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60 THE CIVIL ENGINEER AND ARCHITECT’S JOURNAL. [February 1, 1S6S. exhibited in the Russian department of the Great Exhibition at Paris. It almost follows as a necessity for the actual conversion of mud into stone, that all accumulated masses of this kind should have been deposited at or conveyed to some considerable depth below the common surface of land and water. The pressure of a heavy column of water, and the equable and increased temperature of the interior of the earth, must always be taken into account in considering their origin as rocks, and their sub sequent history. But as all the stones with which we have to deal are now above tho level of the sea, and some of them many hundreds, others even thousands, of feet above it, a further change derived from their mechanical upheaval must have acted upon them. Whatever may have been the nature or mode of action of the upheaving force, it cannot fail to have acted unequally on the different kinds of material. Thus it must have squeezed the softer kinds and crumpled them into folds, broken the harder and more brittle varieties, and cracked and fissured all exposed to its influence. The upheaving force also having not only acted unequally, but having been limited to certain lines of direction and areas, the rocks in certain places have been tilted up at a high angle, while in some spots they have been lifted so uniformly as not to show much evidence of fracture. The action of water percolating the more porous rocks and the fissures has always gone on, and has generally assisted, though it may sometimes have checked, the mechanical action of the upheaving force. This also must have been greatly modified by the mechanical position of the rock. Many disturbing forces have continued for a very long time, they have been always exceedingly slow, and periods of upheaval have generally alternated with periods of subsidence and long intervals of repose. Thus all the materials have to a certain extent interpenetrated each other, and they have been exposed while all this was going on to such chemical action as must happen when water percolates through minerals and groups of minerals of different kinds placed under conditions favourable for change. These conditions are much more numerous and varied than was once thought, and provided time enough be allowed, there seems nothing in the way of modification of the position of the elements that may not have been accomplished. § 5. Weathbbing and protection of Stones.—The effect of exposure after the material composition of stones has been determined by original deposit, and subsequent change, is generally described as weathering. This effect varies ex ceedingly under different circumstances. It varies with climate to a marked degree. It varies with position in refer ence to the sea, and also with reference to prevalent winds and ■compass bearing. It varies with the amount of shelter and the position in the building. The rainfall of the locality affects it seriously. Town and country exposures, even in identical climates, are very different in their influence. This durability for building purposes cannot be determined without many considerations and much observation. It can hardly be deter mined at all in a short time, for many stones that resist des truction at first, and for some years, afterwards decay rapidly, while others that' seem very poor at first become almost per manent when certain early and inevitable results . of exposure have been produced. It is evident that this subject is one of the greatest practical importance, for whatever be the colour, texture, or aspect of a stone,'however easily it can be procured, however cheaply it may be worked, its value must depend on its power of resisting the weather ; and it cannot be denied that wind and rain, heat and frost, are powerful agents con stantly at work, and they tend, if not to reduce the stone to its elementary mud, at least to test very severely its powers of resistance. Thus the architect must study stones in the field and in the quarry, if he would know how they will appear after a few years in his building. There is, however, another effect of exposure, and of an opposite kind. Most stones harden by weathering, and thus, a material that cuts in the quarry like cheese may, if properly used and sheltered for a while, become a very useful material. Our neighbours, the French, have a practice of making a complete facing of thin planks or laths in front of buildings in progress, by which means their soft but beautiful white stone facings, as well as the decorative work, is kept free from the direct action of the weather for some months at least. This practice is thoroughly in accordance with reason, and might be copied by us in England with great advantage, especially in the atmosphere of London and other large towns. It is, however, likely to be most efficacious in a dry climate. In a natural state stones are rarely exposed to weathering except in the two positions of approximate parallelism to the plane of stratification, and at right angles to that plane. The surface of the ground represents the former, and the face of a marine cliff or quarry the latter. The surface of the ground, when nearly horizontal, is often so far sheltered by the accumulation of soil that it offers no evidence of the relative durability of the stone underneath, although under certain circumstances, when the rock is moderately hard and there is little cultivation, a great deal may be learnt by careful local investigation. This is especially the case with hard stones, such as granite and certain varieties of gritstone. It must be remembered that the minimum of weathering takes place when a vertical section of stone is exposed, and the maximum when, the slope of the bedding is considerable, and tends to carry the drainage out on the vertical face laid bare. Thus a cliff does not necessarily exhibit the weathering, especially as the natural process of destruction that takes place in all cliffs takes place to a great extent by undermining and eating away soft beds at intervals, and thus letting the hard and little weathered bands of stone fall down, leaving an unweathered or little weathered face. The nature of weathering, and the kind of injury stones suffer by exposure, is not difficult to understand. All build ing stones in common use are fissured, to some extent porous, and partially soluble in pure water. They are still more easily acted on when the water contains minute quantities of mineral acid, either sulphurous acid, liberated in the atmos phere on the burning of coal containing sulphur, or carbonic acid derived from animal or vegetable life. Carbonic acid gas is always present in the atmosphere, and sulphurous acid cannot be absent in the air of towns; thus, whenever rain falls and is driven by wind, water containing acid enters the exposed surface of stone. Once there it cannot but dissolve a minute part of such soluble minerals as exist in the stone; it eats away the surface, or it destroys the cementing medium, or it carries away the thin bands that occur between insoluble parts. Thus there is and must be direct chemical action, to some extent, on the occasion of every shower that falls. In a climate like that of England, where rain is frequent, this action is con siderable ; it is most considerable in such parts of the country where rain is most frequent, and in such aspects as arc most exposed to the rainy and prevalent winds ; thus a south-west exposure is the worst everywhere in England, and the western parts of our island are generally worse off than the eastern. But as the sulphurous acid is chiefly confined to places where much coal is burnt, towns arc worse off in this respect than the open country; large towns are worse than small towns, and manufacturing towns worse than those where there are fewer chimnies. On the whole, London, Manchester, Birming ham, Leeds, Newcastle, and Liverpool, are among the worst of all, because of the vast extent of area covered with houses, and as being places where most coal is burnt. But there is another source of destruction independent of this: alterations of temperature involve expansion and con traction, and as water attains its minimum volume at the tem perature of about 39° F., any frequent changes above and below this limit cannot fail to disintegrate any surface of stone exposed to it. All stones are more or less absorbent, and are cracked at the surface, although the fissures are often very minute ; into the stone by absorption, or by these fissures, water penetrates, conveyed by that principle of suction which gives to the sponge its peculiar properties. Cooled down to the smallest volume when the external cold approaches the freezing point, any rapid change above or below tho tempera ture of greatest density must produce sudden expansion, and this in its turn involves disintegration ; thus every frost does something to destroy the surface of exposed stone, and the more delicately it is sculptured the more rapid and complete the destruction must be. The public buildings in the University of Oxford afford examples of rapid disintegration that are ex ceedingly striking, owing to the poor quality of the stone used ; but all our buildings exhibit it more or less clearly, Bucking ham Palace, built of Bath stone, having been perhaps as much injured as any. The Houses of Parliament, so often referred to, are, in certain exposures, and with regard to certain stones,