will grow drier, whereas, if placed in a vessel which has no attraction for water, it will remain moist.

When vegetable and animal substances absorb water, they generally augment in volume. This fact explains many phenomena of daily observation.

If a large sheet of paper be moistened, it increases in size, and again contracts when dried. This property is employed by draughtsmen to stretch paper on boards. The paper is moistened, and after being allowed to expand, its edges are glued to a drawing-board; on drying it is stretched, forming a smooth surface for drawing upon. The same property causes the paper to peel from the walls of a room when exposed to moisture.

When a workman would bend a piece of wood, he dries one side and moistens the other. The side which is dried contracts, and the opposite side expands, so that the piece is curved. It is the absorption of moisture that causes the wood-work of houses, furniture, &c., to swell and shrink with atmospheric changes, and which necessitates their being painted and varnished. Paints and varnishes, by filling the pores, prevent absorption.

If two different liquids be separated by a membranous partition, a current will be set up from each liquid to the other through the membrane, and after a time it will be found that there is a mixture of both liquids on each side of the partition. These currents are generally unequal, so that there is an actual gain of substance on one side and a corresponding loss on the other. The current that acts to produce an increase on one side is called endosmose, and the opposite current is called exosmose. Thus, if a bladder filled with strong syrup be tied to the end of a glass tube, and the whole plunged into a vessel of water, the syrup soon becomes diluted by the flowing in of water, and the mixture rises in the tube; at the same time a portion of the syrup flows out and mixes with the water. The flowing in of the water is endosmose, and the flowing out of the syrup is exosmose. Similar results are obtained by using other liquids. The phenomena of endosmose and exosmose enable us to explain many interesting facts in animal and vegetable physiology.

What is the effect of imbibition? On paper? Application. Effect on wood? Application. What are endosmose and exosmose? Illustrate.

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 655 of an inch in diameter, formed of the metals indicated:

Iron
Copper

Platinum

00

Silver

Gold

Lead

187

150"

27

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 tenacious? 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 equilibrium 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 100th of its original volume. Slight as is

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