While investigating the effects of silica dust and coal dust respectively in the production of respiratory disease in coal miners, we have been struck by the sharp contrast between silicotic coal miners in South Wales as compared with silicotic gold miners in South Africa in their respective liability to pulmonary tuberculosis. Coal miners have always been noted for their relatively low tuberculosis mortality, while gold miners and other workers in hard rock, but exempt from simultaneous exposure to coal dust, are conspicuous for their marked liability to fatal pulmonary tuberculosis in late middle age. And yet the recent findings of Cummins and Sladden (1930) and of the Medical Staff of the Welsh National Memorial Association (1930) indicate that both in the pathological, histological and chemical characters of the lungs of Welsh coal miners and in the X-ray appearances observed in long-service colliers of over forty years of age, there is nothing to distinguish them from Band gold miners suffering from silicosis except the added presence of large amounts of coal dust in the lung tissue of coal miners and their relatively low tuberculosis mortality. This contrast has been discussed by one of us (S.L.C.) in a recent paper (1931) and the suggestion made that the well-known adsorption power of finely divided carbon particles for colloidal substances might be a factor in reducing the liability of the coal miner to pulmonary tuberculosis.