=96. Specific Gravity.=—The density of a soil depends on its composition, the fineness of its particles and upon the packing which it has received. It has in other words an apparent and a real specific gravity. It is easy to see that a soil in good tilth would weigh less per cubic foot than one which had been pressed closely together, as in a road or well-pastured field. Ordinary soils in good tilth have an apparent specific gravity of about 1.2, and when entirely free from air, a real specific gravity of about 2.5. If the apparent specific gravity of a soil sample were 1.2 and the air were removed, leaving a vacuum in the interstices of the soil, the apparent specific gravity would not be sensibly increased. The figure 1.2 is the apparent specific gravity of a mixture of soil material which is about 2½ times heavier than water, and of an extremely small proportion by weight of air which is about 1000 times lighter than water. The figure 2.5 is about the true specific gravity of the real soil material, and shows that this material is about 2½ times heavier than an equal volume of water. The weights of a cubic foot of different kinds of soil as given by Schübler[71] are as follows; Pounds. Sand 110 Sand and clay 96 Common arable soil 80 to 90 Heavy clay 75 Vegetable mold 78 Peat 30 to 50 In general the specific gravity of soil decreases inversely as its content of humus. =97. Determination of Specific Gravity.=—The ordinary method of proceeding to determine the true specific gravity is by means of a pyknometer. The pyknometer should have a capacity of from twenty-five to fifty cubic centimeters. From ten to fifteen grams of earth dried to constant weight at 100° are taken, boiled for a time with a few cubic centimeters of water to remove air and poured into the pyknometer. All soil particles are washed out of the vessel in which the boiling took place into the pyknometer with freshly boiled distilled water and after cooling to the temperature at which the calibration took place, the pyknometer is filled with distilled water at the given temperature and weighed. If the soil contain materials soluble in water, alcohol of definitely known specific gravity may be employed and the number thus obtained calculated to a water basis. The calculations when water is used are made as follows: