STONE, a detached piece or fragment of rock. The word is thus applied to the small fragments scattered in the ground or on roads, to the water-worn pebbles of the sea-shore or river beds, and to the hewn, dressed or shaped rock used as building mate rial, with which this article deals. A qualifying word generally ac companies "stone" when the term is applied to pieces of rock cut to a particular size and shape and used for a specific purpose, e.g., "mill-stone," "hearth-stone," "grave-stone," etc. The term "pre cious stone" is used of those minerals which, from their beauty of colour, etc., their rarity and sometimes their hardness, are valued for their suitability for ornaments (see GEM). The word is also often applied to many objects resembling a stone or pebble, such as the hard kernel of certain fruits, as of the cherry, plum, peach, etc., or the calculi or concretions sometimes formed in the gall or urinary bladder or the kidneys. (See BLADDER DISEASES and KIDNEY, DISEASES OF.) The "stone" has been a common measure of weight in north-western Europe. In Germany the "Stein" was of 20 to 22 lbs. In the British system of weights the "legal" stone, or "horseman's" weight is of 14 lb. avoirdupois; in weighing wool it was also of 14 lb., but is now usually 16 lb. The "cus tomary" stone for fish or butcher's meat is of eight pounds.
The disintegration of stone is caused by internal stresses. They may be set up, as in the case of granite, by heat, which causes the quartz or silica constituent to change its chemical form with an accompanying expansion. Similarly quartz or silica pebbles used as an aggregate in concrete cause its disintegration when sub jected to extreme heat. Again the internal stresses may be set up by the solution and recrystallization of minerals, or chemical sub stances, in the pores of the stone, for all stone is more or less porous. In cold climates certain porous stones may be compara tively rapidly destroyed by alternate freezing and thawing—the mere expansion of the absorbed water in freezing being suffi cient. Certain argillaceous limestones contain an earthy or clayey mineral now known as beidellite which has the property of ab sorbing or possibly chemically combining with water. This is accompanied by expansion, and causes apparently sound stone to rapidly disintegrate when exposed to the weather. Smoke and
waste gases from heating and industrial plants are carried by rain into the pores of the stone. Marbles and limestone, being com posed largely of the carbonates of lime and magnesia, are very susceptible to the solvent action of these acids. The mortar used to bed the stones has been held responsible for disintegration since the lime may be dissolved out by the action of rain water, to he recrystallized in the interior pores of the adjacent stone. The physical and chemical tests for stone are thus as follows: (I) Resistance to crushing; (2) acid test ; (3) absorption test ; (4) microscopical examination; (5) freezing and thawing test or its equivalent.
The resistance to crushing varies to an enormous extent with the different kinds of stone, from a little over 6o tons per sq.ft., which is the limit for a weak limestone, up to a load of over 1,300 tons necessary to crush the hardest granites. In general practice the load placed upon stone should not exceed one-tenth of the crushing weight as found by testing. The effect produced by soaking pieces of stone for some days in a 1 % solution of sulphuric and hydrochloric acids will decide roughly whether it will be durable in a city atmosphere. The microscope is the best means of determining the structure of a stone, and of recognizing the presence of matter likely to affect its usefulness adversely. Should iron pyrites be discovered in any quantity the stone should be rejected, as this impurity easily decomposes on exposure, and stains and splits the stone. The freezing and thawing test is slow and requires a rather elaborate refrigerating apparatus. The specimens of stone to be tested are alternately oven-dried, soaked in water and subjected to a freezing temperature, thawed at room temperature, dried, and the cycle repeated as often as desired. The accumulative loss in weight of the dried sample of ter each cycle is the measure of the stone's durability or resistance. This test may be simulated with some degree of accuracy by the sodium-sulphate test, in which the specimens of stone are oven dried, soaked in a saturated solution of sodium sulphate and allowed to dry out at room temperature. In drying out the sodium sulphate crystallizes in the pores of the stone, in a manner similar to the freezing or crystallizing of water. This test is apparently more severe than the freezing and thawing test and requires little apparatus. Laboratory experts have not yet agreed upon the exact relation of these tests to the actual behaviour, or life, of stone in structures. Dolomites, known to be fairly durable as building stone, are not correspondingly resistant to these tests.