A PRELIMINARY announcement has already been made by the Ministry of Supply on the sale of radioactive products for industrial radiography (see Nature, May 7, p. 717). A paper dealing with the supply and uses of radium and radon for industry is now being distributed by the Ministry to trade associations, professional institutes and research centres. Points of interest arising from this paper are the different physical properties of radium and radon. Though they both emit gamma-rays of identical quality, the radiation of the former is constant, whereas that of the latter has a half-life period of 3·8 days. This means that the effective life of radon is a little more than a week, but this is an advantage in, say, field-work where the loss of a capsule would not be serious, nor comparatively dangerous (in contrast to radium). The disadvantage of the variable strength of radon is further offset by the fact that, for equal intensities, it is of much smaller bulk than radium ; hence, for radiographs of equal resolution, radon can be used much closer to the object under test, with a corresponding reduction in exposure-time. Radium may be obtained, on hire only and after notice of two months, for periods of not less than six months ; capsules containing 50 mgm. and 250 mgm. are hired for £11 5s. and £56 5s. per year, with an additional fabrication and testing charge of £20 and £100, respectively. Radon is sold outright, typical costs being £15, £20 and £25 for sources of 250, 500 and 750 millicuries, respectively. Both materials can be obtained from the Radiochemical Centre, White Lion Road, Amersham, Bucks. Gamma-radiography requires skilled personnel for the handling of materials, interpretation of radiographs, etc., and, so far as is known, the only institution in Britain at present offering training facilities is the Kodak School of Engineering Radiography, Wealdstone, Harrow, Middlesex. Advice on the use of gamma-radiography (demonstrations are given) may be obtained from the Superintendent of Radiology Research, Ministry of Supply, Armament Research Establishment, Woolwich, London, S.E.18, and also from the Director, National Physical Laboratory, Teddington, Middlesex.