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122 INDUSTRIAL HYGIENE AND OCCUPATIONAL MEDICINE the determination of quartz to warrant discussion. It may be mentioned, however, that the rapid, highly specific spectrographic examination may aid the analyst by giving an indication of what interfering crystalline materials may be present. The three most widely used methods for the determination of quartz depend on preferential solubility in various solvents. The Knopf5 method treats the sample with hydrochloric acid, hydrofluosilicic acid and finally with hydrofluoric acid. The method is tedious, time consuming, and because of the large number of manipulations required, very prone to technical error. The method of Line and Aradine6 substitutes fluoboric acid for the hydrofluosilicic acid in the Knopf method but does not overcome the difficulties mentioned. Both methods apply an empirical correction factor for the solution of quartz during the analysis. This correction factor does not take into account the variation in rate of solution due to particle size differences as shown by Moke 7 and Harris.8 The phosphoric acid method proposed by Durkan 9 reduces some of the disadvantages of the Knopf and the Line and Aradine methods but does not completely eliminate any of them. Other difficulties are introduced, such as the insolubility of various silicates, the necessity for the prediction of quartz loss in hydrofluoric acid treatment and the necessity of determining particle size so that the proper correction factor may be applied. X-ray diffraction methods for quartz determination have been described by many authors. Sproull10 described the Hull-Debye-Scherrer method and the Laue procedure, both widely used photographic x-ray technics for qualitative crystal analysis. Gross and Martin*11 and Hicks, McElroy and Warga12 have adapted these methods to quantitative measurements. While these methods, with their very high degree of specificity, eliminate many of the difficulties of the petro graphic and chemical procedures, they introduce other difficulties peculiar to them selves. Many variables in film exposure and processing must be reduced to experirnental constants and be held under close control. Owing to the poor resolu tion of many x-ray cameras, the problem of background interference and line super position are difficult to evaluate. The photographic technic does offer two distinct advantages over the methods described below. These are, first, the fact that the exposure time of the film is sufficiently, long to level out any fluctuations in x-ray 5. Knopf, A.: The Quantitative Determination of Quartz in Dusts, U. S. Pub. Health Rep. 48:183, 1933. 6. Line, W. R., and Aradine, P. W.: Determination of Quartz in the Presence of Silicates, Indust. & Engin. Chem. (Analyt. Ed.) 9:60, 1937. 7. Moke, C. B.: The Solubility of Quartz in Hydrofluosilicic Acid, J. Indust. Hyg. & Toxicol. 18:299, 1936. 8. Harris, W. B.: Solubility of Quartz in Hydrofluoboric Acid, J. Indust. Hyg. & Toxicol. 19:463, 1937. 9. Durkan, T. M.: The Determination of Free Silica in Industrial Dust, J. Indust. Hyg. & Toxicol. 28:217, 1946. 10. Sproull, W. T.: X-Rays in Practice, ed. 1, New York, McGraw-Hill Book Company, Inc., 1946, pp. 391-437. 11. Gross, S. T., and Martin, D. E.: Quantitative Determination of Crystalline Materials by X-Ray Diffraction, Indust. & Engin. Chem.'(Analyh Ed.) 16:95-98, 1944. 12. Hicks, V.; McElroy, 0., and Warga, M. E.: Quartz in Industrial Dusts and Deposits on Human Lung Tissues: X-Ray Diffraction, Chemical and Spectrographic Studies, JIndust. Hyg. & Toxicol. 19:177, 1937.