V.-PROPERTIES OF SOLIDS DEPENDENT ON MOLECULAR ACTION.

Tenacity.

64. TENACITY is the resistance which a body offers to rupture when subjected to a force of traction; that is, a force which tends to tear the particles asunder.

The tenacity of a body may be determined in pounds. For this purpose it is wrought into a cylindrical form, having a given crosssection; its upper end is then made fast, and a scale-pan is attached to the lower end; weights are then placed in the pan until rupture takes place. These weights measure the tenacity of the body.

Metals are the most tenacious of bodies, but they differ greatly from each other in this respect. The following table exhibits the weights required to break wires of 855 of an inch in diameter, formed of the metals indicated:

It has been shown by theory and confirmed by experiment, that of two cylinders of equal length and containing the same amount of material, one being solid and the other hollow, the latter is the stronger.

This latter principle is also true of cylinders required to support weights; the hollow cylinder is better adapted to resist a crushing force than the solid one of the same weight, and hence it is that columns and pillars for the support of buildings are made hollow. This principle also indicates that the bones and quills of birds, the stems of grasses and other plants, being hollow, are best adapted to secure a combination of lightness and strength.

(64) What is Tenacity? How is it measured?

What bodies are most tena

cious? Examples. What is the form of greatest strength? Application to grasses, quills, bones, &c.

Hardness.

65. HARDNESS is the resistance which a body offers to being scratched or worn by another. Thus, the diamond scratches all other bodies, and is therefore harder than any

of them.

After the diamond come the sapphire, the ruby, rockcrystal, &c., each of which is scratched by the preceding one, but scratches the succeeding one.

Hardness must not be confounded with resistance, to shocks or compression. Glass, diamond, and rock-crystal are much harder than iron, brass, and the like, and yet they are less capable of resisting shocks and forces of compression; they are more brittle.

An alloy or mixture of metals is generally harder than the separate metals of which it is composed. Thus, gold and silver are soft metals, and, in order to make them hard enough for coins and jewelry, they are alloyed with a small portion of copper. In order to render block-tın hard enough for the manufacture of domestic utensils, it is alloyed with a small quantity of lead.

The property of hardness is utilized in the arts. To polish bodies, powders of emery, tripoli, &c., are used, which are powders of very hard minerals. Diamond being the hardest of all bodies, it can be polished only by means of its own powder. Diamond-dust is the most efficient of the polishing substances.

Ductility.

66. DUCTILITY is the property of being drawn out into wires by forces of extension.

Wax, clay, and the like, are so tenacious, that they can easily be flattened by forces of compression, and readily wrought between the fingers. Such bodies are plastic. Glass, resins, and the like become tenacious only when heated. Glass at high temperatures is

(65.) What is Hardness? What body is hardest? What bodies come next? What are brittle bodies? What is the effect of alloying bodies? Explain the operation of polishing? How is the diamond polished? What is the best polishing substance? (66.) What is Ductility? Give examples of plastic bodies?

so highly ductile, that it may be spun into fine threads and woven into fabrics. Many of the metals, as iron, gold, silver, and copper, are ductile at ordinary temperatures, and are capable of being drawn out into fine wires, by means of wire-drawing machines.

The following metals are arranged in the order of their ductility: platinum, silver, iron, copper, gold, zinc, tin, lead.

Malleability.

67. MALLEABILITY is the property of being flattened or rolled out into sheets, by forces of compression.

This property often augments with the temperature; every one knows that iron is more easily forged when hot than when cold. Gold is highly malleable at ordinary temperatures. Gold is reduced to thin sheets by being rolled out into plates by a machine: these plates are cut up into small squares, and again extended by hammering until they become extremely thin. They are then cut up again into squares, and hammered between membranes, called goldbeater's skins. By this process gold may be wrought into leaves so thin, that it would take 282,000, placed one upon another, to make an inch in thickness. These leaves are employed in gilding metals, woods, paper, and the like. Silver and copper are wrought in the same manner as gold.

The following metals are amongst the most malleable under the hammer gold, silver, platinum, iron, tin, zinc, copper, lead.

When metals are alloyed, they are generally harder and less malleable, as well as less ductile.

Is gold ductile ? When? Give examples of ductile metals. (67.) What is Malleability? Effect of temperature? How is gold formed into sheets? What is the order of malleability of metals? Effect of alloying.

CHAPTER II.

MECHANICS OF LIQUIDS.

1.-GENERAL PRINCIPLES.

Definition of Hydrostatics and Hydrodynamics.

68. THE Mechanics of Liquids is divided into two branches: HYDROSTATICS, which treats of the laws of equi librium of liquids, and HYDRODYNAMICS, which treats of the laws of motion of liquids.

Properties of Liquids.

69. The following properties are common to all liquids:

1. The molecules of liquids are extremely movable, yielding to the slightest force.

There is very little cohesion between the molecules of liquids, whence their readiness to slide amongst each other. It is to this principle that they owe their fluidity.

2. Liquids are only slightly compressible.

Liquids are so slightly compressible, that for a long time they were regarded as absolutely incompressible. In 1823, ERSTED demonstrated, by an apparatus which he contrived, that liquids are slightly compressible. He showed that for a pressure of one atmosphere, that is, of 15 lbs. on each square inch of surface, water is compressed the 1000gth of its original volume. Slight as is

(68.) Define Hydrostatics. Hydrodynamics. (69.) What is the first property of Liquids. Illustrate. Second property? Illustrate.

4

the compressibility of water, it is nevertheless ten times as compressible as mercury.

3. Liquids are porous, clastic, and impenetrable, like other bodies.

That liquids are porous, has already been shown (Art. 9). That they are elastic, is shown by their recovering their volume after the compressing force is removed. It is also shown by the fact that they transmit sound. Their impenetrability is shown by plunging a solid body into a vessel filled with a liquid. If there is no imbibition, a volume of water will flow over the vessel just equal to that of the solid introduced.

Upon these three properties of liquids depends their property of transmitting pressures in all directions.

Transmission of Pressures.-Principle of Pascal.

70. Let a bottle be filled with water and corked, as represented in Fig. 43. If the cork be pressed inwards, the pressure will be transmitted to the molecules in contact with it; these molecules will in their turn press upon the neighboring ones, and so on until the pressure is finally transmitted to every point of the interior surface of the bottle.

It is shown by experiment that the pressure thus "transmitted is equal to that applied to the cork; that is, the pressure upon each square inch of

Fig. 43.

Third property? Illustrate. (70.) What is the Principle of Pascal?