again be collected into a body without change of form. | Mercury, water, and many other substances, may be converted into vapour, or distilled in close vessels, without any of their particles being lost. In such cases, there is no decomposition of the substances, but only a change of form by the heat; and hence the mercury and water assume their original state again on cooling. When bodies suffer decomposition or decay, their elementary particles, in like manner, are neither destroyed nor lost, but only enter into new arrangements or combinations with other bodies. When a piece of wood is heated in a close vessel, such as a retort, we obtain water, an acid, several kinds of gas, and there remains a black, porous substance, called charcoal. The wood is thus decomposed, or destroyed, and its particles take a new arrangement, and assume new forms; but that nothing is lost is proved by the fact, that if the water, acid, gases, and charcoal, be collected and weighed, they will be found exactly as heavy as the wood was, before distillation. In the same manner, the substance of the coal burnt in our fires is not annihilated; it is only dispersed in the form of smoke, or particles of culm, gas, and ashes or dust. Bones, flesh, or any animal substance, may in the same manner be made to assume new forms, without losing a particle of the matter which they originally contained. The decay of animal or vegetable bodies in the open air, or in the ground, is only a process by which the particles of which they were composed change their places and assume new forms. vidual be standing in the same position as formerly, the tendency which his body has to move forward-for it acquired the same motion as the carriage by which it was borne along—will cause him to fall in the opposite direction. The following is a familiar example of the inertia of matter:-Upon the tip of the finger let a card be balanced, and a piece of money-say a shilling-laid upon it. Let the card then be smartly struck, and it will fly from beneath the coin, leaving it supported upon the finger. This arises from the inertia of the metal being greater than the friction of the card which passes from beneath it. Coursing, or hare-hunting, affords a striking illustra tion of inertia. In that field sport, the hare seems to possess an instinctive consciousness of the existence of this law of matter. When pursued by the greyhound, it does not run in a straight line to the cover, but in a zigzag one. It doubles, that is, suddenly changes the direction of its course, and turns back at an acute angle with the direction in which it had been running. The greyhound, being unprepared to make the turn, and therefore unable to resist the tendency to persevere in the rapid motion which it has acquired, is impelled a considerable distance forward before it can check its speed and return to the pursuit. But, in the mean time, the hare has been enabled to shoot far ahead in the other direction; and although a hare is much less fleet than a greyhound, by this scientific manoeuvring it often escapes its pursuer. Those who have witnessed horseracing, may have observed that the horses shoot far past the winning-post before their speed can be arrested. This is also owing to the inertia of their bodies. The decay and decomposition of animals and vegetables beneath the surface of the earth, fertilise the soil, which nourishes the growth of plants and other vegetables; and these, in their turn, form the nutriment of We have now arrived at a most important property, animals. Thus is there a perpetual change from death attraction, which it is desirable should be carefully to life, and from life to death, and as constant a succes-studied. It is a fundamental law of nature, ascertained sion in the forms and places which the particles of matter assume. Nothing is lost, and not a particle of matter is struck out of existence. The same matter of which every living animal and every vegetable was formed in the earliest ages, is still in existence. As nothing is lost or annihilated, so it is probable that nothing has been added, and that we ourselves are composed of particles of matter as old as the creation. In time, we must in our turn suffer decomposition, as all forms have done before us, and thus resign the matter of which we are composed, to form new existences. Inertia means passiveness or inactivity. Thus, matter is perfectly passive in submitting to any condition in which it is placed, whether of rest or motion. When at rest, it shows an inability or reluctancy to move; and when in motion, it shows an equal inability or reluctancy to come to a state of rest. It is obvious that a rock on the surface of the earth never changes its position in respect to other things on the earth. It has of itself no power to move, and would therefore for ever lie still, unless moved by some external force. Now, it is just as true that inert matter has no power to bring itself to rest when once put in motion, as that it cannot put itself in motion when at rest; for having no life, it is perfectly passive both to motion and rest, and therefore either state depends entirely upon external circumstances. by Sir Isaac Newton, that every atom or particle of matter has a tendency to approach or to be attracted towards another atom or particle. This forms one of the leading principles in modern natural philosophy. Experience and observation demonstrate that this power of mutual attraction pervades all material things, and, though unseen except in its results, is ever present with us is the cause of particles of matter adhering to each other, and forming solid masses of these masses assuming in many instances a round or globular form -of the falling of bodies to, and their stability on, the earth-and is one of the causes of the whole of the planetary bodies moving in their paths in the heavens. Attraction is of different kinds, although some of these may be merely modifications of others, and has received different names according to the circumstances under which it acts. The force which keeps the particles of matter together, to form bodies, or masses, is called attraction of cohesion. That which inclines different masses towards each other, is called gravitation, or attraction of gravitation. That which causes liquids to rise in tubes, or in very confined situations, is called capillary attraction. That which forces the particles of different kinds of matter to unite, is called chemical attraction. That which causes the magnetic needle to point constantly towards the poles of the earth, is magnetic attraction. And that which is excited by friction in certain substances, is known by the name of electrical attraction. Many instances might be given of the tendency which matter has to remain in the condition in which it happens to have been already placed. The following are Attraction of cohesion acts only at insensible disamong the most instructive:-When the sails of a ship tances, as when the particles of bodies apparently touch are loosened to the breeze, slowly and heavily at first each other. This kind of attraction may be described the vessel gets into motion, but gradually its speed in- as the quality in nature which causes matter to cohere creases, as the force by which it is impelled overcomes or stick together. It is much stronger in some bodies the inertia of its mass. A great force is necessary at than in others. It is stronger in the metals than in first to set a vehicle in motion; but when once this is most other substances, and in some of the metals it is effected, it goes onward with comparative ease, so that, stronger than in others. In general, it is most powerful in fact, a strong effort is necessary before it can be among the particles of solid bodies, weaker among those stopped. If a person be standing in it when it is sud- of fluids, and least of all, or almost entirely wanting, denly set a-going, his feet are pulled forward, whilst among elastic fluids, such as air and the gases. Thus, his body, obeying the law of inertia, remains where it a small iron wire will hold a suspended weight of many was, and he accordingly falls backwards. On the other pounds, without having its particles separated; the parhand, if the vehicle be suddenly stopped, and the indi-ticles of water are divided by a very small force, while the oppuaa warted up the met espasse seems Kistence of reyboard those of air are still more easily moved among each When the particles of a body can be suspended in The force by which small tubes, or porous substances, of treatises, they do not require particular notice here, and we proceed to consider the kind of attraction which seems to unite all ordinary masses and particles of matter. Reference is here made to the attraction of gravitation. As the attraction of cohesion unites the particles of matter into masses or bodies, so the attraction of gravitation tends to force those masses towards each other to form others of still greater dimensions. The force of attraction increases in proportion as bodies approach each other, and by the same law it must diminish in proportion as they recede from each other. Attraction, in technical language, is inversely as the squares of the distances between the two bodies; that is, in proportion as the square of the distance increases, in the same proportion attraction decreases, and so the contrary. Thus, if at the distance of 2 feet, the attraction be equal to 4 pounds, at the distance of 4 feet it will be only 1 pound; for the square of 2 is 4, and the square of 4 is 16, which is 4 times the square of 2. On the contrary, if the attraction at the distance of 6 feet be 3 pounds, at the distance of 2 feet it will be 9 times as much, or 27 pounds, because 36, the square of 6, is equal to 9 times 4, the square of 2. The intensity of light is found to increase and diminish in the same proportion. Thus, if a board a foot square be placed at the distance of one foot from a candle, it will be found to hide the light from another board of two feet square, at the distance of two feet from the candle. Now, a board of two feet square is just four times as large as one of one foot square, and therefore the light at double the distance being spread over four times the surface, has only one-fourth the intensity. The gradual diminution of attraction as the distance increases, is exemplified in the following table. In the upper line, the distance is expressed by progressive numbers; in the lower corresponding squares the diminution of attraction is indicated by the common arithmetical fractions. Distance Attraction It is here seen, that at the distance of 8, the attractive force is diminished to a 64th part of what it was at 1. The attractive force of matter is also in proportion to the numbers of the atoms of matter which a body contains: the attraction, therefore, does not proceed from the mere surface of a body, but from all the particles which individually compose it. Some bodies of the same bulk contain a much greater quantity of matter than others: thus, a piece of lead contains about twelve times as much matter as a piece of cork of the same dimensions; and therefore a piece of lead of any given size, and a piece of cork twelve times as large, will attract each other equally. The attractive power of any mass acts from the centre. At all equal distances from the centre, the attractive power is equal; for instance, in a body perfectly spherical, the attraction to the centre would be the same at all parts of the surface. The distance of the centre of a sphere from its surface is called the semi-diameter of that sphere-that is, the It is this kind of attraction which is supposed to be half of its thickness. At a point as far from the surone of the causes of springs of water in the earth. The face of a sphere as its semi-diameter, its attractive water creeps up by capillary attraction through porous power is diminished to a fourth. At three distances, beds of sand, small stones, and crevices of rocks, and the attraction is a ninth; at four distances, a sixteenth in this manner reaches the surface even at great heights. and so on. When we wish, therefore, to ascertain the The lower parts of the walls, and also the earthen relative amount of the attraction which any mass of floors of cottages, are in the same manner apt to matter exercises over another, the rule is, to inquire become damp, by the attraction of the moisture upwards how many semi-diameters of the one the other is disfrom the ground. Hence the necessity for clearing away tant from it, and then to multiply that number by itself. all wet earthy matter from the foundations of houses. The result shows how many times the attraction at this Besides these varieties of attraction, there are, as distance is less than at the surface of the former. The already said, chemical, magnetic and electric attrac-moon, for instance, is distant 240,000 miles from the tion, but as these are respectively alluded to under the earth, or as much as sixty semi-diameters of the earth; heads Chemistry and Electricity in the present series 60 multiplied by 60 gives 3600; consequently, the at traction exercised by the earth upon the moon is a 3600th part of what it would exercise upon the same mass at its own surface. If the earth were a perfectly spherical body, its attraction would be equal every where at the level of the sea. As the surface at the pole is thirteen miles nearer the centre than the surface at the equator, the attraction is stronger at the former than at the latter place: it gets proportionally weaker as we advance towards the equator, on account of the increase of distance from the centre. Hence, a mass of iron which is considered a pound weight in Britain, would be less than a pound on the coast of Guinea, and more than a pound in Greenland, for weight is only a result of attraction. If we ascend a mountain, the effect is the same as if we proceed towards the equator: we are always getting farther from the centre of attraction, and consequently weights become lighter. On the top of a hill four miles high, a ball of four thousand pounds weight would be found to be two pounds lighter. Pressure downwards, or weight, is in philosophical language termed GRAVITY, and under that head it is hereafter treated, in connexion with the phenomena of falling bodies. The attraction of bodies is mutual, and in proportion to the quantity of matter they contain. Therefore, any body, however small, exerts some degree of attraction upon the mass of the earth. Any body which comes immediately under our observation, is so small in comparison to the earth, that its attractive force is altogether unappreciable; but if the body were of great density, and of dimensions approaching to those of the earth, then we should see the earth rise to meet the body, or fall towards the body. The heavenly bodies, when they approach each other, are drawn out of the line of their paths, or orbits, by mutual attraction. It is found by experiment, that a plumb-line suspended in the neighbourhood of a mountain, is sensibly attracted towards the mountain from the true vertical line. The mutual attraction of matter is exemplified by the diminution of the weight of bodies as we penetrate into the earth. At the depth of a mile, a body weighing a pound would be found to be lighter than at the surface. This is in consequence of the attraction of the matter of the shell of the earth, which is exterior to the point, being nothing, in consequence of the attractions of its particles on this point counteracting each other; and hence the only efficient attraction on it arises merely from the smaller sphere below the point; and, therefore, the nearer the point is to the centre, the less is this internal sphere, and the less therefore is its attraction on the point. Were we to proceed to the centre of the earth, we should there find that weight altogether ceased, because the attractive power would be equal on all sides. Were there a cavity at the earth's centre, the body would hang suspended in space. The attraction of the earth's mass performs an important function, in binding the atmosphere, which is an elastic fluid, around the surface of our planet, and in causing the air to perforate every open crevice and pore in the superficial substances of the globe. The attractive force, in this respect, produces what is called atmospheric pressure, the air being pulled or pressed down by a force equivalent to about 15 lbs. on the square inch, at the level of the sea, and diminishes in proportion to the distance above that common level. THE REPULSIVE QUALITY IN MATTER—HEAT. While attraction tends to unite and compress the particles of matter, there is another and equally universal principle, known in familiar language by the appellation of heat, the tendency of which is to keep the particles of matter at a certain degree of expansion. Heat is often, in scientific works, named caloric, from the Latin word for heat. Heat pervades all things, but some in greater degrees than others. Even ice has been found to contain a certain portion of heat. In fact, there is no such thing in nature as positive cold. The things which seem cold to us, are only under a low degree of heat, The absolute nature of this universal principle is unknown. We only know it by its effects, and the sensations it produces. Some have conjectured that it is a fluid; others think it is a quality or affection of matter, resulting from electrical action. From its producing no sensible difference in the weight of any substance, it has been called an imponderable body. When the heat of any particular substance, as ice, stone, or wood, is not sensible to us, it is called latent (that is, concealed) heat. We may very readily detect its presence in a piece of wood or metal by rubbing or friction. If a button, for instance, be rubbed on a table, it will soon become too hot to be held by the fingers. In like manner, the axle of any carriage-wheel soon becomes hot, unless the friction is prevented by grease. Heat, in its extreme form, becomes fire. Thus, if an ungreased wheel be rapidly turned for a long time on its axle, so much heat will be excited that both wheel and axle will burst into a flame. The effects of powerful friction are known to savage nations, among whom it is common to produce fire by rubbing two sticks together. Two pieces of flint struck together, or a flint struck hard upon a piece of iron, evolve sparks of fire. By such means, many important purposes are served; for instance the discharge of fire-arms. Fire can also be evolved from the common atmosphere, by compressing a quantity of it suddenly in a tube, at the bottom of which a piece of tinder has been placed. The evolution of heat by these means, and other circumstances, lead to the conclusion that heat is an element mixed up with the atoms of matter, which it serves to keep at a lesser or greater distance from each other. Thus, as we squeeze the pores of a sponge together, and disengage the liquid which they held in cohesion, so, when squeezing or rubbing a portion of matter, do we disengage the heat which it retained amongst its component atoms. In all cases of the development of heat by pressure, hammering, and friction, the cause is the squeezing together of atoms which had been kept asunder by the latent fluid, and which fluid must, as a matter of necessity, come forth and make itself sensibly felt or seen. Heat, then, is a principle of repulsion in nature, and in this capacity its uses are as obvious as those of terrestrial gravitation, to which it apparently acts as a counterpoise. The force of attraction is so powerful, that, unless for a counteracting principle of repulsion, all bodies would hasten into close contact; there would be no air, no water, no vegetable or animal life; all would be an uniform dead solid mass, and the earth itself might perhaps be reduced to a small portion of its present bulk. Heat, by pervading all things, modifies attraction, and, according to circumstances, regulates the density or solidity of bodies. Hence we possess in nature a beautiful variety of substances, some solid and hard, like stone and marble; others soft, or of the jelly form; a third class liquid, like water; and a fourth kind aëriform, or gaseous. Heat expands most bodies in proportion as it is increased in quantity, and they become solid in proportion as it is withdrawn. Water may thus be either expanded into the form of vapour or steam, or hardened into ice. When withdrawn, the process of cooling is said to take place; cold being simply a state of abstraction or comparative absence of heat. Heat is diffused or communicated by conduction and radiation. When it passes slowly from one portion of matter to another in contact with it, it is said to be con ducted; and the process, in scientific language, is termed the conduction of caloric. Metals are the best conductors, then liquids, and lastly, gases. Gold, silver, and copper, are the best conductors among solids; glass, bricks, and many stony substances, are very bad conductors; and porous spongy substances, as charcoal, hair, and fur, are the worst. Clothing is generally made of bad conductors, that the heat of the body may not be conducted quickly to the surrounding air. Furnaces, where great heat is required, are built with porous bricks, which are very effectual in preventing the principle i and the setred that is -on of mati is prog y substans When the ne, or word concealed esence in 11 ction. I it will sea like ma comes L escape of heat, and do not readily communicate the Heat is said to radiate when it is emitted from a fire The rising of mercury in the tube of the thermometer offers a familiar example of the repulsive power of heat in expanding or dilating bodies. Common experience affords many such examples. A bar of iron is longer and thicker when hot than when it is cold. The iron rim of a wheel slips easily into its place when hot, and gripes or binds fast when it becomes cool. When heated from 32 to 212, air expands 3-8ths of its volume, alcohol 1-9th, water 1-22d, and hammered iron 1-273d. In these, and all similar instances, the expansion arises from the fluid of heat lodged among the atoms of matter pressing outwards on all sides, according as it is excited." When the temperature of the atmosphere falls below the freezing-point (32), which it does principally from the weakness of the sun's rays in winter, the phenomenon of frost, or freezing, ensues. Freezing is a process of congelation, or properly crystallisation, produced by the withdrawal of heat, and by which water assumes the form of ice. When the temperature of the atmosphere rises above the freezing-point, the ice melts, and is resolved into its original element. When the temperature of the atmosphere is below the freezing-point, the particles of water which are upheld in the clouds are frozen in their descent, and reach the earth in the form of flakes of snow. If this freezing take place after the particles have become united into rain-drops, we have hail instead of snow. When the descending flakes of snow come into a temperature above the freezingpoint as they approach the earth, they are apt to melt, and in such a case fall in the shape of sleet, which is half-melted snow or hail. Heat has a constant tendency to preserve an equili brium in all situations; and hence its diffusion through nature, and many of the ordinary phenomena in relation to temperature. When we touch a cold substance with our hand, a portion of the heat of the hand rushes into the substance, and leaves the hand so much deficient of its former heat. On the same principle, when we touch a substance which is warmer than the hand, some of the heat rushes into the hand, and renders it hot. When we pour a quantity of hot water into that which is cold, an equalisation of the two temperatures immediately ensues. When the air at any particular place becomes heated or rarefied, it ascends by virtue of its greater lightness, leaving a vacancy which the neighbouring air rushes in to supply. This is one of the chief causes of winds. The same principle is observable in the case of heated apartments. If the door of a heated room be thrown open, a current of cold air immediately rushes in to supply the deficiency in the rarefied atmosphere. Heat is unequally distributed over the globe. At and near the equator, where the rays of the sun are sent in the greatest degree of directness, the greatest heat prevails. In the parts of the earth adjacent to the north and south poles, he transmits his rays so slantingly as to have little power; and there, accordingly, the air is seldom of a genial mildness. The higher we ascend in the air, the colder it becomes; the summits of very high mountains are always covered with snow. In penetrating into the body of the earth, after gaining a certain depth, the heat becomes greater in proportion as we descend. The interior of the globe is by many believed to be at a very elevated degree of heat, if not in a state of ignition. On the surface, great expanses of sea tend to equalise and temper the degrees of heat and cold in their neighbourhood, and great continents have the contrary effect. The degrees of heat and cold in the atmosphere are called its temperature; and for ascertaining this correctly, with reference to a standard, a very ingenious instrument has been invented. This is called the thermometer (a word signifying heat-measurer). It is a glass tube with a bulb at the bottom, into which mercury or quicksilver is put, with a scale of figures along the tube to mark the rising of the quicksilver. This instrument differs from the barometer, in as much as the quicksilver is sealed up close from the air. The atmospheric heat, however, affects the metallic fluid in the bulb, and, according to its warmth, causes it to expand and rise in the tube. The degree of temperature is indicated by the figures to which it ascends. Our common thermometer has a graduation from No. 1, near the bulb, to 212, the degree of heat of boiling water. In the scale of figures, 32 is marked as the freezing-point -that is to say, when the mercury is at the height of 32, water freezes; and the more it is below that point, the more intense is the frost: 55 is reckoned moderate heat, and 76 summer heat, in Great Britain: 98 is the heat of the blood in the average of living men. 210 200 190 180 170 160 150 150 180 110 100 90 90 70 60 40 32 50 20 $10 Fig. 1 Evaporation is always accompanied by the withdrawal of heat, or production of cold, when no heat is directly applied; the heat necessary for the production of the vapour is then derived or radiated from surrounding objects, as is mentioned above in the case of dew forming on plants. In the great operations of nature, the withdrawal of heat to produce intense cold, and the application of heat to produce great warmth, ordinarily take place gradually. Thus, although water freezes at a temperature of 32, it is some time before frost is completely effectual in changing the aspect and condition of liquid bodies; and when the temperature rises a few degrees above 32, after a frost, the ice and snow which have been formed do not vanish immediately; indeed, ice will remain unthawed for several days after the temperature has risen some degrees above the freezing-point. By this slow process, either in the absorption or evolution of heat, the animal and vegetable worlds are not liable to the injury which would ensue from instantaneous changes in the condition of their elementary fluids. Water is increased in volume by freezing, which circumstance explains the ordinary phenomena of the bursting of water-pipes, and other similar occurrences, during frost. When a vessel of moderate strength is filled with water, its expansion, when it is converted into ice, by exposure to a freezing temperature, causes the vessel to burst. If the vessel is not brittle, but | forty-hundredth parts of a time heavier than water. possessed of considerable tenacity, as a leaden water-If there be three figures, thousandths of parts of a time pipe, the rupture will seldom be observed during the are meant; if four figures, ten thousandth parts; and continuance of the frost while the water remains in a so on. Common air is sometimes taken as a standard solid state, but it readily appears when thaw takes place, with which to compare gases, being a more simple mode as the water is then forced out with a velocity corres- of comparing the relative weights of aërial substances. ponding to the vertical height of the column of water But all the solids and liquids are estimated with refein the pipe. The fissures of rocks, too, are widened by rence to water as the standard. the freezing of the water which may happen to lodge in them before frost; and this process, therefore, is a powerful agent in the disintegration of rocks. Portions of steep banks, also, from a similar cause, tumble down | after thaw; for the moisture in them expands when frozen, and thus rends them to pieces, which, however, during the frost, are bound together as by cement, and fall down whenever thaw dissolves the moisture. Any body of greater specific gravity than water, will sink on being thrown into water; but it will float on the surface, if its specific gravity be less than that of water. A body, such as a piece of wood, after floating a certain length of time on water, will imbibe such a quantity of liquid that its specific gravity will be gradually increased, and in the course of time it may sink to the bottom. Heat has a powerful effect in causing certain bodies Porosity is the quality opposite to density, and means to shrink and diminish in volume. This happens with that the substance to which it is applied is porous; that those substances which do not liquefy, such as wood and is, full of small pores or empty spaces between the parclay. The contraction arises from the heat carrying ticles, and that the body is comparatively light. The off the watery particles from the bodies, and thus allow- instances of porosity are numerous in every departing the constituent atoms to come more closely together.ment of the material world, but those which are conAs wood becomes drier, its fibres are sometimes split asunder, so as to emit loud cracking noises, which, in the case of household furniture, are ascribed by the ignorant to supernatural causes. nected with animal and vegetable bodies are the most remarkable. Bone is a tissue of pores or cells, and, when seen through a microscope, may be said to resemble a honey-comb. Wood is also a tissue of cells or Heat is further treated of under the articles Che-tubes. If the end of a cylinder of straight wood be immistry, Pnuematics, and Meteorology. ACCIDENTAL PROPERTIES OF MATTER. mersed in water, whilst the other is forcibly blown into, the air will be found to pass through the pores of the wood, and rise in bubbles through the water. When a gas is comparatively light, it is said to be rare, or to pos Having shown how the beautiful and extensive variety of form in bodies-solid, liquid, gaseous, and the diffe-sess rarity. rent modifications of them-are to be traced to the operation of chiefly two great leading principles in nature, attraction and repulsion, we have now to mention the peculiar forms or characters which bodies assume from the influence of these and other causes, and which are usually classed under the term accidental properties of matter. The following are these properties: Density, Porosity or Rarity, Compressibility, Elasticity, Dilatation, Hardness, Brittleness, Malleability, Ductility, and Tenacity. By compressibility is meant that quality in virtue of which a body allows its volume to be diminished, without the quantity or mass of matter being diminished. It arises, of course, from the constituent particles being brought nearer to each other, and is effected in various ways. All bodies are less or more capable of being diminished in bulk, which is a conclusive proof of their porosity. Liquids are less easily compressed than solid bodies; nevertheless they, to a small extent, yield, and go into smaller bulk by great pressure. The water at Density signifies closeness of texture, or compactness. the bottom of the sea, by being pressed down by the Bodies are most dense when in the solid state, less dense superincumbent water, is more dense or compact than when in the condition of liquids, and least dense of all it would be at the surface. Atmospheric air and gases when gaseous or aëriform. In this manner the degree are much more easily compressed than liquids, or even of density is in agreement with the closeness of the atoms than many solids. Air may be compressed into a hunto each other. The density of bodies may generally be dredth part of its ordinary volume. When at this state altered by artificial means, as is afterwards mentioned. of compression, it has a great tendency to expand and The metals, in particular, may have the quality of den-burst the vessel in which it is confined. sity increased by hammering, by which their pores are made smaller, and their constituent particles are brought nearer to each other. The more dense in substance that a body is, it is the more heavy or weighty. In speaking of the density of different solid and liquid bodies, the term specific gravity is used to denote the comparison which is made. Thus, the specific gravity of a lump of lead is greater than an equal bulk of cork; or the specific gravity of water is greater than that of an equal quantity of spirituous fluid. For the sake of convenience, pure distilled water, at a temperature of 62°, has been established as a standard by which to compare the specific gravity or relative weights of bodies. Water, as the standard, is thus said to be 1. When, therefore, any body, bulk for bulk, is double the specific gravity of water, it is called 2, and so on to 3 and 4 times, up to 22 times, which is the specific gravity of platinum, the heaviest known substance. In almost every case of comparison there are fractional parts, and these are usually written in figures, according to the following arrangement: Fractional parts are divided into tens, hundreds, thousands, and so on. If, in addition to the figure expressing the main part of the specific gravity, there be one other figure, with a dot or point between them-thus 2.5-the additional figure signifies tenths, and the body is two times and five-tenth parts of a time more dense or heavy than water. If two figures occur thus, 10-40-hundredths are signified, and the body is ten times and Some bodies have the power of resuming their former volume or shape when the force which diminished it is withdrawn. This quality is termed elasticity. Steel is one of the most elastic of metallic bodies, but its elasticity is not nearly so great as that of Indiarubber, which, though twisted, drawn out, or compressed in different ways, always resumes its original form. The aëriform fluids, such as atmospheric air, and the gases, are all exceedingly elastic; and so are liquids, such as water, but to a smaller extent. Dilatability is that quality of bodies by which they are enabled to be expanded or enlarged in their dimensions, without any addition being made to their substance. Hardness is the quality which is the opposite of softness, and does not depend so much on the density of the substance, as the force with which the particles of a body cohere, or keep their places. For instance, glass is less dense than most of the metals, but it is so hard that it is capable of scratching them. Some of the metals are capable of being made either hard or soft. Steel, when heated to a white heat, and then suddenly cooled, as by immersion in water, becomes harder than glass; and when cooled slowly, it becomes soft and flexible. Brittleness is that quality by which bodies are capable of being easily broken into irregular fragments, and it belongs chiefly to hard bodies. Iron, steel, brass, and copper, when heated and suddenly cooled, become brittle. Malleability is the quality by which bodies are capable of being extended by hammering. Some of |