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A source mechanism for the noble gases in the earth's atmosphere may be the solar-wind bombardment of extraterrestrial dust. Calculations based on estimates of recent influx of dust to the earth, quiet solar-wind intensity, argon abundance in the solar wind, and assumed Poynting-Robertson lifetimes of small particles in space agree within a factor of 5 with the present amounts of argon-36 and argon-38 in the atmosphere.

Measurements of excess Ar(36) + Ar(38) ( released mainly at 1200 degrees C) in magnetic concentrates of Pacific sediments and in a dense concentrate of Greenland dust agree within an order of magnitude with expected concentrations implanted by solar-flare ion streams of energy less than 10 Mev per atomic-mass unit. The agreement implies that more than 10 percent of each concentrate may be extraterrestrial, depending on size distribution and flare spectra. Rare-gas measurements on fine-grained dust can provide data on: solar-flare "paleo-ion" fluxes, energy spectra, and isotopic abundances; identification, mineralogy, and chemistry of interplanetary dust; influx rates to Earth and sedimentation rates of oceanic cores; and lunar-surface residence and mixing times.

RECENT measurements of hydrogen diffusion-rates in α-iron by Johnson and Hill1 gave approximately 2 × 10−7 cm.2/sec. for the value of D at room temperature; and in steel by Frank, Lee, and Williams2 gave approximately 1 × 10−7 cm.2/sec. Johnson and Hill measured re-emission of hydrogen from cylinders previously soaked in hydrogen at high temperatures and pressures, while Frank et al. used a mass spectrometer to measure the emission of hydrogen and deuterium from thin sheets. Frank, Swets and Fry3, observing the build-up of hydrogen permeating through thin sheets of steel, obtained for D at room temperature values about an order of magnitude higher.

IN a recent communication, Tamers1 has suggested that tritium in iron may decay radioactively with a time short compared with its twelve-year half-life, perhaps with a half-life of several days or less. He bases this suggestion principally on some misinterpretations of tritium measurements in iron meteorites. It may be useful to summarize the experimental meteorite evidence, which, if examined in detail, cannot be used to support such a suggestion. I should like here to present some of this evidence and to indicate briefly and in preliminary form a simple, plausible and physically reasonable explanation for the tritium observations in the iron–nickel phases of meteorites. A discussion of the other physical reasons for not believing Tamers's suggestion seems completely superfluous.

SEVERAL points urgently require clarification following my recent exchange of communications with Mr. Tamers1,2. First and most important, the limited available evidence does indeed suggest that a difference of several orders of magnitude exists between γ- and α-phase of iron for diffusion constants of hydrogen at room temperature. It is, of course, the room-temperature behaviour which is relevant for the problem of loss on Earth. Mr. Tamers quotes the paper of Sykes, Burton and Gegg3 as showing that these two diffusion constants differ by only a factor of 10 at 650° C. The identical equations and numbers from this paper demonstrate a difference of a factor of one million at room temperature (20° C).