IT is USEFUL to recall that selenium is a congener of sulphur, tellurium and arsenic. Selenium is a rare element, being slightly more abundant than gold, but is found neither in large quantities nor in the elemental form. The principal sources are flue-dusts from sulphuric acid works using seleniferous pyrites, flue-dusts from smelting processes and particularly the anode sludge of the electrolytic refinement of copper which may contain up to 15 per cent. Its current price is £12 per lb. Traces of selenium occur in rocks or soils all over the world. The mesozoic soils of the great plains of the U.S.A. (Dakota, Montana, Wyoming) have concentrations up to 30 parts per million, and plants growing in such soils absorb selenium which may subsequently be ingested by animals and humans in amounts sufficient to give rise to chronic and sometimes to fatal poisoning. Wheat grown on soil containing 30 parts per million may be toxic even after milling, though the toxicity is mainly associated with the protein moiety. Cattle and horses grazing on soils with concentrations of 10-20 parts per million may be the subjects of chronic poisoning. This, in horses, is shown by a loss of mane and tail hair and by abnormal hoof growth. In more severe cases, horses may develop blind staggers, erosion of bone, focal neurosis of liver, with cirrhosis—if of long standing—and death from respiratory failure.' The danger of seleniferous soils is minimised by high rainfall and possibly by the presence of arsenic, which in traces may inhibit the toxic effects of selenium. There is little evidence that selenium of vegetable origin is ingested in injurious quantities by man, but it can be found concentrated in the lungs, liver, kidney and spleen. Normal urine contains minute amounts, the principal source of which is peas and beans. Selenium intake predisposes to dental caries; the garlicky odour of the breath is due to dimethyl selenide. The industrial uses of selenium have increased rapidly in recent years. The common commercial forms are grey metallic selenium and selenium dioxide. These are used in 1, rectifiers, to convert an alternating current to direct as required for battery charging or electrolytic processes,