when it contains the same quantity of matter in a less space, or more matter in the same space; and the fact of one body being more dense than another is indicated, as will be shewn hereafter, by the difference in the weight of equal volumes of the two substances. Thus platinum and mercury are known to be more dense than copper and water, because a cubic inch of the former substances weighs more than a cubic inch of the latter.

10. Porosity. From the experimental fact that all substances may be rendered more dense, we infer the porosity of all bodies, that is, that the component particles or constituent atoms either touch each other at very few points, or, what is more probable, that they are not in contact at all; for it seems reasonable that the space by which the volume is diminished must before the diminution consist of pores. The animal and vegetable kingdoms teem with evidences of a porosity of bodies, independently of the fact of their compressibility. The mineral poison which pervades the whole frame, and the sap circulating through every part of the smallest plant, afford clear indications of this property, and we have evidence of the porosity of hard solid substances, whenever an air bubble is driven from a lump of sugar or a piece of chalk immersed in water, and in every stream which filtrates through the rocky sand stone. There is a curious semi-transparent precious stone termed the hydrophanous agate, sufficiently hard to emit sparks on being struck against steel, which when plunged in water discharges a great number of small air bubbles, becomes transparent, and receives an augmentation in its weight. On being dried it loses the water which it had imbibed, and returns to its natural opacity. The preceding quality of bodies is an immediate consequence of the theory of their molecular constitution, and the pores will be of greater or less magnitude, according to the intensity of the attractive forces to which the particles are subject. This method however of viewing the question involves refinements and considerations somewhat foreign to an elementary treatise; it

is better therefore to place it on the evidence of known experimental facts.

11. Elasticity. The elasticity of substances is the quality in virtue of which they resume their former state, after either a change of volume or a change of form, when the cause to which this change was owing has ceased to act. This quality exists in very different degrees, and must be considered as resulting partly from a change of bulk, and partly from a change of form; when air is compressed by pressing down a tumbler inverted over water the elasticity arises from a change of volume, but when a steel spring is bent it results from a change of form; in general, the elasticity is connected with both these changes. A substance is considered perfectly elastic when the force of restitution is exactly equal to the force of compression; common air and all the gases possess this property nearly in perfection; hence they are termed elastic fluids. The elasticity of liquids is seen at once from the rebound of a stone or cannon ball from their surfaces, and from the way in which they rebound when poured from one vessel into another. When solid bodies, as two ivory balls, impinge on each other, the degree of elasticity is measured by the magnitude of the rebound after impact. The two forementioned kinds of elasticity, so to speak, though referable to the same principles, are well exemplified in the case of Indian rubber. This substance is known to be very elastic, it yields readily to compression and recovers itself immediately; here the elasticity is due almost entirely to a change of figure; for if the same Indian rubber be enclosed in a space which it accurately fills, it will resist compression with great violence. Here then the same substance illustrates the two kinds of elasticity, there is a feeble effort of recovery from a state of altered figure, and a very violent one from a state of compressibility or altered dimension. It is to this quality of bodies that we owe so much of the use of steel in the arts. Our comfort is consulted, and our carriages are preserved, by the use of springs; it is from the elasticity of a

spiral spring that our watches derive all their motion and regularity; thus furnishing an accurate measure of time.

12. Dilatability.-By the dilatability of a substance is meant, the quality in virtue of which it has a tendency to increase in volume without any increase in the quantity of matter. This is an essential property of elastic fluids, which by virtue of their elasticity dilate, as the pressure to which they are subject decreases. But it is a property of all bodies equally to change in volume under the influence of heat; all bodies dilate or expand when their temperature is raised, and contract when their temperature is lowered, and return to exactly the same volume under the same temperature. This is one of the most regular and beautiful laws of nature; at every hour both of night and day the heat varies, and all bodies at the surface of the earth participate in these variations. They are alternately in the act of dilating or contracting, and can never have the permanent dimensions which we assign to them. These constant alternations are however productive of no irregularity or confusion, for all nature experiences them equally. These alternations arise from the motion of all the constituent particles, both internal and external; hence if we learn from the porosity of bodies that their constituent particles are not in contact, we may learn from their dilatability that they are never at rest, but are continually changing their relative positions and distances. Whence we may conclude that matter, which seems to us the most inert, is the subject of ceaseless change, ceaseless activity.

CHAP. III.

HARDNESS-TOUGHNESS-DUCTILITY, &c.- -FRICTION

CRYSTALLOGRAPHY.

13. In the preceding chapter we treated of certain properties which are common to all matter in whatever state it may exist; but there are some properties distinct from the preceding peculiar to solids, and closely connected with their most curious constitution, which must be briefly noticed.

The hardness of a solid expresses that peculiar character which gives rise to a difficulty of displacing the particles among themselves. Thus steel is harder than iron, and

diamond harder than steel. There is no invariable connexion between hardness and compressibility; for the compressibility of steel and of soft iron is nearly the same, but the hardness widely different. The hardening of metals is one of the most remarkable and delicate operations in the arts; the degree of hardness is indicated by a particular succession of colours, so that the artist can see at once of what temper his tool is. If a body be too much hardened it becomes brittle; but brittleness and elasticity may exist together, as in glass, which is very brittle, but also highly elastic. The hardness of any solid must be carefully distinguished from its resistance to compression. The diamond is the hardest known substance, not because if it be laid on a piece of metal and hammered it will penetrate the metal rather than yield, but because it cannot be itself scraped or striated by any known substance, whereas

it will striate all others. Glass, for instance, is much harder than marble, because it can scratch or striate marble, and less hard than rock crystal or diamond, because it is readily scratched by these substances. The hardness of a body is also tested by its resistance to wear; thus diamond, being the hardest known substance, can only be polished by its own dust. The degree of hardness is very different in substances whose chemical constitution is the same, but it evidently depends on their physical constitution; thus, white marble is much harder than chalk, and we have reason to believe that white marble is nothing else than chalk, whose constituent particles are in much closer contact. Softness is the opposite quality to the preceding, and a soft metal, as lead or gold, can be scratched very easily.

Toughness is very distinct from hardness, being the property in virtue of which solids will endure very heavy blows without breaking; it implies a certain yielding of parts combined with a great general cohesion and very different degrees of elasticity. Some woods are exceedingly tough, and the toughest known substances are cast iron and steel.

Malleability is that property of some metals in virtue of which they may by a blow be deprived of their figure without exerting any powerful effort to preserve it, and without fracture; thus some metals can be reduced to thin plates by hammering. Gold is the most malleable of metals, and every one must have seen the operation of making gold leaves and gold-beaters' skin, which is the skin between which the metal is hammered out into so beautiful a thinness, that two thousand are not much thicker than a sheet of paper. This property is possessed in very different degrees by other metals; some break at once like a piece of glass.

Ductility is the property which some metals possess of being drawn out into wires; it is very different from the preceding, since metals which cannot be hammered far