character of crystallised water, see par. 246. A knowledge of these forms is an essential part of the science of mineralogy, and is of importance to the geologist.

Mineral substances without regular structure, such as glass, flint, wrought-iron, are said to be amorphous. A solid that was at first amorphous may become crystalline, if it is long subject to vibration. When the axle of a railway-carriage is new, the iron is fibrous in texture, and tough; but after long use it is found to have become crystalline and brittle, and hence sudden breakages.

The crystalline arrangement would seem to be the natural arrangement, which the molecules of matter assume when they can. All the facts of the case lead to the supposition that molecules do not exert the cohesive force equally on all sides, but only in certain definite directions; and that there are differences in this respect between the molecules of one substance and those of another.

25. Resistance to Deformation.-In solid bodies, cohesion manifests itself chiefly in resisting changes of form-deformation or strain. A body may be deformed in various ways; it may be broken, it may be bent, twisted, extended, compressed.

26. Tenacity. When an attempt is made to break a body by pulling, the force with which its particles. resist being torn asunder, is called its tenacity. In respect of this quality, bodies differ immensely. If a thread of steel of a certain thickness require a weight of 100 lbs. to break it, a thread of silver of the same thickness will break with 29 lbs., and one of lead with 2 lbs. But many circumstances modify the tenacity. The threads now spoken of were supposed prepared by drawing;' but if the same threads are

annealed, that is, heated to redness and then allowed to cool slowly, they will break with 54 lbs., 16 lbs., and 2 lbs. respectively. Similar phenomena are observable in resistance to bending and cross fracture. But all three forms of resistance are modified by the property of

27. Elasticity.-A rod of india-rubber may be pulled out to several times its original length, or it may be squeezed end-ways into a fraction of that length; and in either case, when the force is removed, the body returns to its former shape. Similarly, a blade of steel, a rod of whalebone, a quill, may be bent largely, and recover its original form. Smear a flat smooth surface of marble or iron with a sticky coloured substance, and drop an ivory ball upon it from a height of a few feet; a considerable round patch of the ball will be found coloured, showing that the convex surface had been flattened to that extent, and yet is now as round as ever. This property in a body of yielding to a stress and then returning to its former figure when the force is withdrawn, is called Elasticity. Substances in which the property is very marked are styled elastic, and contrasted with non-elastic substances. But there is no such thing as absolute non-elasticity. Within certain limits, every body recovers from distortion; only in one substance, such as lead, the extent of this recoverable distortion is so small as to be unobservable, and in another, like steel, very wide. The amount of strain that may be recovered is called the limit of the body's perfect elasticity. When that limit is exceeded, the body remains permanently extended, or compressed, or bent.

28. Compressibility.-There are two kinds of elasticity -elasticity of shape, and elasticity of bulk; and the same body by no means exhibits the two qualities in

equal degree. out in length, it becomes smaller in thickness; when compressed in length, it bulges out in the middle; but when compressed in a mould, it requires great force to make it yield a very little. It is easy to understand how a body with visible pores, like a sponge, should lose in bulk by compression; but a piece of sound cork seems without pores, and yet it can be squeezed between the fingers into less than half its bulk, returning to its former dimensions when released. A gold coin is sensibly less in bulk than the blank metal was before stamping, although the gold from the first is quite solid and continuous even under the microscope. In this case the strain has gone beyond the limit of elasticity, and the loss of bulk is not recovered.

When a rod of india-rubber is drawn

29. Compressibility of Gases.-It is in gases that compressibility is seen in greatest perfection. Take a cylinder full of air or any other gas, and insert an airtight piston at the top. Before being inclosed, the gas was under a pressure of 15 lbs. to the square inch (see par. 198); now add another pressure equal to an atmosphere, and the gas will shrink into half its bulk; add yet another, and it will be reduced to a third. When the extra pressure is removed, the gas returns at once exactly to its former bulk; in other words, it is perfectly elastic. A gas follows the same law in expanding beyond its original bulk when the normal pressure is lessened; under half the compressing force, it fills twice the space. This fact, that the volume of a gas varies inversely with the pressure, is known as 'Boyle's Law.'

30. Cohesion in Liquids.—A solid is distinguished by maintaining a certain shape; and this it does in virtue of the cohesion of its molecules. A liquid always has its

free upper surface level; and for the rest, has no shape but that of the vessel holding it. Yet the force of cohesion is not altogether wanting in liquids; witness a drop of water suspended from a rod, or rolling about on a vegetable leaf. Liquids owe their fluidity to the ease with which their molecules slide over one another, while still cohering with a certain force. It is this that enables them to transmit pressure equally in all directions, a property which is the basis of the science of Hydrostatics.

31. Viscosity. But all liquids are not equally fluid. Water poured into a vessel becomes level at once; treacle or tar remains heaped up at first, and requires time to spread itself out; there is sensible resistance to the sliding of the particles over one another. Such liquids are said to be viscous. Red-hot lava is an example of a viscous fluid. Yet there is no hard and fast line between perfect liquids and viscous liquids. Alcohol or ether is more fluid than water; in other words, water is more viscous than alcohol or ether. On the other hand, it is sometimes difficult to say whether a given substance is a viscous fluid or a solid; as in the case of a piece of sealing-wax gradually heated. Even substances so hard as to break across, sometimes show themselves to have fluidity. Cold pitch can be broken into angular pieces like bottle-glass. Take a piece, and leave it lying free on a flat surface at the ordinary temperature; after a time, it will be found spread out into a cake, as if it had been poured out molten; and yet all the while a sudden bend or blow would have made it break like glass.

32. Adhesion.-While cohesion acts between the particles of the same substance, adhesion acts between dissimilar kinds of matter. It is the same molecular force in both cases, but it is convenient to have a distinct name for each case. Chalk-marks on a wall, and sealingwax on paper, are instances of adhesion.

33. Adhesion of Liquids to Solids takes place much more readily than that of solids to solids, because in the

case of a liquid and a solid the surfaces come into more complete contact. When the hand or a rod of metal is dipped into water, a film of the water adheres to the surface, and is borne up against its own weight; nor can any force shake it all off. Plunge a bit of gold, or silver, or lead, into mercury, and a portion of the mercury will in like manner adhere. Wherever we have wetting, we have a case of adhesion of a liquid to a solid.

But liquids do not always wet solids, or adhere to them. A rod coated with grease, or the wing of a waterfowl, remains dry when plunged in water. Mercury does not adhere to a porcelain cup, or to a rod of iron or platinum. The explanation is simple. There is in every case an attraction between the solid surface and the liquid, but it is opposed by the attraction of the particles of the liquid for one another, and there can be actual adhesion only when the first is stronger than the other. When the adhesive force is able to overcome the attraction of the liquid for its own particles, a part of it is separated and carried off on the surface of the solid; if the cohesion of the liquid is the stronger of the two, there is no wetting of the surface.

34. Capillary Attraction is only a particular effect of adhesion. A tube with

a small bore, like a hair, is called a capillary tube, from capillus, the Latin word for a hair. If the end of such a glass tube is dipped in water, the water is seen to rise in the tube above the level of the rest of the surface.

Fig. 5.

In a series of tubes of

different diameters, the liquid ascends highest in the