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Values for the surface area of portland cements were obtained by three different methods: the Wagner turbidimeter, air permeability, and a statistical calculation based on size-distribution data. In the Wagner turbidimeter (1), from the change with time in transmission of a beam of light through a suspension of the material, size distribution of the material and surface area are calculated. The turbidimeter analyses were made in the laboratories of J C. Pearson, Lehigh Portland Cement Co., who also kindly supplied the cements. Redetermination of these materials in the same laboratories about a year later gave surface area values that were consistently lower by about 5 per cent; we have taken for comparison the original values as being probably the more accurate.
Roller, Paul S.
Physical Chemist, Eastern Experiment Station, U. S. Bureau of Mines, College Park, Md
Roundy, P. V.
Cooperative Fellow, American Instrument Co., Silver Spring, Md