Method.—A 1,500-gram sample, dried to constant weight, is placed in the tin bucket, which is then placed in the iron cylinder; the cast-iron lid is put in place and the air exhausted. The water is then slowly admitted until the stone is covered, when the lid is removed. The stone is allowed to soak for one-half hour (the average time of the immersion of material in the void test); the bucket is removed and hung on the arm of the balance (e), so that it is immersed in the water contained in the vessel (f), and the weight of the immersed tin bucket and contained material is measured.

The amount of water absorbed is measured by weighing the material after surface drying with a towel and again after drying it to constant weight over a hot plate. The original dry weight is used in determining the loss of weight in water and the final dry weight in determining the unit absorption. The original dry weight is used because small particles are apt to be lost during the surface drying.

Computations. The quantities measured and recorded are (1) the weight of the original dry stone, (2) the weight of the stone and tin vessel suspended in air and (3) in water, (4) the weight of the surfacedried stone after the test, and (5) the weight of the thoroughly dried stone after the test. All these weights are in grams. The difference between the last two weights gives the weight of water absorbed, and the weight of the original dry stone divided by the loss of weight of the stone in water corrected for absorption gives the specific gravity.

METHOD USED WITH MATERIAL THAT PASSES THE ONE-FOURTH INCH SCREEN.

Apparatus. With screened material a 3-inch wrought-iron pipe, 12 inches long, shown in Pl. VI, A, is used. It is permanently capped at the bottom and has a removable screw cap at the top. The valve at a is connected with the air pump, and water is admitted through the funnel and the stopcock in the top. A glass flask with a long neck graduated to one-fifth of a cubic centimeter is used to hold the material. The Le Chatelier specific-gravity bottle (Pl. VI, A) may be used.

Method. About 55 grams of the material, dried to constant weight, is put in the flask and weighed. The flask is then placed in the cylinder and the air exhausted, after which water is admitted to the flask through the funnel. The vacuum prevents foaming when limestone is used and also approximates the condition under which the voids are measured. When the material is entirely covered with water, the flask is removed from the cylinder and allowed to stand one-half hour. Its weight is then taken and the volume read on the neck of the flask. Owing to the delicacy of this test the temperatures of air and water must be controlled. The amount of

water absorbed is determined by weighing the material after surface drying with blotters and filter paper, and again when dry.

Computations. The weight in grams of the flask is known and the following quantities are measured and recorded, the weights being in grams and the volumes in cubic centimeters: (1) Weight of flask and sand; (2) weight of flask, sand, and water; (3) weight of surface-dried sand; (4) weight of dry sand; (5) volume of sand and water, read on the neck of the flask.

From these data the original weight in grams of the dry sand, the weight in grams of water absorbed per gram of dry sand, and the total weight in grams of water absorbed are computed. The difference between the volume of the sand and water read on the neck of the flask and the volume of the water in cubic centimeters in the flask corrected for absorption gives the volume of the sand in cubic centimeters. Finally, the ratio between the weight in grams and the volume in cubic centimeters of the dry sand gives the specific gravity.

PERCENTAGE OF SILT.

Apparatus.-The apparatus used in this determination is shown in Pl. VI, B. The glass percolator (a) is 13 inches long, with an upper inside diameter of 3 inches and a lower inside diameter of 21 inches. The vessel (b) is placed with its outlet (c) 3 feet above the top of the percolator. The opening (d) at the bottom of the percolator is one-half inch in diameter, and is fitted with a perforated porcelain disk to prevent the passage of the material placed in the vessel. The glass tube (e) is placed with its lower end 10 inches above the surface of the material.

Method. The vessel (b) and the percolator (a) are filled with clean water, and a 100- to 200-gram sample, depending upon the probable amount of silt present, is put in the latter. The upper stopper of the percolator is put in place, and the current of water is started. The silt is carried by the current of water over into the vessel f. The flow of water is continued until the effluent is clear, when the upper stopper is removed, the material stirred, and the current again started. This operation is repeated until the effluent is clear immediately after the material is stirred. The water containing the silt is evaporated on a water bath, when the silt is scraped and brushed onto a watch glass and permitted to remain uncovered until it attains the temperature of the room so that it will be under the same conditions as the original sample. It is then weighed. The silt thus obtained is chemically analyzed.

Computations. The percentage of silt is the weight of the silt divided by the weight of the material placed in the percolator and multiplied by 100.