In the article ATMOSPHERIC INFLUENCE will be found a short review of the present state of our knowledge with respect to the resistance of building stones to the particular causes of decay presented by that agent. There are other causes occasionally at work, however, to which it may be advisable to call attention, for the stability of a building may often depend on their action. Thus in bridges, it is found that some stones do not satisfactorily resist exposure to tidal immersion, though they may stand, as workmen say, very well when kept constantly either wet or dry ; the Portland stone is an illustration of this remark. Other stones again may stand well in fresh water, but they disintegrate in salt water ; others may stand well in the atmosphere of the interior of a country, and yet yield rapidly on the sea shore, or in certain towns. Some stones resist the action of fire, such as the commonly-called fire stones of the subcretaceous series; whilst the limestones, and the ordinary sandstones, are very Injuriously affected by great heat ; and are either partially calcined, or split and cracked by it in a manner to destroy their powers of cohesion. Where the stones employed are intended to receive heavy crushing loads, they must be selected from amongst the denser varieties ; and the portions of the stone work of a building which might take up water by capillary action must be executed of such materials as should limit its range and its chemical effect. And here it may be as well to remark that, although in positions of this kind, the non-absorptive powers of a stone are great recommendations for its use, yet that in the plain wall spaces of the upper structures externally, and in floors and wall linings internally, there is a disadvantage in the use of the decidedly nou-absorbent stones. They are in fact rapid conductors of heat, and if any moisture should exist in the atmosphere it condenses on their surfaces in a manner which is unsightly, and often injurious to the health of the inhabitants of the building wherein this phenomenon occurs. Some stones are exposed to a very disagreeable action from the efflorescence of the salts they may contain ; others again part with their consti tuents iu a manner to compromise the solidity of the mass, and even to produce injury to the materials around them, as for instance, in tho case of the magnesian limestone, which under the action of London rain parts with the sulphate of magnesia in such quantities as to destroy, upon its subsequent efflorescence, the materials into which it filters. It would seem, moreover, that some of the limestones are liable to chemical changes of a peculiar nature ; for the marbles, such as the Carrara marble, pass from the subcrystallino to the saccharoid state on exposure to gentle heat, or to the effects of the atmosphere ; whilst on the other hand, the Portland stone, and other varieties of the carbonates of lime, which have not been affected in situ by plutonic agencies, harden by exposure. It may be that in the former case, the water of crystallisation evaporates ; and thus allows the base to resume an amorphous character; and that in the latter, the hardening proceeds from the change in the proportions of carbonio acid present, the atones being originally protocarbonates of lime, and subsequently passing into the perearlonatcs. The best sand.
stone...also harden by expeenro to the weather ; and It may be observed that this indurating process takes place the most rapidly when the water taken up by the sandstones may contain any lime in solution.
The resistance of building stones to physical forces depends greatly upon the homogeneous nature of their grain, and the direction of their planes of stratification and cleavage ; the resistance to disintegration and decay depends also very much on the same causes, especially when the atones are employed in buildings. Thus the atones which have been produced by the deposition of diluvial, or alluvial currents, present distinct traces of bedding, in consequence of the inequalities iu the transporting powers of those currents at particular periods ; and if those stones should be used in such a manner as to leave the planes of bedding vertical, and parallel to the exposed face of the building, they would be exposed to exfoliation of a very dangerous character. The atones which have been produced by the deposition of substances in suspension in former oceans, such as the oolites, the chalk, and the freshwater, or marine limestones of the tertiary series, often have planes of bedding produced by the periodic abundance of the animal remains they may contain ; or they may have distinct laminations iu consequence of their mode of crystallisation. In either of these cases the stones would be exposed to crush with greater ease in the direction of the divisional planes, and the effects of the atmosphere would be most energetically exercised when those planes were placed in a direction to favour the absorption of moisture by capillary action, or to allow the stone to exfoliate in a direction parallel to the face. In fact, the most important rule to be observed in the practical use of stone is to place It " in the natural way of the bed," as workmen say ; and though there may be some stones in which it is very difficult to distinguish the bed, and others in which there is less reason than usual for adhering to this law, yet even in them it is preferable to place the bed of the stones in its natural position.
A very elaborate report was drawn up in 1839, by a commission named for the purpose of selecting tho building-stone to be employed iu the Houses of Parliatnent ; but in consequence of the marked failure of the material then selected, the recommendations of that report have lately been treated with neglect. This is unfortunate; because there
is much information of value in the report, and the methods of exami nation it was based upon are substantially correct ; and, as it now appears that the stone actually used in the Houses of Parliament, was not the one recommended by the commissioners, the latter ought not to be blamed for the failure which has occurred. The errors which, however, do exist in the report consist in these points :-1. The resis tances to atmospheric action were appreciated by the state of the stone in certain ancient buildings, which were mostly in the neighbourhood of the quarries from whence those stones had been extracted. 2. In the attempt to judge of the durability of both new and old stones by what is known as Brard's test, or exposing them to the effects of the efflorescence of a solution of the sulphate of soda. Now it is notorious that the atmospheres of large towns contain many elemeuts which do not exist in the atmosphere of the couutry, and nothing but a long exposure to the former would justify an absolute opinion as to its possible effects; even the Cacti stone itself resists tolerably well in Lower Normandy, but it decays rapidly at Havre, and more rapidly still in London. Tho objections raised by the French engineers to Brard'a test have already been referred to under ATMOSPHERIC IN FLUENCE.; and it may suffice here to observe that practically it has been shown to be of very little use as an indication of the durability of a building-stone.
Before closing these remarks, it may be as well to state that some classes of atones are known by the name of freestone, when they are susceptible of being worked freely by the axe, or by the mallet and chisel ; the ordinary sand and lime stones are of this description, but the granites, whinstones, slates, and basalts, cannot be worked in this manner. Another technical distinction in the mode of conversion arises from the description of saw used; the softer stones are cut by the toothed saw, the harder ones are cut by the plate saw and grit. The marbles are susceptible of receiving a polish ; the ordinary sand and limestones remain dull whatever labour may be expended on their faces. The atones which strike fire (or the eilicious ones) are usually harder and more durable than the calcareous stones ; but the labour upon then is much more expensive ; and it is almost always the case that the stones which present on the surfaces of their fracture, a number of asperities and sparkling facettes are more difficultly worked than are the stones which present even, dell faces. Of materials of this description the lighter coloured ones are generally softer than the darker ones, and it may be considered a safe rule in judging of new atones, that if they absorb water easily they are not fitted to resist the action of the atmosphere. If the stones should yield a clear ringing sound, they may be considered to be of a fine uniform texture, and free from flints, earthy nodules, vents, or shakes; and if in working they should emit a sulphurous smell they would usually prove to be durable. One of the simplest and best precautions to be observed in the use of building-stones, is to quarry them some long time previously to their application, and to expose them to the weather, in order that they may lose gradually what workmen call their " quarry damp." In buildings, the denser and harder stone should bo used in the lower courses, the lighter ones in the superstructure; the non-absorbent atones should be used in the ground and in plinths, cilia, string courses, and weather beds of cornices, 3:c.; the softer and more absorbent stones may be used forItalia walling.
Mr: Hodgkinson found that the resistance of stone columns to crush ing weights was nearly uniform when their heights were less than 12 times the dimension of their sides ; when their heights were 15 times their sides, the resistance was manifestly diminished ; when their heights were 24 times their sides, the resistauce diminished in the ratio of 138 to 93 nearly ; when they were 30 times the side, the resistance became reduced from 138 to 75; and when they were 40 times their side, the resistance was reduced from 188 to 52. Stone columns are considered by Mr. Hodgkinson to yield first at the ends, in consequence of the tendency of rigid bodies to split into wedges, with sharp points at their apices, under the action of superincumbent weights. The weights stone-columns can bear depend at all times on their smallest section.
Mr. Adie published in the ' Transactions of the British Association for the Advancement of Science,' 1834, some interesting observations on the expansion of stone under the influence of heat. From these it would appear that within a range of 1S0* Fahr., the expansion of the Craigleith stone is of its length, or the same as the expan sion of glass, and very nearly the same as the expansion of cast-iron, as ascertained by Lavoisier. The expansion of the black Galway marble was found to be 0.00043855 of its length ; that of white marble seems to have been rather less. [HEAT; RESISTANCE OF MATERIALS.] (Consult Itondelet, l'Art de Bdtir ; Claude!, Formula a l'uslge des Ingaieurs; Dumauet, Cours de Construction ; De la Beche'a Geological Observer; Report on Building Stones, 1839; les Anuales des Mines, and lea Annales des Ponta et Chaussees, passim; les Annales des Trarau.c Publiqucs de la Belyive ; Transactions of the British Association ; Builder (journal ), passim ; Transactions of the Society of Arts, 1860.)