WEIGHT OF STONE.
Weight or density is an important property, since upon it depends to a large extent the strength and durability of the stone.
If it is desired to find the exact weight per cubic foot of a given stone, it is generally easier to find its specific gravity first, and then multiply by 62.4—the weight, in pounds, of a cubic foot of water. This method obviates, on the one hand, the expense of dressing a sample to regular dimensions, or, on the other hand, the inaccuracy of determining the volume of a rough, irregular piece. Notice, how ever, that this method determines the weight of a cubic foot of the solid material, which will be a little more than the weight of a cubic foot of the stone as used for structural purposes. In finding the specific gravity there is some difficulty in getting the correct dis placement of porous stones,—and all stones are more or Tess porous. There are various methods of overcoming this difficulty, which give slightly different results. The following method, recommended by General Gilimore, is most frequently used: All loose grains and sharp corners having been removed from the sample and its weight taken, it is immersed in water and weighed there after all bubbling has ceased. It is then taken out of the water, and, after being compressed lightly in bibulous paper to absorb the water on its surface, is weighed again. The specific gravity is found by dividing the weight of the dry stone by the difference between the weight of the saturated stone in air and in water. Or expressing this in a formula, in which W,, represents the weight of dry stone in air, W, the weight of saturated stone in air, W, the weight of stone immersed in water.
The following table contains the weight of the stones most frequently met with.
The apparent hardness of a stone depends upon (1) the hard ness of its component minerals and (2) their state of aggregation. The hardness of the component minerals is determined by the resistance they offer to being scratched; and varies from that of talc which can easily be scratched with the thumbnail, to that of quartz which scratches glass. Many rocks composed of hard materials work readily, because their grains are loosely coherent; while others composed of softer materials are quite tough and diffi cult to work, owing to the tenacity with which the particles adhere to each other. Obviously a stone in which the grains adhere closely
and strongly one to another will be stronger and more durable than one which is loose textured and friable.
The toughness of a stone depends upon the force with which the particles of the component minerals are held together.
Both hardness and toughness should exist in a stone used for stoops, pavements, road-metal, the facing of piers, etc. No experi ments have been made in this country to test the resisting power of stone when exposed to the different kinds of service. A table of the resistance of stones to abrasion is often quoted, but as it contains only foreign stones, which are described by local names, it is not of much value.
Under this head will be included (1) crushing or compressive strength, (2) transverse strength, (3) shearing strength, (4) elasticity. Usually, when simply the strength is referred to, the crushing strength is intended.
The crushing strength of a stone is determined by applying measured force to prisms until they are crushed. The results for the crushing strength vary greatly with the details of the experiments. Several points, which should not be neglected either in planning a series of experiments using the results obtained by experiment, will be taken up separately, although they are not entirely independent.