Stones.--Most of the stones and other minerals employed for purposes requiring strength, are sufficiently incorruptible to last for ages, without requiring any particular protection from the chemical agency of the atmosphere. The granite and marble of some of the oldest Grecian and Roman structures retain their smoothness at the present day. Bricks and terra cotta are equally indestructible. Nevertheless, it sometimes happens that the strongest rocks become disintegrated in time, and fall to pieces, in consequence of their containing iron pyrites, or some other substance, upon which chemical action easily takes place. This defect is observed in some varieties of sienite and of freestone. A rock should always be rejected in building, which in its natural situation is found to be soft and friable at its surface, however hard its interior may be. Sometimes the surface of buildings is found to be disfigured by the oxidation of iron, in spots or streaks, upon the stone. The only remedy, where corruptible materials of this sort have been used, appears to consist in keeping them covered with a coating of paint, sufficient to exclude air and moisture.

Metals.--The precious metals, as gold, silver, and platina, being incapable of oxidation under common circumstances, require no process to keep them from decay. But many other metals become speedily corroded by exposure to the air and moisture, and require an artificial surface to protect them from decay. Copper protects itself by forming, when exposed to the atmosphere, a superficial coating of carbonate of copper, which gives it a dark color, but prevents the further action of the air on the internal parts. It is from this cause that the copper coins and the bronze statues and ornaments of antiquity remain nearly unimpaired at the present day. But the most useful of all metals, iron, is speedily rusted by exposure to the weather, and pure malleable iron decays more rapidly than cast iron. A mass of iron filings is speedily converted, by druggists, into carbonate of iron, by causing a small quantity of water to trickle gradually through it, the air being also admitted. Some curious facts are attendant on the rusting of iron. If a bar of this metal be frequently agitated, it rusts apparently much less, than a similar bar at rest in the same place. This fact is observed in rail-roads, and is noticed by Mr. Wood, in his treatise. A polished surface resists the action of the atmosphere longer than a rough surface, because less of the metal comes in contact with the air. Certain galvanic combinations are found to retard the chemical change, or decay, of metals. Sir Humphrey Davy discovered that the copper sheathing of ships may be preserved from corrosion or decay, by placing over it bars of some oxidable metal, such as lead, zinc, and especially cast iron. But in vessels thus guarded, although the copper remained entire, yet the bottoms became exceedingly foul by the adhesion of seaweed, shells, and marine animals. It is probable that these living beings are kept off by the poisonous nature of the copper, which becomes operative only when the metal is oxidated or corroded, and that, therefore, the usefulness of the copper depends upon its decaying.

To prevent the corroding and tarnishing of metals, it is customary, in the arts, to cover them with some less destructible material which may exclude the air. The more permanent metals, such as gold, silver, and tin, are applied to protect those which are less permanent, and hence have arisen the arts of gilding, plating, and tinning. Large and coarse objects are generally protected by a coating of paint, varnish, or oil, as in iron railings and large machinery. A valuable varnish for iron is made from coal tar. Where the lustre of a polished metal is intended to be preserved, a transparent varnish, or lacker, is employed. Rubbing, or scouring, is a temporary, though often necessary expedient, which removes a dull surface, at the expense of wearing out the material.

When cast iron is buried in the earth, especially if imbedded in clay, its decay is extremely slow, as is seen in aqueducts and gas pipes made of this material.

Organic substances.--The compounds which are spontaneously formed by organic bodies, both vegetable and animal, are of a different nature from those which exist in unorganized matter. They are the peculiar results of vital processes, and neither their structure nor composition can be imitated by art. During life, the elements of organic bodies are held together by vital affinities, under the influence of which they were originally combined. But no sooner does life cease, than these elements become subject to the laws of inert matter. The original affinities, which had been modified, or suspended, during life, are brought into operation; the elementary atoms react upon each other, new combinations are formed, and the organized structure passes sooner or later into decay.

The rapidity with which decomposition takes place in organic bodies, depends upon the nature of the particular substance, and upon the circumstances under which it is placed. Temperature, moisture, and the presence of decomposing agents, greatly affect both the period and extent of this process. By regulating, or preventing, the operation of these causes, the duration of most substances may be prolonged, and many materials are rendered useful, which, if left to themselves, would be perishable and worthless. The preservation of timber, of fibrous substances, of leather, of food, and of various objects of art, are subjects of the highest importance, and have received, at various times, much attention from scientific experimentalists.

Temperature.--The influence of temperature, in accelerating or retarding the decay of organized substances, is generally known. Cold tends to check the progress of destructive fermentation, and when it extends so far as to produce congelation, its preservative power is complete. Bodies of men and animals have been found frozen, in situations where they had remained for years, and even ages; and the recent discovery of an extinct species of elephant, in the ice of Siberia, shows that the period of this preservation is unlimited. On the other hand, in warm seasons and in hot climates, every thing tends to corruption and decay. Both animal and vegetable substances pass rapidly into the putrefactive fermentation; alimentary substances are difficult to preserve, and when moisture is combined with heat, ships, houses, and other structures of wood, as well as cordage, canvass, and clothing, have the period of their duration greatly abridged.

Dryness.--Although certain degrees of heat, especially when combined with moisture, tend greatly to promote decomposition, yet if the degree of heat, and the circumstances under which it acts, are such as to produce a perfect dissipation of moisture, the further progress of decay is arrested. The exertion of chemical affinities usually requires that one of the agents at least should be in a fluid state. And while a body is in a state of perfect dryness, no internal chemical change is likely to befall it. The beams and furniture of houses, often remain entire for centuries. In the arid caverns of Egypt, the wood of sarcophagi appears to have undergone no alteration in the lapse of two or three thousand years; the fibres of linen textures are found distinct and perfect, though weakened in strength, and the dried flesh of the mummies themselves discovers no marks of decomposition. In cabinets of Natural History, the specimens, so long as they are kept perfectly dry, undergo no alteration from spontaneous decay. They are, however, extremely liable to the depredations of insects, from which they require to be protected, either by impregnating them with poisonous substances, or by enclosing them in cases which are hermetically tight. In damp seasons and situations, an artificial dryness may be produced by keeping a shallow vessel of quicklime within the cases, and renewing it as fast as it becomes saturated.

Wetness.--Some materials, especially wood, are capable of lasting for a long time, if kept continually immersed in water, especially at low temperatures. Thus the lower part of a pump log is much more durable than the upper, if kept always under water. The effect of pure water is to dissolve and carry off the soluble parts, leaving the fibrous structure in a state less liable to fermentation than before. Some animal substances, likewise, such as leather, bear immersion in water for a considerable time. It must be observed, however, that the effect of wetness upon most organized bodies, is to soften their texture, and render them less able to support mechanical violence, than when dry. Wood, after having been long immersed, if taken out and dried, is found to be more brittle than it was before.

But the state which most rapidly promotes decay, is that of alternate moisture and dryness, attended with exposure to the atmospheric air. It appears, in regard to wood, that in each wetting, a sensible portion of substance is dissolved, and that in each drying, a new portion of soluble matter is formed. In a ship, under common circumstances, the parts which first decay, are those which are situated between wind and water, or are subjected to alternate dryness and moisture. So also in a post standing in the earth, the part which first decays, is usually that which is nearest the surface of the ground. Exposure to the vicissitudes of weather, is also one of the most common and active causes of decomposition.

Antiseptics.--A certain class of substances has received the name of antiseptics, from their power, when present, of resisting putrefaction in organic bodies, as well as in their products. Such are charcoal, tannin, resins, camphor, bitumen, sugar, chlorine, alcohol, oils, acids, and salts of various kinds. The manner in which they exert their preservative agency is not fully understood. It appears, however, that in some cases they combine with the substance to be preserved, forming a less perishable compound, as in the instance of leather; and probably in other instances they unite with and qualify the decomposing agents which are present.

Timber.--A vast expense is every year created by the premature decay of wood, employed in ships and other structures, which are exposed to vicissitudes of weather, and especially if they are subjected to the influence of warmth combined with moisture. Trees of different species, vary greatly in the durability of their wood, yet none of the species commonly employed, are capable of withstanding, for many years, the effect of unfavorable exposures and situations. The decay in timber is sometimes superficial, and sometimes internal. In the former case, the outside of the wood first perishes and crumbles away, and successive strata are decomposed, before the internal parts become unsound. In the other species, which is distinguished by the name of the dry rot, the disease begins in the interior substance of the wood, particularly of that which has not been well seasoned, and spreads outwardly, causing the whole mass to swell, crack, and exhale a musty odor. Different fungous vegetables sprout out of its substance, the wood loses its strength, and crumbles finally into a mass of dust. This disease prevails most in a warm, moist, and confined, atmosphere, such as frequently exists in the interior of ships, and in the cellars and foundations of houses. Its destructive effects, in ships of war, have given rise to numerous publications. Some writers consider that the dry rot is not essentially different from the more common kinds of decay, but there seems to be sufficient reason for the distinction which has usually been drawn. The prevention of the evil has been attempted in various ways, and with some degree of success.

Felling.--It is agreed, by most writers, that the sap of vegetables is the first cause of their fermentation and decay. Hence it appears desirable, if there is any season, in which the trunk of a tree is less charged with sap than at others, that this time should be selected for felling it. The middle of summer and the middle of winter, are undoubtedly the periods when the wood contains least sap. In the months of spring and fall, in which the roots prepare sap, but no leaves exist to expend it, the trunk is overcharged with sap; and in many trees, as the maple and birch, sap will flow out at these seasons, if the trunk is wounded. In summer, on the contrary, when the leaves are out, the sap is rapidly expended, and in winter, when the roots are dormant, it is sparingly produced; so that no surplus of this fluid apparently exists. From reasoning a priori, it would seem that no treatment would be so effectual in getting rid of the greatest quantity of sap, as to girdle the tree, by cutting away a ring of alburnum, in the early part of summer, thus putting a stop to the further ascent of the sap, and then to suffer it to stand until the leaves should have expended, by their growth, or transpiration, all the fluid which could be extracted by them previously to the death of the tree. The wood would thus probably be found in the driest state to which any treatment could reduce it in the living state. Buffon has recommended stripping the trees of their bark in spring, and felling them in the subsequent autumn. This method is said to harden the alburnum, but the cause is not very apparent, nor is the success at all certain.

Seasoning.--At whatever period timber is felled, it requires to be thoroughly seasoned, before it is fit for the purposes of carpentry. The object of seasoning is partly to evaporate as much of the sap as possible, and thus to prevent its influence in causing decomposition; and partly to reduce the dimensions of the wood, so that it may be used without inconvenience from its further shrinking. Timber seasons best, when placed in dry situations, where the air has a free circulation round it. Gradual drying is considered a better preservative of wood, than a sudden exposure to warmth, even of the sun; for warmth abruptly applied, causes cracks and flaws from the sudden and unequal expansion produced in different parts. Two or three years' seasoning is requisite to produce tightness and durability in the wood work of buildings. It must be observed, that seasoning in the common way only removes a portion of the aqueous and volatile matter from the wood. The extractive and other soluble portions still remain, and are liable to ferment, though in a less degree, whenever the wood reabsorbs moisture. Such, indeed, is the force of capillary attraction, that wood, exposed to the atmosphere in our climate, never gives up all its moisture. Seasoning by stove heat, in buildings constructed for the purpose, has been found to answer well, and to save much time, especially in boards partly seasoned before.

Preservation of Timber.--When wood is to be kept in a dry situation, as in the interior of houses, no other preparation is necessary than that of thorough seasoning. But when it is to be exposed to the vicissitudes of weather, and still more when it is to remain in a warm and moist atmosphere, its preservation often becomes extremely difficult. Numerous experiments have been made, and many volumes written, upon the preservation of timber, and the prevention of the dry rot; but the subject is not yet brought to a satisfactory conclusion. The methods which have hitherto been found most successful, consist in extracting the sap, in excluding moisture, and in impregnating the vessels of the wood with antiseptic substances.

For extracting the sap, the process of water seasoning is recommended. It consists in immersing the green timber in clear water for about two weeks; after which, it is taken out and seasoned in the usual manner. A great part of the sap, together with the soluble and fermentable matter, is said to be dissolved or removed, by this process. Running water is more effectual than that which is stagnant. It is necessary that the timber should be sunk, so as to be completely under water, since nothing is more destructive to wood, than partial immersion. Mr. Langton has proposed to extract the sap by means of an air-pump, the timber being enclosed in tight cases, with a temperature somewhat elevated, and the sap being discharged in vapor by the operation of the pump.

It appears extremely probable, that if trees were felled in summer, and the buts immediately placed in water, without removing the branches, a great part of their sap would be expended by the vegetative process alone, and replaced by water. It is well known that branches of plants, if inserted in water, continue for some days to grow, to transpire, and to perform their other functions. This they probably do at the expense of the sap, or assimilated fluid, which was previously in them, while they replace it by the water they consume. This state of things continues until the juices are too far diluted to be capable of any longer sustaining life.

The charring of timber by scorching, or burning its outside, is commonly supposed to increase its durability, but on this subject the results of experiment do not agree. Charcoal is one of the most durable of vegetable substances; but the conversion of the surface of wood into charcoal, does not necessarily alter the character of the interior part. As far, however, as it may operate in excluding worms, and arresting the spreading of an infectious decay, like the dry rot, it is useful. Probably, also, the pyroligneous acid which is generated when wood is burnt, may exert a preservative influence.

The exclusion of moisture, by covering the surface with a coating of paint, varnish, tar, &c., is a well-known preservative of wood which is exposed to the weather. If care is taken to renew the coat of paint, as often as it decays, wood, on the outside of buildings, is sometimes made to last for centuries. But painting is no preservative against the internal or dry rot. On the contrary, when this disease is begun, the effect of paint, by choking the pores of the wood, and preventing the exhalation of vapors and gases which are formed, tends rather to expedite, than prevent the progress of decay. Paint, itself, is rendered more durable, by covering it with a coating of fine sand. Wood which is not thoroughly seasoned, should never be painted.

The impregnation of wood with tar, bitumen, and other resinous substances, undoubtedly promotes its preservation. It is the opinion of some writers, that "woods abounding in resinous matter, cannot be more durable than others," but the reverse of this is proved every year in the pine forests of this country, where the lightwood, as it is called, consisting of the knots and other resinous parts of pine trees, remains entire, and is collected for the purpose of affording tar, long after the remaining wood of the tree has decayed. A coating of tar or turpentine, externally applied to seasoned timber, answers the same purpose as paint in protecting the wood, if it is renewed with sufficient frequency. Wood impregnated with drying oils, such as linseed oil, becomes harder and more capable of resisting moisture. It is frequently the practice, in this country, to bore a perpendicular hole in the top of a mast, and fill it with oil. This fluid is gradually absorbed by the vessels of the wood, and penetrates the mast to a great distance. Animal oils, in general, are less proper for this purpose, being more liable to decomposition.

The preservative quality of common salt (muriate of soda) is well known. An example of its effect is seen in the hay of salt marshes, which is frequently housed before it is dry, and which often becomes damp afterwards from the deliquescence of its salt, yet remains unchanged for an indefinite length of time. In the salt mines of Poland and Hungary, the galleries are supported by wooden pillars, which are found to last unimpaired for ages, in consequence of being impregnated with the salt, while pillars of brick and stone, used for the same purpose, crumble away in a short time by the decay of their mortar. Wooden piles, driven into the mud of salt flats and marshes, last for an unlimited time, and are used for the foundations of brick and stone edifices. In canals, which have been made in the salt marshes about Boston and other places, trunks of oak trees are frequently found with the heart wood entire and fresh, at a depth of five or six feet below the surface. At Medford, Mass., the stumps of trees are found standing in the gravelly bottom of the salt marsh where the tide rises in the canals four or five feet above them. This bottom must originally have constituted the surface of the ground, and must have settled long enough ago for the marsh mud to have accumulated, as it has done for miles round, apparently since that period.

The application of salt in minute quantities, is said rather to hasten than prevent the decay of vegetable and animal bodies. Yet the practice of docking timber, by immersing it for some time in sea-water, after it has been seasoned, is generally admitted to promote its durability. There are some experiments which appear to show, that after the dry rot has commenced, immersion in salt water effectually checks its progress, and preserves the remainder of the timber.[A] In some of the public ships built in the United States, the interstices between the timbers in various parts of the hull, are filled with dry salt. When this salt deliquesces, it fills the pores of the wood with a strong saline impregnation, but it has been said, in some cases, to render the inside of the vessel uncomfortably damp. If timber is immersed in a brine made of pure muriate of soda, without the bitter deliquescent salts which sea-water contains, the evil of dampness is avoided.

A variety of other substances, besides common salt, act as antiseptics, in preventing the dry rot, and the growth of the fungus which attends it. Nitre and alum have been recommended for this purpose, and some of the metallic salts are considered still more effectual. Of these, the sulphates of iron, copper, and zinc have the effect to harden and preserve the timber. Wood boiled in a solution of the former of these, and afterwards kept some days in a warm place to dry, is said to become impervious to moisture. Lime-water has recently been found a powerful antiseptic. Corrosive sublimate, as recommended by Sir H. Davy, is perhaps the most powerful preservative of organized substances from decay, and proves destructive to parasitic vegetables and animals; but its safety, in regard to the health of crews, if used in large quantities about the wood of a ship, may be considered as doubtful.

An opinion has been supported in this country, that the decay of timber in ships, by dry rot, is owing to the impure atmosphere generated by bilge water, and that it is to be remedied by constructing ships with a view to their free and effectual ventilation.

Preservation of Animal Textures.--The solid and fibrous portions of organic bodies, such as wood, bone, shell, horn, hair, cotton, &c., are most easy of preservation. But the soft and succulent parts, such as the pulp of vegetables, and the flesh of animals, are extremely perishable, owing to the decomposing influence of their fluid contents; and require the assistance of art to communicate to them any degree of durability. These substances, when they cannot be dried, are usually preserved by enveloping or impregnating them with antiseptics. For alimentary substances the antiseptics used are sugar, alcohol, salt, and the acetous and pyroligneous acids; while, for scientific specimens and preparations, alcohol, oil of turpentine, resinous and bituminous varnishes, alum, and corrosive sublimate, are found most effectual.

[A] The British frigate Resistance, which went down in Malta harbor, and the Eden, which was sunk in Plymouth Sound, were both affected with dry rot. These ships, after remaining many months under water, were raised, and it was found that the disease was wholly arrested. Every vestige of fungus had disappeared, and the ships remained in service afterwards, perfectly sound from any further decay.--Supplement to the Encyclopedia Britannica, iii. 682.

Embalming.--As the art of embalming can hardly be ranked among the useful arts, any further than it can be made subservient to the promotion of anatomy, or natural history, it is not much cultivated at the present day. The ancient Egyptians converted the dead bodies of their friends into mummies, by removing the viscera from the large cavities, and replacing them with aromatic, saline, and bituminous substances, particularly asphaltum; and also enveloping the outside of the body in cloths impregnated with similar materials. These impregnations prevented decomposition, and excluded insects, until perfect dryness took place. In times comparatively modern, embalming has been practised with great success, particularly where bodies have remained at a low and uniform temperature, and have been protected from the access of the air. The body of King Edward the First, of England, appears upon record to have been embalmed. He died in July, 1307, and was buried in Westminister Abbey. In 1770, his tomb was opened, and the contents examined, and after this lapse of four hundred and sixty-three years, the body of the monarch remained entire. The flesh upon the face was a little wasted, but not putrid. The body of Canute, King of Denmark, who invaded England in 1017, was found very fresh in 1776, by the workmen employed in repairing Winchester Cathedral. The bodies of William the Conqueror, and of Matilda his wife, both buried at Caen, were found entire in the sixteenth century. In like manner, the remains of various other princes, and persons of note, have been discovered to be undecayed some centuries after their decease. In certain cases, bodies not embalmed have been preserved, merely by the exclusion of air, and a uniform, low temperature.

But the most perfect of all the modes of preserving the animal body, without continued immersion, appears to be, a thorough impregnation with corrosive sublimate. This may be performed, by saturating the soft solids with a strong solution, consisting of about four ounces of bichloride of mercury to a pint of alcohol. This is injected into the blood-vessels, and after the viscera are removed, the whole body is immersed for three months in the same solution. At the end of this period, it easily dries, and is afterwards nearly imperishable.

In what are called by anatomists wet preparations, the objects are kept immersed in alcohol, and last for an indefinite time. Oil of turpentine answers the same purpose, and in the Museum of Natural History in Paris, there is a head prepared in this way, more than a hundred years ago, by the celebrated Ruytch, which preserves all the vivacity of its colors. In cold weather, the liquid becomes opaque, but is again rendered transparent in the spring.

An artist at Florence is said to have discovered a mode of petrifying animal substances, but his method has not been communicated to the world.

Tanning--The skins of animals, when prepared by merely drying them, are stiff, incapable of resisting water, and liable to decay. If, however, they are impregnated with the tannin which is found in astringent vegetables, that substance combines with the gelatin of the skin, and forms a durable compound, which is no longer soluble in water. Common tanned leather is prepared in this way. The skins are previously prepared by soaking them in lime-water, which facilitates the separation of the cuticle and hair. A slight degree of putrescency assists the same object. They are then immersed in the tan-pits, in a strong infusion of some astringent vegetable. Oak bark, from its cheapness, and the quantity of tannin it contains, is commonly employed in the preparation of leather, both in this country, and in Europe. The bark of the hemlock spruce, and of the chestnut, the leaves of the different species of sumach, and various other astringent vegetables, are used in sections of country where oak is scarce. The strength of the astringent infusion is increased from time to time, until the skin is saturated with tannin. A portion of extractive matter likewise combines with the hide, and to this the brown color, which is common in leather, is owing. The presence of this extractive is supposed to render leather more tough and pliable.

When strong or saturated solutions of tannin are used, the leather is formed in a much shorter time, but it is observed that leather tanned in this way is more rigid and more liable to crack, than that made in the common manner, with weaker infusions, gradually increased in strength. But sole leather, the most important requisites of which are firmness and resistance to water, is immersed in an infusion kept nearly saturated by alternate strata of bark. The full impregnation requires from ten to eighteen months.

The currying of leather is performed by covering the skin or leather, while yet moist, with common oil, which, as the moisture evaporates, penetrates into the pores of the skin, giving it a peculiar suppleness, and rendering it, to a certain extent, water proof. During the process, it is pared, washed, and rubbed, to increase its flexibility. The black color is also imparted by the currier, by rubbing the outside with a solution of copperas, or any solution of iron, which immediately turns it black, by combining with the tannin in the leather.

Tawing is the method by which skins are dressed of a white color, and it is performed without the use of bark. The skins are first prepared by steeping them in lime-water, and subjecting them to various processes of scraping and fulling. They are then fermented with wheat bran, and afterwards impregnated with a solution of alum and common salt. Before being dried, they are fulled with wheat bran and yolks of eggs, and are thoroughly trodden, steeped, and washed. In this process, the place of tannin appears to be supplied by some principle extracted from the alum.

As examples of the foregoing processes, common sole leather is simply tanned, the upper leather of boots and shoes is tanned and curried, the white leather for gloves is tawed, and fine morocco leather is tawed, and afterwards slightly tanned with sumach, and dyed. Chamois, and other kinds of wash leather, are steeped in lime pits, and afterwards fulled with oil. Before the dressing is finished, the superfluous oil is scoured out with an alkaline liquor.

Parchment.--Parchment used for writing, is prepared from the skins of sheep and goats. These, after being steeped in pits impregnated with lime, are stretched upon frames, and reduced by scraping and paring, with sharp instruments. Pulverized chalk is rubbed on with a pumice stone resembling a muller, which smooths and softens the skin, and improves its color. After it is reduced to something less than half its original thickness, it is smoothed and dried for use. Vellum is a similar substance to parchment, made from the skins of very young calves.

Catgut.--The strings of certain musical instruments, the cords of clock weights, and those of some other machines and implements, are made of a dense, strong, animal substance, among us usually denominated catgut. It is derived from the intestines of different quadrupeds, particularly those of cattle and sheep. The manufacture is chiefly carried on in Italy and France. The texture from which it is made, is that which anatomists call the muscular coat, which is carefully separated from the peritoneal and mucous membranes. After a tedious and troublesome process of steeping, scouring, fermenting, inflating, &c, the material is twisted, rubbed with horsehair cords, fumigated with burning sulphur, to improve its color, and dried. Cords of different size, and strength, and delicacy, are obtained from different domestic animals. The intestine is sometimes cut into uniform strips with an instrument made for the purpose. To prevent offensive effluvia during the process, and to get rid of the oily matter, the French make use of an alkaline liquid called eau de Javelle.

Goldbeaters' Skin.--This delicate membrane is also manufactured from the intestines of animals. The workman strips off that part of the peritoneal membrane which surrounds the cæcum. He then takes about two feet o£ it in length, turns it inside out, and leaves it to dry. It is afterwards steeped in a weak solution of potash, cleansed by scraping, and cut open. It is then stretched to dry upon wooden frames, and notwithstanding the tenuity of the membrane when dry, every piece of it is double or consists of two membranes glued together. It is finished by washing it with a solution of alum, and coating it with isinglass and whites of eggs, together with some aromatics to repel insects.

Specimens in Natural History.--Preparations of animals intended to show their external form and characters, are made by detaching their skins, and stuffing or mounting these so as to represent the natural figure and attitudes of the animal. Quadrupeds and birds are preserved by extracting the body through an opening on the under side, at the same time inverting the skin. The fleshy parts of the limbs are extracted through the same opening, also the neck, brain, and eyes, leaving the scull , if the animal be small. Care is taken not to injure the hair, or plumage. When the fleshy parts are removed, the inside of the skin is rubbed with some poisonous substance, usually arsenic,[A] to destroy insects. The skin is then returned to its natural situation, and filled with cotton or tow; or, what is still better, an artificial body, shaped out of wood, cork, or dried clay, may be introduced within the skin. The opening is sewed up, and wires are passed longitudinally through the legs and neck. These are afterwards bent into the proper position to give the attitude desired. Glass eyes are inserted, and the hair and feathers rendered as smooth as possible, and retained, while drying, in paper bandages.

Reptiles, and fishes without scales, are extracted by carefully separating the bones of the neck through an opening in the throat, or gills, and inverting the skin. In serpents, the whole body is easily extracted through the mouth. Fishes with scales cannot be turned without injury; it is therefore necessary to detach the skin carefully, without doubling it. Insects may be killed, without hurting their texture, by the fumes of burning sulphur, or prussic acid, or, in many cases, by pinching the breast. They are then secured by pins, and placed to dry with the wings and legs in the natural attitudes. Arsenic, or corrosive sublimate, is generally necessary to secure them from the depredations of other insects.

[A] The following is the arsenical soap of Becoeur, much used in France: Camphor, five ounces; powdered arsenic, two pounds; white soap, two pounds; salt of tartar, twelve ounces; lime, four ounces, melted and triturated together.

An Herbarium, or collection of dried plants, is usually formed by subjecting the plants, while fresh, to a sufficient pressure between folds of paper, to preserve their natural smoothness and regularity, until they become dry. The plants should be gathered at a time when their characters are most perfectly developed. A specimen in flower should be preserved, and, if possible, one also in fruit. The plant must be carefully spread out on smooth bibulous paper, so that the leaves, petals, &c, may be displayed as perfectly as possible. In this situation it is retained, and another sheet of paper turned gradually over it, commencing at one side, till the whole is covered. Several sheets of paper are then to be added to each side, and the whole placed to dry under a strong, equal pressure. In this way many plants may be preserved without further trouble, especially if the weather be warm and dry. The process, however, may be expedited by shifting the papers, or by passing over them occasionally a warm iron. These precautions are more necessary for succulent plants, or for others in cold and damp weather.

Appert's Process.--A method brought into notice by M. Appert, for preserving articles of food unchanged for several years, deserves to be noticed among the practical improvements of the present century. This method was partially known at a much earlier period, but its most successful modes of application were undoubtedly discovered by M. Appert. It consists in a very simple process. The articles to be preserved are enclosed in bottles, which are filled to the top with any liquid; for example, with the water in which the article, if solid, has been boiled. The bottles are closely corked, and cemented, to render them hermetically tight. They are then placed in kettles filled with cold water, and subjected to heat till the water boils. After the boiling temperature has been kept up for a considerable time, in some cases an hour, but varying with the character of the article to be preserved, the bottles are suffered to cool gradually. In this state, meats, vegetables, fruits, milk, and other substances, are preserved perfectly fresh, without any condiments, for long periods, of from one to six years. Instead of bottles, tin canisters are sometimes used, and rendered tight by soldering.

The remarkable effect of this process has been explained, by attributing the preservation of the articles to the total exclusion of atmospheric air. But as air, in common cases, is always present in sufficient quantities to excite fermentation, it is supposed that the application of heat serves to fix the small portion of atmospheric oxygen which is present, by combining it with some principle in the other substances; so that it is no longer capable of producing the fermentative action, which in parallel cases leads to decomposition.

WORKS OF REFERENCE.--CHAPMAN'S Treatise on the Preservation of Timber, 8vo. 1817;--TREDGOLD'S Elementary Principles of Carpentry, 4to. 1820;--McWILLIAM, on the Dry Rot, 4to. 1818;--Article Dry Rot, in the supplement to the Encyclopedia Britannica;--AIKEN'S Chemical Dictionary, article Leather;--LABARRAQUE, l'Art du Boyaudeur; Bulletin, de la Société de l'Encouragement pour l'Industrié, 1822;--LETTSOM'S Naturalist's Companion, 8vo.;--Taxidermy, or the Art of Collecting, Preparing, and Mounting Specimens in Natural History, London, 1823;--APPERT, Art of Preserving Animal and Vegetable Substances, London, translated, 1812;--Edinburgh Review, vol. xxiii. p. 104;--URE'S Dictionary of Arts, article Putrefaction.