several days and nights undisturbed, in a very high temperature; for, if perfect rest and a temperature of from 120° to 190° be not afforded, regular crystals of candy will not be obtained.

The manufacture of barley-sugar is a familiar example of crystallization. The syrup is evaporated over a slow heat, till it has acquired the proper consistence, when it is poured on metal to cool, and when nearly so, cut into lengths with shears, then twisted, and again left to harden.

Why are small, radiant, and tree-like crystallizations seen on dirty windows in London ?

Because of the great number of coal fires in the metropolis: these crystals (of sulphate of ammonia, or at least, sulphite of ammonia, which becomes sulphate by exposure to the air) being an abundant product of the combustion of coal.-Brande.

Why do some springs petrify objects by their spray? Because their water is impregnated by means of its carbonic acid, with a large portion of carbonate of lime, which it deposits on issuing into the air. At Clermont, in France, there is such a spring, where Mr. Scrope saw the stuffed skins of a horse and a cow, birds, fruit, flowers, &c. undergoing this petrifying process. Its incrustations have also formed an elevated natural aqueduct, 240 feet in length, and terminating in an arch thrown across the stream it originally flowed into, 16 feet high and 12 wide.—Scrope's Memoir on the Geology of Central France.

Why is fluor spar so called?

Because it has the property of increasing the fusibility of other mineral substances. It has also been called vitreous spar, because when fused it has the appearance of glass.

HEAT.

Why is heat considered one of the chief agents in chemistry?

Because its most obvious sources are chiefly referred to the general head of chemical combination. Thus, fire, or the combustion of inflammable bodies, is nothing more than a violent chemical action attending the combination of their ingredients with the oxygen of the air. Animal heat, is, in like manner, referable to a process bearing no remote analogy to slow combustion, by which a portion of carbon, an inflammable principle existing in the blood, is united with the oxygen of the air in respiration, and thus carried off from the system: fermentation is nothing more than a decomposition of chemical elements loosely united, and their reunion in a more perfect state of combination. Friction, as a source of heat, is well known: we rub our hands to warm them, and we grease the axles of carriage-wheels to prevent their setting fire to the wood. Again, Count Rumford has established the extraordinary fact, than an unlimited supply of heat may be derived by friction from the same materials.-J. F. W. Herschel.

Savages light their fires by rubbing two pieces of wood: Count Rumford made great quantities of water boil, by causing a blunt borer to rub against a mass of metal immersed in the water; and Sir Humphry Davy quickly melted pieces of ice by rubbing them against each other, in a room cooled below the freezing point. Instances have occurred, where whole forests have been burned down by fires kindled from the violent friction of the branches against each other by the wind.

Why is it conjectured that there is a difference between solar and terrestrial heat?

Because the rays of the first pass through glass without heating it, while the rays of the latter are stopped by the glass, which becomes hot when opposed to them.

Why may heat be considered as a power opposed to attraction?

Because it tends to separate the particles of bodies; and whenever a body is heated, it is also expanded.— Brande.

Why is the term caloric used?

Because it may distinguish the cause of heat from the sensation which we call by the same name; but the terms caloric and calorific fluid seem to imply the material nature of heat, which has not yet been proved.

Why is caloric considered a subtle material fluid, the particles of which mutually repel each other?

Because this supposition appears to give a plausible explanation of most of the phenomena dependent upon heat, as the expansion, fusion, and vapourization of bodies, on the idea that the particles of caloric when interposed between the particles of bodies, in sufficient quantity, produce these effects. It is natural to suppose when a body is enlarged in bulk, that the enlargement is occasioned by the introduction of the particles of other matter, by which the particles of the expanded body are repelled to a greater distance from one another; and this repulsion becomes so great, in consequence of the introduction of a large quantity of heat, as to enable the particles of solid bodies to assume the fluid, or aëriform states.

There are, however, other theories of the nature of heat; but the question remains undetermined; and it is fortunate that most of the phenomena connected with the operation of heat, may be explained equally well upon either theory.

Why is heat called latent?

Because, when heat liquefies a solid, or converts a liquid into vapour, the liquid or the vapour is no hotter than the solid or liquid from which it was produced, though a great deal of heat has been expended in producing this effect, and has actually entered into the substance. Hence it continues to exist in the

product, maintaining it in its new state without increasing its temperature, and is thus latent or hidden. This great discovery was made by Dr. Black, who further proved, that when the vapour condenses, or the liquid freezes, this latent heat is again given out from it.

Why does water thrown upon a fire so powerfully repress it?

Because of the great quantity of heat latent in steam;—hence, again, why fire and water are so often adduced proverbially as furnishing a striking contrast. -Arnott.

Why does iron become red-hot by hammering?

Because of the condensation of the metal by the force of the blow. Air may also be condensed by pressure, so as to set tinder on fire.

Why are inorganic bodies, such as gold, iron, &c. found in all parts of the globe?

Because they have no direct dependence, in the present state of the earth, at least, on the solar heat, or its consequences; whereas organic, or living matter, has.

Why does ice, when heated, become water; and the water, when heated further, become steam?

Because the continued addition of heat gradually increases the mutual distance of the constituent atoms of the ice, and their cohesive attraction is overcome; till, at length, the atoms are repelled to still greater distances, and the substance is converted into steam! Abstraction of heat causes return of states in the reverse order; the steam when cooled again, becomes water as before, and the water, when cooled, becomes ice.

Why does a pint of water, when converted into steam, occupy nearly 2000 times the space of the water?

Because the heat merely produces a repulsion

among the particles, and by no means fills up the interstices.-Arnott.

Why are air-expansion engines so much more powerful than steam-engines?

Because heat, when used to dilate air, produces about four times the quantity of expansive power that it does when used to form steam.

Why is not high-pressure steam issuing from a boiler heated perhaps to 300° not hotter than low-pressure steam from a boiler at 212°?

Because, in the instant when the high-pressure or condensed steam escapes into the air, it expands until balanced by the pressure of the atmosphere; that is, until it become low-pressure steam, and it is cooled by the expansion, as air is cooled on escaping from any condensation.

Why does not a generator, if cracked when very hot, immediately let out the vapour or water?

Because the heat repels the water and vapour to a certain distance from the metal, and, virtually, stops the crack, until the temperature is allowed to fall, when the rush of steam by the crack is tremendous. Mr. Perkins, in reasoning upon this effect, says: "The repulsive power of the heated metal is sufficient to retain the vapour and the water equally distant; for, what else is vapour than water in a state of expansion ?"

Why does a Prince Rupert's Drop fly to powder on being simply broken?

Because it is a lump of glass let fall, while fused into water, and thereby suddenly cooled and solidified on the outside, before the internal part is changed; then as this at last hardens and would contract, it is kept extended by the arch of external crust to which it coheres. Now, if a portion of the neck of the lump be broken off, or if other violence be done,