leability. Gold has this last property in a degree far beyond all other substances. Lead, copper, and silver, are also both malleable and ductile wire is drawn out very fine. both from copper and silver: but of gold it is affirmed by Boerhaave, that a single grain of it may be stretched under a hammer till it is big enough to cover a house.

Opposite to ductility is brittleness. Matter which has this quality, consists of parts so posited that they will not be drawn about, but fly asunder when any violence is offered to them. There is a metal which in weight and colour very much resembles gold, but, being incurably brittle, cannot be applied to any of the purposes of gold. Some other metals in their native state are apt to be very brittle and unfit for use, till they are cleared by art of their sulphur, that cold brittle oil, which is mixed with them in too large a proportion. This is often the case with iron, to the ore of which they had some calcarious or alkaline matter to absorb the sulphur, and render the metal malleable. It may seem strange that sulphur, which may be rendered so fluid by heat, should be a cause of brittleness: but when it has only such an heat as will soften it, it has no ductility any more

than

than sand; and, when cold, will crack and fly either with friction or the application of a very gentle heat. Of all matter glass is the most brittle: and it is observed of this and other substances which have the like brittleness, that they generally break with a polished surface superior to any which art can give them, as pitch, rosin, sealing-wax: because their parts are united only by simple contact, not by insertion or implication, as hodies which are tough and fibrous.

Many substances, which are brittle in their ordinary state, may be rendered very ductile by heat; pitch and wax by a very slight degree, rosin by a greater, and glass itself by the heat of a furnace, when it may be extended to any degree by a blast of the breath, and molded into any kind of figure. If it is figured when cold, the parts are abraded by grinding it with water and some hard powder, as sand, emery, or the powder of diamonds, which frets and wears it away.

Elasticity is that property of bodies by which their parts are restored again to their natural figure and position, when they have been driven or pressed out of it by any violence. A sponge squeezed into a smaller compass, will be expanded instantly to its

former

former bulk when the force is withdrawn. A ball of ivory laid upon a polished slab of marble, will touch it but in a few points: but if it be dropped from any height, and means are used, either by oiling or wetting the marble, to mark the contact exactly, it will be found to make a broad round spot: which shews that the parts were pressed inwards by the fall, and are immediately restored again to their proper position. When we press a bladder blown up with air, we say it is elastic; but we know that the elasticity is not in the bladder, but in the medium which it contains within it. And the same is true in other cases: elasticity is not in the solid parts of bodies, but in the fluid within their pores; and therefore, as the condition of this fluid is altered, bodies may have their elasticity, and lose it, and acquire it again The spring of a watch, by the application of fire, may be made as brittle as glass, or straitened, or bended backwards and made to exert its spring the contrary way. The disposition or construction of the solid parts may in many cases contribute to the effect of elasticity, though it is not to be imputed to any power in the parts themselves.

Transparency and opacity are other proper

VOL. IX.

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ties

ties of matter, which respect the passage of the rays of light through their substance.It is evident to reason, that the atoms or units of which bodies are composed, being impenetrable in their nature, must be impervious to the rays of light, how subtile and powerful soever the light may be in its own nature. The light therefore, in its passage, must penetrate the porous vacuities; and it will be differently affected in its passage, as the units are differently figured and differently posited in regard to their pores. The rays of that uniform white light by which all objects are rendered visible proceed in strait lines: but when these rays are broken and dissipated into a variety of directions, they no longer affect us with the sense of light. When the interstitial vacuities of bodies are so disposed that the light can preserve its rectilinear course through them, such bodies appear luminous throughout, and are visible in their internal substance: but when their constitution is such as will not allow a free passage to the light, they are then visible only by those rays which are reflected from their surface, and their internal substance is opaque or dark.

Bodies not transparent in their ordinary state,

Mr.

state, may be rendered so, either by relaxing their parts with heat, so that the light may pass through them more easily; or by giving some new direction, together with an additional force, to the matter of light. Hawksbee, one of the first modern electricians, was very much surprised to find that the sealing-wax and the pitch within-side a glass globe (that is, applied as an inner coating to one hemisphere of the globe) became so transparent when the glass was whirled about and rubbed with the hand, that the fingers might plainly be seen on the other side through the coating*. Oil is condensed when cold into a sort of globules impervious to the light, and becomes as opaque as a solid lump of suet: but when these globules are dissolved, and opened by the action of fire, the oil not only becomes transparent, but appears as bright and shining as if the light were a natural part of its body.

Many heterogeneous fluids grow dark and muddy with cold, but may soon be clarified again by the application of a moderate heat. Red port wine is sometimes as foul as if brickdust were intermixt with it, but will soon become

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* See Hawksbee's Experiments, p. 168, 169. Second edit.