The photo-extinction method for small particle analysis depends on the absorption of light by silt-size particles in suspension. The rate of change of absorption in a sedimentation cell is related by Stokes' law to the particle diameter. By the use of graphical integration, continuous size distribution curves were constructed for samples of silt, loess, and volcanic ash. In order to compare the photo-extinction method with the more familiar pipette, hydrometer, and microscope methods, samples of the loess were analyzed using the 4 techniques. Results obtained with the photo-extinction method are reproducible, compare favorably with data obtained by use of the standard microscope method, and are superior to results obtained by either the pipette or hydrometer method.

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