Arranged in this way the cylinders previously weighed are exposed in the shade or to direct sunlight and reweighed after a stated interval. On account of the small surface here exposed in comparison with the total quantity of soil and moisture it is recommended to weigh the cylinders once only in twenty-four hours. The weighings may be continued for a fortnight or even a month. In soils fully saturated with water the rate of evaporation is at first nearly the same on account of the surface being practically that of water alone. As the evaporation continues, however, the rate changes markedly with the character of the soil. =159. Rapid Method of Wolff.=—In order to expose a larger surface to evaporation and to secure the results in a shorter period of time, Wolff[110] fills square boxes, having wire-gauze bottoms, with fine earth, and after saturating with moisture weighs and suspends them in the open air. The wire-gauze bottoms are previously covered with filter paper to prevent loss of soil. [Illustration: FIGURE 20. APPARATUS FOR DETERMINING COEFFICIENT OF EVAPORATION. ] =160. Estimation of Water Given up in a Water-Free Atmosphere.=—The air-dried sample, in quantities of from five to ten grams in a thin layer on glass, is placed over a vessel containing strong sulfuric acid. It is then placed on a ground glass plate and covered with a bell jar. The sample is weighed at intervals of five days until the weight is practically constant. This method is valuable in giving the actual hygroscopic power of a soil depending on its structure alone. =161. Estimation of the Porosity of the Soil for the Passage of Gases.=—Some further notion of the physical state of the soil known as porosity, may also be derived by a study of the rate at which it will admit of the transmission of gases. A method for estimating this has been devised by Ammon.[111] Air is compressed in two gas holders by means of a column of water of proper height to give the pressure required. The tubes through which the air passes out of the gas holders are each furnished with a stop-cock and united with a glass tube having a side tube set in at right angles for carrying off the air. The use of two holders makes it possible to carry on the experiment as long as may be desired, one holder being filled with air while the other is emptying. The common conducting tube is joined with a meter which is capable of measuring, to 0.01, the volume of air passing through it. The pressure is regulated by means of the stop-cocks. The air passing from the meter is received in a drying tube filled with calcium chlorid.