From the drying tube the air enters a drying flask filled below with concentrated sulfuric acid and above with pumice stone saturated therewith. Next the dried air passes through a worm, eight meters long, surrounded with water at a given temperature. The dried air of known temperature next enters the experimental tube. This tube is made of sheet zinc 125 centimeters in length and five centimeters in internal diameter. It is placed in an upright position, and about six centimeters from its upper end carries a small tube at right angles to the main one for connection with a water-filled manometer. The upper and lower ends of the tube are closed with perforated rubber stoppers carrying tubes for the entrance and exit of the air. In the inside of the zinc tube are found two close-fitting but movable disks, of the finest brass wire gauze, between which the material to be experimented upon is held. The layer of fine soil is held between these disks and may be of such a depth as is required for the proper progress of the experiment. With soils of firm texture opposing a great resistance to the passage of the air the column of earth tested should be shorter than with light and very permeable soils. The experimental tube is surrounded with a water jacket, which may also be made of sheet zinc, carrying small tubes directed upwards for holding thermometers. The water jacket should be kept at the same temperature as the air which is used in the experiment. The process of filling the tube, the amount of pressure to be used and the air and soil temperature, will naturally vary in different determinations. The volume of air at a given pressure and temperature which passes a column of soil of a given length in a unit of time will give the coefficient of permeability. =162. Determination of Permeability in the Open Field.=—A method for determining the rate of transmission of a gas through the soil in the field has been devised by Heinrich.[112] A box C (Fig. 21) is made of strong sheet iron and has an opening below, ten centimeters square, and a height of about twenty centimeters. At exactly ten centimeters from the bottom, the box has a rim at right angles to its length so that it can be placed only ten centimeters deep in the soil. The box holds a volume of earth equal to 1,000 cubic centimeters. [Illustration: FIGURE 21. METHOD OF HEINRICH. ] The part of the box above ground is connected with the bottle B by a glass tube as indicated in the figure. The bottle B should have a capacity of about ten liters. The air in B is forced out through C by water running in from the supply A and the pressure in B is recorded by the manometer D. The experiment should be tried on a soil thoroughly moist. In measuring the pressure in B the water pressure should be cut off by the pinch-cock between A and B, and the pressure on the manometer observed after the lapse of one to two minutes. MOVEMENT OF WATER THROUGH SOILS: LYSIMETRY.