made, has been little understood or attended to. During a few months' residence in the metropolis, I was induced, by the nature of my mineralogical pursuits, to pay some attention to the different stones of which the pavements and public edifices are constructed. On walking into the court of Somersethouse, after some weeks of dry weather, I was particularly struck with the appearance of the columns on the left band, facing the west. The stones, in three columns, were some of them en tirely coated with soot, when the stones above and below were perfectly white. In other parts, a white stone was between two black ones, and the division of colour as distinct as if the one had been painted white and the other black. These stones were all equally exposed, and the variation of colour could not be explained by their situation. At first I conceived, that this difference of colour might be occasioned by some substance entering into the composition of the black stone, that had a chemical affinity for ammonia, which is contained in soot; but, on examining some of the stones that were within my reach, I found that those which were covered with soot, had a hard, smooth, surface, and the white stones were evidently decaying. The particles on which the soot had fixed, were fallen off, and had laid bare the natural colour of the stone, as per fectly as if they had been recently scraped with a chissel. In other parts of the building, I observed the visible decomposition of the stone, by moisture, particularly in the upper part of the alto relievo figures.

This edifice, like most of the modern buildings in the metropolis, is constructed of Portland-stone; a peculiar kind of lime-stone, which I shall afterwards more particularly notice. It is evident, how. ever, that this stone which is considered of the same kind, and comes from the same place, varies much in its property of durability.

When the same stone is constantly exposed to the action of water, the difference in its qualities of resisting decay is more apparent, as is evident from an inspection of London, Blackfriars, and Westminster, bridges, at low water."

The best kind of Portland stone is ill suited to resist the decomposing effects of water, the two former of these magnificent bridges, constructed at so much expence, are perishable monuments of the neglect of mineralogical science.

In stones of the argillaceous genus, more striking instances of rapid decay occur. I have seen stones of this kind, in their native beds, or quarries, some 'hundred feet under the surface of the earth, so extremely hard, that they resisted the point of the pick-axe, and could only be removed by blasting with gunpowder; yet, when the same stone was exposed to the air for a few months, it became soft and shivered into small pieces. The cause of this sudden decay, I shall afterwards explain. It rarely happens that builders or architects have any acquaintance with mineralogical and chemical science, to enable them to anticipate the changes which will be effected in the materials they select, by the action of the agents to which they are to be exposed. The loss and disappointment which this ignorance has occasioned in the construction of inany public works, is well known.-A remarkable instance of this kind lately took place at Paris. A gentleman was walking with an eminent mineralogist in one of the newly-erected public edifices; they were pleased with the appearance of soine large columns in the interior; when the latter had examined them more closely, he predicted, from the nature of the stones, that they would perish in less than three years, About ten months after, the gentleman happened to pass the same place, and observed the stones of these columns were shivering so rapidly, that workmen were then engaged in replacing them; which had become necessary to secure the roof. In forming the tunnel of the Huddersfield canal, which is three miles in length, the workmen in one part had to cut through a bed of stone of considerable extent, so hard that they were obliged to remove it by blasting." It appeared so compact and firm, that it was thought unnecessary to wall and arch the passage; but, in a few months after the access of air to it, it shivered and fell in; and the removal and repair occasioned much delay and expense. It was a dark compact argillaceous stone, containing oxyd of iron, and resembling some kinds of basalt; but its shistose or slaty structure was soon apparent, and it became as soft as the bituminous shale which accompanies coal. Some kinds of

stone

at Charnwood forest, in Leicestershire. The granite is chiefly used for paving in the neighbourhood; it is small-grained, or what some mineralogists would call, secondary granite; and others sunite as it contains small crystals of hornblende. Its specific gravity I found 2.77. It is extremely hard, and is worked by blasting. If it can be raised in blocks of sufficient magnitude, it might be employed with great advantage in constructing the foundation of the new bridges; as it is only one hundred miles distant from London, from whence there is direct carriage by water. It is, I believe, the most durable stone that can be found in any great quantity at the same distance from the metropolis. At the same place is procured another stone, resting upon the granite; it is a species of greenstone, and hornblende porphyry, containing small crystals of felspar, in a basis chiefly of hornblende. The workinen informed me, it is broken with more difficulty than granite; it is not so hard, but is very in frangible. Its specific gravity I found 2.88. It contains some oxyd of iron, like other stones of this species, which may perhaps render it improper to be used, where it can be acted upon by water; but it has every appearance of being a very durable stone, and might deserve the attention of builders, where great strength of materials is required. In silicious sand-stones, the coarseness or fineness of the grains is of less importance than the substance in which they are imbedded. Those which have a basis of ferruginous clay, are soft and perishable; but when the basis itself is of a silicious kind, the stones are almost equally durable with granite. The upper strata of many of the highest hills in Yorkshire and Derbyshire, are of this kind, which Mr. Whitshurst calls a millstone grit. It is of a finer-grained stone, of the same kind that Kirkstall Abbey, near Leeds, is built. Though the Abbey is a ruin, the stones which remain are little decayed. After the lapse of six hundred years, they preserve their angular sharpness, and the impression of the chissel, as fresh as if they had been recently worked. There is a quarry of this stone in the neighbourhood; and I have observed some of the stones in the London docks, are of a similar kind. Some silicious sand-stones appear to be of alluvial formation, and have their parts so imperfectly cemented, that they are unfit for the purposes of architecture; of this kind are the rocks on which the town

of Nottingham is built;` and the red-sand rock of Cheshire. The former may be considered more properly as indurateď gravel, intermixed with rounded quartz pebbles; but, though it is too loose to be applied to purposes of architecture, it may be excavated to a considerable extent, without the necessity of supporting the sides and roof. The granite of Charnwood forest, and the loose sandrock of Nottingham, are not more than twenty miles distant, but they may be considered as forming the two extremities of the scale of aggregate silicious stones, from the very hardest to the softest kind. Argill, or clay, is never found pure in any kind of building-stone. It is soft, smooth, and unctuous to the touch, and will absorb more then 2 times its own weight of water; and, as Mr. Kirwan has observed, it commuuicates, in some degree, these properties to stones, if it is combined in a proportion of from 20 to 30 per cent. In a greater proportion it destroys the qualities of silicious stones. Many argillaceous stones contain a considerable quantity of iron, not perfectly oxydated. When exposed to the atmosphere, they speedily decay. It is in stones of the argillaceous kind, that the greatest caution and mineralogical knowledge are required, in the selection for purposes of durable architecture. I have seen a hillock at the mouth of a lead-mine, supporting a luxuriant vegetation, which a respectable miner informed me he had twenty years before blasted from a compact bed of toadstone, or basalt, that resisted the pickaxe, and no soil had been since thrown upon it. An instance of this kind was the occasion of an action at the last York assizes, between the proprietors of the Barnsley canal, and the engineer. On the part of the proprietors it was contended, that the hill through which he had to cut a tunnel, was a soft marble, On the part of Mr. Pinkerton it was stated, that, though the part exposed was now soft, it was, when first opened, a very hard and compact rock, extremely expensive and difficult to work; and the truth of this statement he offered to prove, by perforating any part of the hill where the stone had not before been laid bare to the action of the atmosphere. The argillaceous sand-stone which accompanies or lies over coal, is used for buildings in coal countries, as in some parts of the West Riding of Yorkshire; but it is not suited for public buildings, or works intended to be durable. Stones

of

of the calcareous genus, comprise all the various marbles and limestones. These are generally more free from admixture with other earths, than stones of the silicious, or argillaceous kind; and their relative degrees of excellence for purposes of architecture are more easily ascertained by their external character. Besides pure lime, they contain from 45 to 50 per cent. of carbonic acid and water. Lime when pure is soluble in five hundred times its weight of water; and, even when united with carbonic acid, it is in a less certain degree soluble in river waters, owing to the minute portion of different acids, which is generally contained in them. On this account they are ill suited to form the foundations and piers of bridges, or to be employed in the construction of works exposed to the action of the water. The durability of marbles and lime-stones might, I be lieve, with some certainty be determined by their relative degrees of hardness, and by observing the time required to dissolve an equal cube of each kind of stone in marine acid, of the same strength, diluted with five times the quantity of water at the same temperature. The sediment remaining will also determine the quantity of silex or clay with which the lime is combined. Magnesia enters into the composition of some lime-stones in the proportion of two-fifths, and renders the softest stones of this kind less soluble in acids than the hardest marbles, on which account it will be necessary to ascertain by chemical experiments, whether the slowness with which lime-stone is soluble, proceeds from the presence of magnesia; but I believe it will also be found that a mixture of this earth, where it occurs in lime-stone, not only renders it less soluble in acids, but communicates to it a degree of durability which is not to be found in other lime-stones of the same degree of hardness. The high comparative degree of preservation observable in the exterior of York Minster, and other public edifices which are built of this stone, may serve to prove its excellence for purposes of architecture.

Portland Stone is a peculiar kind of lime-stone, which some mineralogists call roe-stone. When examined with a magnifying lens, it will be found to contain a number of small round globules, resembling in appearance the roes of fishes, imbedded in a calcareous basis, from whence it derives its name. It also contains fragments of shells, and minute Calcareous crystals. It varies in its qua

lities of hardness and compactness, and in its properties of durability, as may be observed in many of the public edifices in London, which are built of this stone. In the construction of St. Paul's, some attention appears to have been paid to the selection of the stones for the exterior; which are more perfect than those in many buildings of a recent date; but they are evidently perishing in the upper part of this magnificent structure.

Portland-stone contains carbonate of lime, united with a small portion of silex and clay. Its solution in diluted muriatic acid gives a dark-blue precipitate, with the Prussian alkali, indicating the presence of oxyd of iron, to which it owes its brownish tint; but the quantity of iron is too small to affect its quality for the use of the architect. It burns to a white lime, losing more than eight parts in twenty of its weight, during calcination. According to Professor Jameson, roestone is never used for architecture, on account of its speedy disintegration; but his observations appear to have been confined to the varieties of this stone in Germany, and inapplicable to those in our own island. Two stones called by the same name, from different situations, are seldom exactly similar in all their properties; which indeed rarely happens with stones from different layers of the same bed. And where strata of calcareous stone are separated by other kinds of stone, the upper and lower strata, almost invariably differ in hardness and specific gravity; on which account it would be very desirable, that a mineralogical examination of stones should be made in their native quarries, and that those which are intended for the external part of buildings, should be judiciously selected from the others. Of all stones of the calcareous genus, there cannot be a doubt that compact marbles, which can receive the highest. degree of polish, would be the most beautiful and durable for the exterior of buildings; but their scarcity in this country prevents their application to this purpose, Alabaster, which is composed of lime, united with sulphuric acid, from its beauty and the facility with which it can be worked, is used for ornamental architecture and sculpture; but the solubility of this stone renders it ill-suited to resist the agency of water.

Dr. Watson relates, that he suspended two ounces of this stone in a pail of water for forty-eight hours, changing the water several times, and found that it

had

had lost one-thirtieth part of its weight. I suspect this alabaster was one of the softest kind; but the experiment may serve to show, that this stone will not bear exposure to rain. There are no other stones deserving the attention of the architect, but those of the silicious, argillaceous, and calcareous, genus, in the latter of which we may class magnesian lime-stone, the only building-stone into which magnesia enters in any considerable proportion.

The decomposing and disintegrating agency of water, air, and change of temperature, on stones employed in architecture, is the same by which Nature is constantly operating to convert solid rocks into soil. The fiat of Omnipotence "commands the hardest stones to be made bread," or to become the means of supporting vegetable and animal life, but the processes by which this effect is produced, are slow and gradual. The earths of which all stones are composed, are either to a certain degree soluble in water, or are capable of being mecha nically suspended in it when minutely divided. A drop of water, constantly running across the hardest stone, soon marks its path, by cutting a furrow in the surface; hence, the well known adage "Non vised sæpe cadendo." This effect, how. ever, is slow, compared with that of other causes, which are constantly operating. Water insinuates itself into the pores and ninute crevices of stones, and being expanded by increase of temperature, separates the parts from each other, but it produces this effect in a much greater degree when expanded by freezing. Frost is the most potent agent by which Nature operates en masse,' frequently splitting the hardest rocks, and levelling immense portions of nmountains in a single night. In building-stones which have a tendency to a slaty structure, the destroying effects of frost are most likely to be soon perceptible, from the facility with which water can insinuate itself between the lamina. In stones of the argillaceous genus, the joint effects of water and air frequently produce a speedy decompo sition, even of those of the hardest kind. If a stone have a strong earthy smell, when breathed upon, its durability may be suspected. Iron frequently enters in large proportions into argillaceous stones, in a state not perfectly oxydated, and azterwards combines with a further por tion of oxygen, forming a brown incrus tation to a certain depth, which becomes soft and falls off, or is washed away, and

the process of decomposition goes on till the whole stone is changed. The argill, or clay, in these stones, is also frequently capable of absorbing a greater portion of water; and the stone may be rendered soft by the combined operation of these two causes. It is in these stones that the different earths are combined, in the proportions best suited to the support of vegetable life.

Lichens and mosses fix themselves on the surface of stones, and, by insinuating the minute fibres of their roots, tend to accelerate their decay and prepare a vegetable mould for plants of a larger growth. The decomposition of many argillaceous stones, which are most necessary for the support of vegetable life, is most speedily effected by natural causes; hence, more caution and skill are necessary in their selection for architecture. No stones of this genus should ever be employed, which have not had the test of time, without a careful mineralogical and chemical examination of their nature and contents. This exami nation would always precede the applica tion of stones for public works or edifices, intended to endure for ages, were engi neers and architects as attentive to their future fame, and the interests of posterity, as to present emolument. Besides the chemical examination of stones, it is necessary to try the effect of various degrees of heat, and of boiling water, upon them, applied for a considerable time, and to note carefully their encrease or loss of weight and other changes. In this manner we may sometimes anticipate with certainty, in a few days, what will be the effects of less powerful, but longcontinued natural operations in a series of years. For purposes of durable archi tecture, no stones but those of the sili cious genus should be employed in the exterior parts of buildings. In our moist and variable climate, all kinds of stone but the silicious will perish sooner than in countries where the rains are less fre quent. The present state of our churches and public buildings proves that the se lection of building-stones has been left to ignorance or chance to determine, The antients, in their public works, appear to have had a just regard to perpetuate the glory of the era in which they lived, and to leave to posterity durable and useful monuments of their skill, which should secure their gratitude and veneration. The public architecture in this country appears constructed on calculations of false economy and present onvenience