cutting of the Mount Cenis tunnel-is now effected by means of diamond-pointed drills, driven at a very high speed by powerful engines.

Crystals of quartz, and common flint, will also scratch glass; and even a piece of hard steel, such as a corner of a newly-broken triangular file, may serve in place of a diamond for writing on glass.

51. Hardness depends, however, on some circumstances not yet thoroughly understood; for the same body when subjected to different treatment, may be hard or soft.

Steel owes its hardness to tempering, that is, to a sudden cooling in oil or water when the steel has been heated to a certain temperature. If the same steel be allowed to cool slowly, it retains all the softness and flexibility of iron. Iron does not possess the property of being hardened by sudden cooling. The discovery of this fact respecting the tempering of steel is, perhaps, second in importance to few discoveries which man has made; for it has given him all the edge tools and cutting instruments with which he now moulds every substance to his wishes, and to which he owes all his modern mechanical improvements. A savage would work for months with fire and sharpened flints in order to fell a tree or carve a rough canoe, when a modern carpenter with his tools of hard steel could accomplish the same object better in a few days.

"Brittle."

52. It is a general rule that very hard bodies are also very brittle or liable to break. The special intensity of cohesive power which constitutes hardness, seems to be accompanied with a condition which allows the molecules no play of position.

A comparatively slight force across the direction in which the cohesion is concentrated, will rupture the body, especially if the force be applied sharply.

Glass scratches pure iron-proving that it is harder than iron; yet glass is the very type of fragility.

Hard-tempered steel is very brittle; those steel chisels and tools used for shaping stones and metals, require of course to be exceedingly hard; but they thereby lose in strength and are often broken. Cast-iron, a complex crystalline substance which is much harder than malleable or wrought iron, is correspondingly more brittle :. while soft iron and steel are the toughest materials in nature.

53. In general the fragility is greater the more sudden the change

of temperature to which any material has been subjected. Giass vessels, if rapidly cooled when blown, are so brittle that the slightest scratch with flint or a grain of sand will break them.

Rupert's drops are tadpole-shaped pieces of green bottle glass, which have been formed by melting the glass and allowing it to drop into cold water. Only the surface or skin of the glass in this case has had time to solidify; the interior particles, not having settled in their natural position, are in such a state of tension that the mere breaking off of the tail of the drop is sufficient to cause the whole to fly into a thousand pieces; but a hard blow may be given to the thick part of the drop without breaking it.

Brittleness, in the making of metallic casts or of glass vessels, is obviated by the process of annealing, i.e. by re-heating and very slow cooling, which may have to extend over days or even weeks, that the articles may be fit for handling and for resisting ordinary changes of temperature.

"Elastic."

54. An elastic body has its molecules so balanced between the extreme positions of no cohesion on the one hand, and of actual contact on the other, that they admit of a certain play under the action of any force, always returning to their original position when the strain is removed.

Elastic bodies vary much in the amount of this play of their particles, and also in the readiness and extent of their return towards their first arrangement.

Thus, india-rubber is very elastic, for it yields far; but it is by no means perfectly so, for when stretched much or often, it becomes permanently elongated. Glass, again, is almost perfectly elastic, though only within small limits; for it will retain no permanent bend ; but, unless in very thin plates or in fine threads, it will not bend far without breaking.

A steel sword, or any long strip of good steel, may be bent till its ends meet, and yet when allowed will return to perfect straightness; a piece of inferior steel, or of any other metal, will either break in the bending, or retain a bend. A steel watch-spring, although so much and so constantly bent, resumes its original form when set free at the end of a century; yet often, from some invisible flaw it will suddenly give way while in action.

The elasticity of steel is of the greatest utility: time-pieces, gur

22

Plancy.

Flexibility.

locks, door-locks, carriage springs, railway buffers, steel pens, spring balances, are some of the hundred cases where it finds application. 55. When a billiard-ball strikes another, or rebounds from a marble slab, there is more than a mere meeting of the surfaces at a single point. For if the slab be slightly smeared with oil or ink, a large round spot will be left on both the slab and the ball, proving that there had been a sudden flattening of the ball at the instant of striking. It is the force with which the particles of ivory are drawn back into their original position that throws the ball up in the air again, or causes it to rebound.

The elasticity of ivory is so good, that billiard-balls scarcely lose even their polish by long use, though the touching parts yield at every stroke.

56. In bending a rod of steel or a plank of wood, we have to pull the molecules on the upper (or convex) side of the rod asunder, and to press those on the lower (or concave) side closer together. We are thus doubly resisted by the cohesion of the particles, and there are two forces conspiring to bring the rod to its original shape when we leave it to itself.

57. Elastic bodies may be either hard or soft; but the harder they are, then the less the extent of strain they allow within the limits of elasticity. If we exceed this limit, we produce a set or permanent change in the shape or size of a body. A sprain of a muscle, for example, is a stretch beyond the limit of its elasticity.

The hard bodies-steel, glass, ivory, stone, &c.—have a much smaller elastic limit than the soft ones, such as caoutchouc, silk, catgut, &c.

"Pliant."

58. Animal and vegetable substances have a cohesion called pliability, rarely met with in the mineral world, by which their fibres admit of sharp or sudden bending without fracture, as if they consisted of a chain of minute hinges or joints.

Silk, bladder, lint, hemp, skin, hair, &c., are very pliable, and owe their value chiefly to this property. Weaving and textile manufactures generally employ the various materials endowed with this quality.

The most remarkable example of this in the mineral world is afforded by asbestos, or amianthus, a fibrous mineral, which owes its name to another and still more remarkable quality, namely, that

Malleability of Metals.

23

of being indestructible by fire. It has been even manufactured into gloves, jackets, and fire-proof clothing. It is a non-conductor of heat; so that a red-hot poker may be for a short time held in the hands covered with a thick asbestos glove. If strongly compressed, there is an unpleasant sensation of warmth felt between the seams. The Chevalier Aldini invented asbestos dresses in 1830 for the use of fire-men; but they were found to be too costly, a jacket costing fifty pounds; and they were so heavy, that if a man fell, he had great difficulty in getting up again without assistance.

"Malleable."

59. The cohesive quality by which bodies allow of their being hammered into thin leaves or plates is termed malleability.

In malleable bodies, the molecules have no special disposition, but will cohere in any way, the attraction acting indifferently all round the molecules, so that they yield to force and glide about among each other without fracture, almost like the particles of a liquid.

60. Gold is the most malleable of all the metals; it may be hammered until it is reduced in thickness to the 300,000th of an inch. In this state it is transparent to light, allowing the green rays to traverse it. For gold-beaters the metal is first formed into rods; these are then rolled or flattened into ribands: the riband is cut into thin strips, which are hammered out to a great width; these are again subdivided, and lastly hammered out to the tenuity described. An alloy of gold with silver, in the proportion of 20 parts of gold to 22 silver (called lemon gold), is equally malleable. This may be reduced to the 300,000th part of an inch in thickness, and it is also transparent to light, allowing the reddish-purple or violet rays to pass through it. No other metals can be reduced to such a degree of tenuity. Tin is malleable, but the thinnest leaves are the 1600th part of an inch in thickness.

Dutch gold, an alloy of copper and zinc, may be brought by malleation to very thin leaves, sometimes mistaken for and substituted for gold. The thinnest leaves are, however, quite opaque, and do not allow any coloured light to traverse them.

Silver and copper may be hammered till rendered very thin. The coppersmith hammers a flat piece of copper into the shape of a vessel without crack or seam, and of uniform thickness throughout. Iron becomes singularly soft and malleable when heated to red

ness. Under the forge hammer it takes any desired form almost as readily as potter's clay. Between rollers, it can be spread into sheets as thin as paper, or lengthened into uniform solid bars for railways.

Platinum is the only other metal which, like iron, is malleable or weldable at a red or white heat.

61. By the same, or an analogous property, metals such as copper, silver, gold, may be made, under the blow of the coining-press, to take impressions as delicate as those of sealing-wax from a signet stone.

"Ductile."

62. The most malleable bodies are not always the most ductile, or easy to draw into wire.

Both properties are, however, alike due to a fluidity of the molecules by which they may be moved past each other without loss of cohesion.

63. To form iron wire, a rod of iron, being reduced in size so as to pass through an opening in a plate of hard steel, is seized beyond the plate by strong nippers, and the whole rod is forcibly drawn through. It is thus reduced, of course, to the size of the opening, and is lengthened accordingly. By repeating the operation through smaller holes in succession, a wire may at last be obtained of the thickness of a hair.

A grain of gold has been drawn into five hundred feet of wire, and from a calculation made by Müncke, the diffusion of a known weight of gold over silver-wire may be carried to such a degree, that one grain admits of subdivision into ninety-five thousand millions of visible parts, i.e. visible under a microscope magnifying a thousand times.

Dr. Wollaston produced a wire of platinum only the 30,000th of an inch in diameter by the following ingenious process :-A small platinum wire was placed in the axis of a small cylinder of silver; the compound wire was then drawn out in the usual way so far as its ductility would admit. It was then placed in nitric acid; the silver was dissolved, and the platinum left in the form of a wire of about half the thickness of the thread of a spider's web. lt could only be distinctly seen when heated to redness.

Platinum is thus the most ductile metal; after it come the other metals in the following order: silver, iron, copper, gold, &c.