700

THEORY AND PRACTICE OF ENGINEERING.

Rook II

2*53 cwt., or 71*06 lbs. on the square inch. The experiments with the stones werecautiously made; the weight on the lever was successively increased by a single pound,and in order to ensure greater accuracy, a minute was allowed to elapse previous to theapplication of each additional weight. The eighth column shows the pressure at which thestone commenced to crack, and the ninth the pressure at which it was crushed. 'Die unitassumed is the pound weight placed at the end of the lever ; the employment of this unityin the table is preferred to stating the calculated weights, l>ccause it is not wished to givea greater appearance of accuracy than can strictly be adjudged to the experiments ; but ifabsolute measures be required, the pressure either upon the face of the cubes employed on1 square inch of the surface may be estimated, as nearly as the means employed enabled itto be ascertained, by multiplying the figures in the table by either of the values of theunit above stated. The results having been obtained with the same press, and under thesame circumstances, it is presumed that no objection can be made to them as comparativeexperiments.

The tenth indicates the specific gravities of the stones, accurately taken by the meansusually employed.

The eleventh contains the specific gravities of the solid materials of which each stone iscomposed, on the supposition that the water absorbed, when the atmospheric pressure isremoved, completely replaces the air which before occupied the pores.

The twelfth shows the bulk of water absorbed by the stones when saturated under theexhausted receiver of an air-pump, their entire bulk being taken as unity. The quantityof water absorbed in this process may be considered to represent space occupied by thepores or interstices in the substance, unless we suppose that in some cases the adhesionbetween air and the solid particles is so great that the entire removal of the atmosphericpressure is not sufficient to counteract the force. It is certain when this pressure is notremoved, long immersion in water will not occasion the displacement of all the air containedwithin its pores.

Sandstones on the continent, as well as in England, are less used in architecture thancalcareous stones ; there are nevertheless some kinds which are sufficiently solid, and inaybe safely employed in those districts where calcareous stones are wanting. At Paris thesandstones found in the environs are only used for paving, in consequence of the difficultywith which they are worked : many of the sandstones, which appear to be very friable, andreadily affected by the air, are used with advantage in constructions under water.

Limestones , or Calcareous Stonesy are the most generally used in the construction ofedifices, and are called calcareous, because, when exposed to heat, they are reducible tolime; they arc also distinguishable by being soluble in acids, in which they strongly effer-vesce : a drop of nitric acid falling on a calcareous stone, it bubbles and hisses, like hotiron plunged into water; when struck with the steel it emits no sparks.

Limestones are most frequently used, not only because most abundant, but also becausemore easily worked than all others, and possessing sufficient tenacity to resist pressure, andpreserve the mouldings, arrises, &c. The varieties are not, however, indifferently used ;some have not sufficient cohesive power, as, for example, chalk, several granular calcareousstones, simple or micaceous, from the primitive and intermediate strata, which do not resistpressure ; others, having their parts sufficiently compact, are too fragile, or, according tothe opinions of the workmen, too dry y such as those which are very compact and formed offine grains having a conchoidal or shelly fracture; these varieties are frequently filled withfissures injurious to their solidity, whether open or filled with calcareous spar. Calcareousstones, most suitable for construction, are the compact, with unequal fracture, flat, orirregular, of an earthy tinge, and these formed of shells, united by a cement, partly earthy,partly crystalline. These varieties abound in the secondary and tertiary formations, in de-posits analogous to those of the Jura, and similar to those in the environs of Paris ; of thesethe greater number of the monuments of the civilised world have been constructed ; thefinest houses of Amsterdam and the mosques of Constantinople are built of this material.

'The stones of the secondary formation are also in general use ; those in the second bedof tertiary formation are abundantly employed in Paris . The workmen class this underseveral varieties according to their application.

The calcareous tufa or travertine used in Italy is whitish or yellowish, and is found inthe substructions of most of the ancient temples, in the Coliseum and other public buildingsin Rome : it must always be placed in the same position as in their natural beds, where itis deposited in horizontal layers; when laid improperly these exfoliate and split vertically ;stone of a very compact and homogeneous structure, and forming beds of great thickness,will alone allow of their natural position being reversed.

Calcareous spar is found in crystalline masses, or in colourless crystals; it is easily dissolvedby muriatic acid; its specific gravity is 2*7; it loses 46 per cent, by the expulsion of car-bonic acid. The stalactitic carbonate of lime, or concretionary limestone, is formed ofzones which have a fibrous structure arising from the successive deposits of the crystallinelimestone from its solvent water. The stalagmite or alabaster limestone does not exhibitconcentric zones, but spreads out in a waving and parallel direction. The stalactites are