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Several attempts have been made to utilize the information obtained by allowing air-dry soil to absorb moisture at various relative humidities. The water absorbed by soil colloids when allowed to come to equilibrium over a sulfuric acid-water mixture containing 3.3 per cent sulfuric acid by weight has been made the basis for a method of estimating the quantity of colloid present in a soil (6). The water held under this condition (99 per cent relative humidity) falls below the hygroscopic coefficient. The British soil workers (8) have also used a determination of the moisture held at 50 per cent relative humidity as a criterion of soil properties. More recently, workers in the Bureau of Chemistry and Soils of the United States Department of Agriculture (5) have made determinations of the amounts of water held over sulfuric acid-water mixtures of various concentrations by different soil colloids. An attempt was made to correlate the ratios between some of the values so obtained with the chemical composition of the soil.The vapor pressure-water content curves of a number of soils have been studied by Thomas (13,14) and by Puri, Crowther, and Keen (11). They covered the entire range from oven-dry at 105°C. to saturation. It was found in both investigations that an inflection point in the curves occurred near 50 per cent of the vapor pressure of pure water. No breaks were found; this indicated no sudden change in the nature of the forces holding the water. Puri, Crowther, and Keen reached the conclusion that the curves were all of the same type, but that the general slopes of the curves were decreased with increases of clay and organic matter content. Thomas

Test plants were grown within a chamber enriched with radon-222 in the atmosphere, in tobacco fields with different sources of phosphate-containing fertilizer, and in culture containing lead-210 in the nutrient solution. Harvested leaves were subjected to three curing conditions. The major portion of the lead-210 in the plant was probably absorbed through the roots. Airborne radon 222 and its daughters contributed much less to the plant's content of lead-210 and of polonium-210. The stage of leaf development and the methods used to cure the leaf affected the final amount of polonium-210 in tobacco leaf.