With increasing knowledge of the effects of inhaled dusts upon the lungs has come an increase in the number of cases of Workmen's Compensation in which pulmonary disease is attributed to exposure to this form of risk during employment. Where a post-mortem examination has been made, and the opinion of a pathologist is sought, no difficulty is encountered in those cases of silicosis in which the disease is well developed, since the lesions are characteristic and easily recognised. The position, however, is by no means so easy where lesser degrees of pulmonary fibrosis are present and, in particular, where it becomes necessary to try to assess the bearing which these may have upon the susceptibility to, and the progress of, intercurrent pulmonary disease—for example lobar pneumonia. In these difficult circumstances recourse is frequently had to the chemical estimation of the quantity of silica recoverable from samples of lung. Certain standards have been set up, and the presence of silica in quantities in excess of the “normal” forms valuable corroborative evidence that a pulmonary fibrosis is attributable to its influence, where the subject in question has been exposed to this risk in his employment, and on the other hand the absence of an excess of silica is evidence that any pathological condition in the lung is due to other causes.