tissues in long-term uptake experiments remains unsettled. Critical studies13,14have shown that even very low doses of radiation may produce some physiological changes in plant cells.

The general outline of the evolution of single stars is well understood but at most stages of evolution important uncertainties remain. This paper contains a very personal view of what are the major uncertainties and of what problems remain to be solved before one can be satisfied with the theory. It is suggested that some problems may be essentially insoluble even with the very large and fast computers that are currently available.

IN a paper which has recently been published, McCance1described the effects of administering hypertonic salt by mouth and intravenously to a dehydrated person. Volumes of urine up to 7–8 c.c./min. were passed, and this urine had a much lower osmotic pressure than the urine passed by the same person before the salt was given. The concentration of sodium chloride in the urine, however, appeared to be at or near the maximum concentration of which the kidney was capable, and it was suggested that the reabsorption of water in the distal tubules was being limited by the concentration of sodium chloride reached within them, a maximum value being attained when the osmotic pressure was still below the level at which it would become the limiting factor. It has now been shown that this conclusion was incorrect, and that it is the total osmotic pressure which is the limiting factor all the time, but that its limiting or maximal value is related to the minute volume, and falls as the latter rises. The evidence was obtained by such experiments as the following.

I WISH to point out a cosmological consequence of the recent reduction in the experimental half life of the free neutron, if this is confirmed.

A time-harmonic expansion of the gravitational tide potential is computed using an ephemeris of high precision for the Moon and the Sun and the latest I.A.U. astronomical constants. The results, which are computed for three different epochs and by novel methods, are compared with Doodson's classic expansion. The chief differences are due to secular trends in large terms and to revised constants which reduce all the solar terms. A new expansion is also given for the radiational tide potential.