matter, whether large or small, are influenced by the power of gravitation, so the molecules which make up the mass are held together with greater or less firmness by the force of molecular attraction. The intensity of this force determines also the physical state of the matter concerned. In solids the power of molecular attraction is most strongly exerted, in liquids it is weaker, and in gases it seems to be overcome by a force of repulsion which tends to separate the molecules.

We distinguish in ordinary usage between the terms cohesion, where molecules of like composition are held together, as the particles of iron in a bar of that metal, and adhesion, where bodies of unlike composition are held together, as when a glass rod is dipped into water, the force of adhesion causes the liquid to adhere to the solid.

Chemical Attraction.-The attraction between atoms which causes them to unite in the formation of molecules is otherwise known as chemical affinity, and will be referred to later.

CHAPTER II.

SPECIAL PROPERTIES OF MATTER.

A. SPECIAL PROPERTIES OF SOLIDS.

It is obvious from the definition of solids as contrasted with liquids and gases (see p. 15) that they must have properties which are distinctive, and are not shared in any notable degree by the other forms of matter.

Hardness is the resistance to wearing by friction, such as scratching or rubbing, shown by a solid. It is possessed in the highest degree by the diamond, which is, therefore, capable of scratching any other solid.* Hard bodies are often used as polishing powders, as diamond dust, emery, pumice, and tripoli. Great hardness may be imparted to steel and other bodies by a process called tempering, that is, cooling them suddenly from a high temperature. Under these circumstances, however, they usually become more brittle.

Brittleness is inability to withstand compression or a blow, and indicates a want of tenacity or cohesive power between the particles of the solid. It may accompany great hardness, as in the diamond and glass.

Tenacity is the resistance to a tearing or pulling strain exerted upon solids. It may vary in different directions in the same body, as in the case of wood, where it is greater in the direction of the fibres than transversely to them. Closely connected with this are two properties chiefly shown in metals, viz., ductility and malleability.

Ductility is the capability of being drawn out into wires or threads, and is possessed especially by certain metals, like gold. platinum, iron, and copper. Glass and waxes when hot can also be drawn out into fine threads.

* The hardness of a body is expressed by referring it to a scale of hardness; that usually adopted is,—

Malleability is the property in virtue of which bodies are flattened into thin sheets or films under the influence of hammering or rolling. It is possessed especially by metals, such as gold and copper.

Elasticity has already been spoken of as a general property of matter (see p. 18) shared by solids, liquids, and gases, although in unequal degrees. The elasticity there referred to was, however, the elasticity of compression, and was regarded as the complement of the general property of compressibility. Elasticity of traction, or that developed by a stretching force, elasticity of torsion, or that developed by a twisting force, and elasticity of flexure, or that developed by a bending force or weight, are other phases of elasticity peculiarly belonging to solids. A familiar application of the elasticity of torsion is the torsion balance now used extensively in practice, and of the elasticity of flexure, steel springs, as for watches, carriage springs, etc.

Structure of Solids.-A solid is distinguished from a liquid or gas by possessing definite shape independent of the containing vessel. When we come to examine more closely the structure of solids, we observe notable differences. Certain substances, for instance, on passing from the liquid to the solid condition, assume characteristic shapes, as alum, nitre, sugar, ice, etc., while others become solid without assuming distinctive shapes, as fats, waxes, and flocculent and gelatinous precipitates. The former are called crystalline bodies and the latter amorphous. Again, crystalline bodies have differences of structure, due to cleavage, as in mica, rock-salt, etc., or to confused crystallization, as in granular minerals like marble, emery, etc. The several systems under which all crystalline bodies may be classified will be referred to later (see under Heat).

B. SPECIAL PROPERTIES OF LIQUIDS.

1. Attraction and Repulsion in Liquids.

Capillary Phenomena.-We have already referred to the adhesion of liquids to solids in speaking of the moistening of a rod of glass with water, as illustrative of the force of adhesion (see p. 24). Not all liquids show this attraction. Some even show a repulsion of the solid, as when a glass rod is dipped into mercury. Instead of a curving upward of the surface of the liquid on all sides of the glass rod, as with water, a depression of the surface occurs immediately around the rod, showing that repul

sion exists between the mercury and the glass. The water is drawn upward around the rod because its adhesion to the glass distinctly exceeds the cohesion of the liquid; the mercury adheres to glass much less strongly than it coheres, and hence it curves away from the rod. These attractions and repulsions have an interesting illustration in the phenomena known as capillary, observed when tubes of relatively fine diameter are dipped into liquids. If a glass tube be dipped into a liquid which wets it (or adheres), as in the case of water, the liquid will rise in the tube to a higher level than the surrounding surface, and the height is the greater the smaller the diameter. If, on the other hand, the tube be dipped into a liquid which does not

wet it (or adhere), as in the case of mercury, the liquid will be depressed in the tube below the surrounding surface, and the smaller the diameter of the tube the greater the depression. These phenomena of rods and tubes are illustrated in Fig. 4. Many natural phenomena, such as the rise of moisture in rootlets and stems of plants, the rise of oil in a lamp-wick, the absorption of water by filter paper or sponges, are to be considered as illustrations of the principle of capillarity.

Diffusion of Liquids.-Closely connected with capillary phenomena are those of diffusion. If two liquids of different densities, but capable of admixture, be placed one above the other in the same vessel, they will begin to mix or diffuse through each other, even if the upper liquid be of less density than the lower. This will also take place though they be separated by porous partitions of various materials. But it is found that the rate of diffusion differs greatly for different substances. Many solids. when in solution will diffuse rapidly, while others will diffuse with great slowness. The former class will be found to include

most crystallizable solids, like salt, sugar, magnesium sulphate, etc., while the latter class includes uncrystallizable or amorphous substances, like starch, gums, gelatin, or glue. To the former class the term crystalloids has been given, and to the latter the term colloids (from the Greek word for glue).

Graham founded upon this property of unequal diffusibility the process of dialysis. A sheet of bladder or parchment paper is stretched tightly over the lower end of an open cylinder or inverted glass funnel. The mixture of liquids to be separated by dialysis is poured in above, and the dialyser supported with the lower end immersed in pure water contained in a larger outer vessel. The crystalloid substances will diffuse through the membrane, and be found in solution in the outer vessel, while the colloids will remain in the inner vessel, or dialyser. The chemist has thus separated the crystalline arsenous acid from the colloid food material with which it may have been mixed in the stomach, and obtained it in a pure state for verification in cases of suspected poisoning.

2. Pressure of Liquids.

Liquids are but slightly compressible, and with the removal of the force causing pressure recover immediately their original volume. For this reason and because of the ease with which their molecules are free to move, they readily transmit pressure throughout their entire mass. This pressure is transmitted throughout the liquid equally in all directions, whether it is that in which the force is applied or at an angle to it.

This is illustrated in the case of the sprinkling nozzle of a garden hose, or similar apparatus, where the water is seen to issue with equal force from all the apertures. Of course, if the pressure in the one case is exerted over a larger area of surface than in the other, a different total force is felt in consequence. Here we must multiply the intensity of the pressure per unit of surface by the area of surface to get the total force exerted. Hence a pressure of five pounds per square inch exerted over a surface of sixteen square inches would be felt as a pressure of eighty pounds upon that surface.

An important application of this principle of transmission of pressure, and exertion of the same over a larger surface than that where it was applied, is found in the hydraulic press. This, as shown in Fig. 5, consists of a small force-pump, in which works a solid piston, P. When this piston is depressed by means of a