if hurried, the crystals are irregular and confused. To obtain very regular crystals, the evaporation must be spontaneous, or that which takes place at common temperature.

Why is the Giant's causeway disposed in angular columns ?

Because it is supposed that the whole body of the rock was once in a state of fluidity, being no other than the lava of a burning mountain; that the prodigious mass cracked in its cooling, into the above forms; that it may since in some measure have been deranged by earthquakes; that these have swallowed up the volcano itself; and that the waters of the neighbouring ocean now roll over the place where once it stood.Parkes.

The most remarkable basalt is the columnar, which forms immense masses, composed of columns, thirty, forty, or more feet in height, and of enormous thickness. Those at Fairhead are 250 feet high. The coast of Antrim, in Ireland, for the space of three miles in length, exhibits a very magnificent variety of columnar cliffs; and the Giant's causeway consists of a point of that coast, formed of similar columns, and projecting into the sea for a descent of several hundred feet. These columns are for the most part hexagonal, (or six-sided) and fit very accurately together; but most frequently not adherent to each other, though water cannot penetrate between them. In the Hebrides are likewise some vast specimens of basalt.

Why are certain bodies porous, or full of small vacant spaces?

Because of the crossing of the constituent crystalline needles or plates in bodies.-Arnott.

Why are crystals mechanically divided only in certain directions, so as to afford smooth surfaces?

Because, in every crystallized substance, whatever

PART V.

t

may be the difference of the figure which may arise from modifying circumstances, there is, in all its crystals, a primitive form, the nucleus, as it were, of the crystal, invariable in each substance, giving rise to the actually outward existing forms.-Parkes.

Why has strong salt and water a pellicle (or film) on its surface?

Because the attraction of the saline particles for each other is becoming superior to their attraction for the water. This is the common criterion of the fitness of a solution for crystallization.

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Sir Isaac Newton seems to have had a very clear idea of the cause of crystallization. "When," says he, a liquor saturated with a salt, is evaporated to a pellicle, and sufficiently cooled, the salt forms in regular crystals Before being collected, the saline particles floated in the liquor, equally distant from each other; they acted, therefore, mutually on each other, with a force which was equal at equal distances, and unequal at unequal distances; so, in virtue of this force, they must arrange themselves in an uniform manner. ‚”—Newton's Optics, Book iii.

Why will not salt crystallize when dissolved in a considerable quantity of water?

Because the particles of the salt are too far asunder to exert reciprocal attraction: in other words, they are more powerfully attracted by the water, than by each other.-Brande.

Why does the salt crystallize upon evaporation of part of the water?

Because some of the saline particles then gradually approach each other, and they will, according to certain laws, become regular solids; another portion of the salt will remain dissolved in the water which is left; this is usually called the mother liquor, or water.

There is a great variety in the form of crystallized salts, and each salt preserves its own peculiar form

thus, common culinary salt generally crystallizes in small tubes, and sulphate of soda in six-sided prisms. Why do certain salts (called freezing mixtures) convert water into ice?

Because, as heat is required to convert solids into liquids, it follows, that in cases of sudden liquefaction, (as when the salts are dissolved in the water) cold will ensue: hence its production during the solution of many saline bodies, and hence, also, the explication of the theory of freezing mixtures

The artificial preparation of ice has occupied much of the attention of modern chemists. The most recent experiments were made by M. B. Meijlink, who, after numerous trials, with different salts, for the purpose of converting water contained in a tin vessel into ice, during their solution, ultimately gave the preference to a mixture of four ounces of nitrate of ammonia, four ounces of sub-carbonate of soda, and four ounces of water. This mixture, in three hours, produced ten ounces of ice; whilst, with the mixture of sulphate of soda and muriatic acid, he obtained ice only after seven hours. -Brande's Journal, 1829.

Why do many salts, when exposed to the air, effloresce, or fall to powder?

Because they lose their water of crystallization,

Why do some salts effloresce more than others?

Because some thus completely lose their water; while others retain different quantities, according to the dryness of the air.

Why do some salts deliquesce, (or become moist or liquid) by exposure to the atmosphere?

Because they attract water from the atmosphere.
Why are not salt boilers made of cast iron?

Because the cast iron would crack by the adhesion of the salt.

Why is salt-petre refined by solution in water?
Because the rough petre, as it is called, is always

contaminated with muriate of soda and other salts. In order, therefore, to separate them, the refiners dissolve the whole in water, and then, by boiling the solution, evaporate a part of the water, and the muriate of soda, &c. fall down, while the salt-petre is held in solution. When the greatest part of these salts is thus separated, the remaining liquor is suffered to cool, and the nitre is obtained in crystals. This process illustrates the difference which there is in the solubility of salts.Parkes.

Why was nitre used in the composition of Greek-fire? Because it fed or kept alive the sulphur, resin, alcohol, camphor, &c. of which the fire was also composed, by conveying oxygen from the atmospheric air to the sulphurous gas, and to the sulphur while burning. Into this composition, when melted, woollen cords were dipped, and rolled up for use. These balls being set on fire, were thrown into the tents of the enemy, and as the combustibles were furnished with a constant supply of oxygen from the nitre, nothing could extinguish them.

Why will a lump of alum in a glass of water, assume a pyramidal shape in dissolving?

Because, at first the water acts with so much energy as to overcome the cohesion of the solid in every direction; but, as the particles of the alum become united with those of the water, the power of the solvent diminishes. The particles of water which combine first with the alum, become heavier by the union, and fall to the bottom of the glass; and the action at the lower extremity ceases, before it is complete at the upper. When the action has nearly terminated, if we closely examine the lump, we shall find it covered with geometrical figures, cut out, as it were, in relief, upon the mass; showing, not only that cohesion resists the power of solution, but that, in the present instance, it resists it more in some directions that in others; and that when the attraction of the solvent is nearly satis

fied, it is balanced by that delicate modification of cohesion, upon which crystalline arrangement depends. This experiment beautifully illustrates the opposite action of cohesion and repulsion.

Why is alum used in making candles?

Because it gives firmness to the tallow.

Nitre has very recently been applied to the improved preparation of candles, by steeping the cotton wick in lime water, in which is dissolved a considerable quantity of nitre. By this means is obtained a purer flame and a superior light; a more perfect combustion is ensured; snuffing is rendered nearly as superfluous as in wax-lights; and the candles thus made do not run, or waste. The wicks should be thoroughly dry before the tallow is put to them.-Brewster's Journal, 1829.

Why is alum used in salt-drying cod-fish.

Because it prevents the salt from dissolving.

Why is alum used for preparing paper for the preservation of gunpowder?

Because it prevents the bad effects of damp atmospheric air upon the powder, and preserves the paper from readily taking fire.

Why are the crystals collected in camphor bottles in druggists' windows always most copious upon the surface exposed to the light?

Because the presence of light considerably influences the process of crystallization. Again, if we place a solution of nitre in a room which has the light admitted only through a small hole in the window-shutter, crystals will form most abundantly upon the side of the basin exposed to the aperture through which the light enters, and often the whole mass of crystals will turn towards it.-Brande.

Why is there rock salt?

Because it is supposed to have been deposited by the sea, or by salt lakes drying up, which formerly