27 



During the coming year it is intended to determine the density of 

 rock by careful measurement with a micrometer screw, and the poros- 

 ity by means of a vacuum pump and hot water, which will be much 

 more accurate than the methods at present employed. 



CEMENTATION TEST. 



The next test in order is the cementation test, which is intended to 

 determine the important cementation or cohesive property of rock, 

 which, when possessed in a high degree, gives an impervious shell to 

 the surface of a road. 



This property is determined by taking 500 grams of the sample 

 broken to pass through a screen of 6 mm mesh, but not through a 

 0.5 mm mesh. The material thus broken is placed in a ball mill 

 (PI. II, fig. 2) ,'ind reduced to a powder that will pass through a 0.25 

 (TOO- inch) mm mesh. This dust is made into briquettes of circular 

 section, 25 mm (0.98 inch) in diameter and 25 mm in height, by mix- 

 ing the dust with distilled water (about 4 cc) on a glass plate and 

 placing it in n steel die of the proper dimensions. This die has a 

 closely fitting steel plug which is inserted over the dust, and the 

 dust is subjected to a pressure of 100 kilograms per square centi- 

 meter (1,422 pounds per square inch) with a machine especially 

 in designed for the purpose, which gives perfectly uniform conditions 

 in making each briquette. (See PI. Ill, also photograph in exhibit 

 case. ) The w r eight of the dust varies with the density and compressi- 

 bility of the rock, but generally it requires about 23 grams (0.8 of an 

 ounce) of dust to make a briquette of the above dimensions. These 

 briquettes are kept in a hot-air bath until their weight becomes con- 

 stant, and are then placed in a desiccator and allowed to cool, after 

 which they are broken by impact. 



The machine especially designed for this test (see PI. IV) consists 

 of a 1 kilogram (2. 2 pound) hammer (H), arranged like the hammer of 

 a pile driver on two vertical guide rods (D). The hammer is raised 

 by a screw (C) and dropped automatically from any desired height. 

 It falls on the flat end plunger (B) of 1 kilogram weight, which is 

 pressed upon the briquette (O) by two light spiral springs held on the 

 guide rods (F). The plunger (B) is bolted to the crosshead (Gr), 

 which is held by two vertical rods (F). A small lever (J), carrying a 

 brass pencil (K) at. its free end, is connected with the side of the cross- 

 head by a link motion, arranged so that it gives a vertical movement 

 to the pencil six times as great as the movement of the crosshead. 

 The pencil is pressed against the drum (A), and its movement is 

 recorded on a slip of silicated paper fastened thereto. The drum is 

 moved automatically through a small angle at each stroke of the ham- 

 mer. In this way a record is obtained of the movement of the hammer 

 after each blow. The standard fall of the hammer for a test is 1 cm. 

 (0.39 inch). This blow is repeated until the bond of cementation of 



