Porosity-Density-Specific Gravity.

15

making the pins used in wooden ship-building, for wedges used in fixing iron rails to the chairs—by compressing it laterally to about half its original bulk, and so making it approach the solidity of metal.

34. It is the buoyancy given by the air contained in its pores that makes wood lighter than water; for if a log of wood be exposed to the pressure of a great depth of ocean, its pores become filled with water, and it sinks as readily as stone. Thus it was with the boat of a whale-fishing ship which had been dragged far under water by a whale when being afterwards drawn up by the rope of a harpoon, it was supposed to be bringing a piece of rock with it.

35. Petrifaction furnishes a striking proof of the existence of pores in such bodies as wood or bone. The usual explanation given of this natural formation is that at some remote period, the wood or bone had been immersed in water, which contained silicious or flinty matter in solution, and that this, penetrating through all the pores of the mass, hardened on the decay of the vegetable or animal matter, and at the same time displaced it. In a fossilized substance, then, we have a fac-simile, an actual cast, moulded by nature in limestone or flint, of the whole system of pores that existed in the animal or vegetable body during life.

"Dense."

36. The quantity of matter which we estimate by its weight, existing within a given space-such as a cubic foot or a gallon-may be very variable.

Thus a cubic inch of lead is nearly forty times heavier than a cubic inch of cork.

If a wine-glass be filled with mercury, we are surprised when we lift it for the first time; it is as heavy as thirteen and a half times the same quantity of water.

37. We commonly estimate the quantity of matter in a body by its specific gravity, or weight, compared with that of an equal bulk of some specific or standard substance, which we adopt as the unit of density. Pure water being so easily procurable at all times and places, and being uniform in its composition, has been almost universally selected as the standard of reference.

38. When, therefore, we say that the metal platinum has a specific gravity of 22, we mean that a cubic inch of it is twenty-two times as heavy as a cubic inch of pure water. So we say gold has a specific

16

Cohesion in Crystalline Bodies.

gravity of 19; mercury, 13; lead, 11 ; iron, 8§; copper, 8; common stone, about 2; wood, from to 1; cork, ; and so on.

39. Density must depend, first on the weights of the individual molecules, and secondly, on their degree of approximation, i.e. on the number which are packed within a given bulk. Hydrogen molecules are lighter than those of any other known substance; and hence chemists take this gas as the unit of reference, to avoid fractions as far as possible in the expression of specific gravities of gases and vapours.

Secondly, density must depend on the temperature, for whatever affects the nearness of the molecules of a body must of course alter its density. Now, as a general rule, heat expands bodies and separates their molecules more widely: hence heat, as a rule, lessens the density by increasing the bulk.

As our standard of density, then, will itself vary according to the degree of warmth or temperature, we must, for accurate comparison, have it at a certain fixed temperature—such as the freezing point of water-or we must know what allowance to make for the difference of warmth.

"Crystalline."

40. Cohesive attraction is not, in most cases, the same all round a molecule; but like the poles of a magnet, it seems to lodge nearer certain sides or ends of the molecule. Thus when the particles are free to move in any direction, and to follow their natural tendencies, they generally assume a more or less regular arrangement, or a form we call crystalline.

Moisture, or watery vapour, freezing on the window-panes, shows beautifully this selective attachment of the particles in passing from the liquid to the solid state.

A flake of snow consists of groups of crystals of a stellated form, in which all the angles are of 60°. Each crystal consists of six prisms radiating from a centre, and exhibits a symmetry of formation as regular as that of a fern-leaf, or a bird's feather.

Water beginning to freeze, shoots delicate needles across its surface these thicken and interlace till the whole mass becomes apparently solid, but the crystalline arrangement remains, and may be detected by allowing a lump of ice to remain for some time in water at, or a little above, 32°.

This crystalline structure of solids is well illustrated by immersing

for a few hours a rough block of alum in a nearly saturated solution of alum in water. It will be found that the alum, instead of dissolving uniformly, will appear as if dissected, traversed by numerous lines and smooth surfaces, always taking the form of the regular octohedron, or eight-sided crystal, which is the natural form of crystallised alum. This demonstrates that the cohesive force is strongest and best able to resist solution in the direction of the crystalline planes.

When any solid, such as a metal, is in the liquid state, its molecules are not within perfect cohering distance; but as it cools these approach gradually, and in the way which their polar attraction dictates. It becomes solid first on the outside of the mass, as it cools from without. If, before the cooling is completed, we break the crust and pour out the remaining liquid, the curious internal crystalline structure will be displayed. This is well seen in melting the metal bismuth in an iron ladle. When the melted metal has solidified on the surface and at the sides, the fluid portion is poured away and the bismuth will be found crystallised in regular cubes striated on the surface. Sulphur may be obtained crystallised in prisms by a similar process. Hollow balls of crystals of carbonate of lime and quartz have been thus produced naturally. They are called geodes. What is called the grain of a metal is the result of this action.

41. The process of crystallisation is most readily exhibited by the solutions of salts. Saltpetre, glauber-salt, copperas (or green vitriol), sulphate of copper (or blue vitriol), alum, borax, and any other salt the solubility of which is much increased by heat, are well adapted for crystallisation. When dissolved in hot water, and the water is allowed to cool, or slowly to evaporate, the salt is deposited in beautiful solid crystals, each salt having a special and invariable form of crystal, with sharp angles and flat polished faces.

If any such crystal be broken, the broken surface appears to the microscope as if regular layers of particles had been disturbed (like a broken stack of bricks or pile of bullets), and the deficient portions of the crystal will be exactly replaced by putting it in the evaporating solution.

42. A drop of a solution of sal-ammoniac in water, placed on a slip of glass, and allowed to evaporate, produces the most beautiful arborescence.

If a piece of copper is thrown into a solution of nitrate of silver, the copper is dissolved, while the silver is rejected. During this

18

Crystalline and Amorphous State.

exchange the silver slowly forms into a beautiful shrub or tree, resting on the remaining copper as its root. This formation is called the arbor Dianæ.

43. All the precious stones have a crystalline structure, and can be well cut and polished only parallel to the natural faces of their crystals. This is called cleavage. Thus a diamond can only be split in layers corresponding to the regular octohedron. The basaltic pillars of the Giant's Causeway in Ireland, and of the Isle of Staffa, which appear like a garden supported on magnificent columns in the midst of the ocean, are natural crystalline arrangements rivalling in regularity and beauty any human work, and in grandeur so surpassing, that superstition might well ascribe them to the handiwork of giant architects.

44. No better example can be given of the power of co-operation of the small particles of matter, than the force with which they unite when crystallisation takes place.

Water, with one or two other substances, becomes more bulky on solidifying: and the united effort of the small molecules of water to arrange themselves in a crystalline form is such as to burst the strongest vessels. A cannon, filled with water and firmly plugged at the mouth, has been burst when the water became frozen. This agency contributes to the gradual breaking down of our Alpine summits, and the destructive falling of their fragments into the valleys.

45. There is an immense variety of crystalline forms, though they may all be referred to a certain number of typical figures or systems, which may be found in any text-book on mineralogy, the science to which the subject of crystallisation specially belongs.

46. When bodies possess no regularity of structure, they are termed amorphous-that is, without definite form.

This may arise, however, not from any want of polarity in the particles, but simply from the circumstances in which they are placed, and the very same molecules may be found at one time amorphous and at another crystalline. Chalk and Iceland spar are similar in composition-carbonate of lime or carbonate of calcium. Chalk is soft, white, opaque, is easily broken in any direction, and has no crystalline form. It is in the amorphous or shapeless condition. Iceland spar is seen in beautiful transparent crystals of a

Hardness.-The Diamond.

19

rhomboidal form: it is colourless, hard, brittle, and breaks only in the three directions of the faces of a rhomboid. Again, iron and all metals are devoid of structure when in a molten state, but become crystalline on cooling: and indeed, as a rule, all crystals descend from an antecedent amorphous condition of fluidity. For the perfect development of the crystalline state, it is necessary that the particles should be all free to move, and this is effected by solution, fusion, or sublimation.

47. The operation of crystallisation is very extended in the inorganic world especially, and leads to peculiarities of structure that usually appear under the guise of special names-as "hard,” "brittle."

"Hard."

48. One body is said to be harder than another when we can scratch the latter with it.

The hardness is not due, as might at first be supposed, to mere density, but to the force with which the molecules retain their polar or crystalline arrangement. Gold and mercury are both among the heaviest of metals, and yet they are also among the softest of them.

49. Mineralogists have a number of type-substances, whose degrees of hardness are very uniform, arranged so as to form a scale of reference for determining the hardness of any other body. In the following or Mohr's scale, which is commonly adopted, the relative hardness of each of the ten substances is cited by the number which gives its place in the list :

1. Talc.

2. Rock-salt.

3. Calc-spar.

4. Fluor-spar.

5. Apatite.

6. Adularia-felspar.

7. Rock-crystal.

8. Prismatic topaz.

9. Corundum.

10. The diamond.

To determine the hardness of any mineral, we simply try, beginning with the hardest, which of these ten it will just scratch; in this way we can say between which two degress of hardness it lies.

It

50. Diamond, which is a crystalline form of carbon-the same substance as charcoal-is the hardest of known substances. cuts or scratches every other body, and can be polished only by its own dust yet it is but about one fourth part as heavy as gold. Glaziers use a small point of diamond, formed by the natural edges of a crystal, as a knife for the cutting and shaping of glass.

The piercing through hard rocks-such as was done in the recent