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 2120

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, vituly, 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,

which jars its substance, the cohesion is destroyed, and the whole crumbles to dust with a kind of explosion.-Arnott.

Why does a loose bladder, tied at the mouth, and held before a fire, gradually swell and appear fully inflated? Because the small quantity of air contained in the bladder is then so much dilated by the heat, that it occupies a considerably increased space, and fills the bladder, of which it before occupied only a small part.

Why does change of temperature retard or hasten the decomposition of dead vegetable and animal substances?

Because the functions of life bring into combination, to form the curious textures of organic or living bodies, chiefly four substances, viz. carbon, or coal; the ingredients of water, or oxygen and hydrogen; and lastly, nitrogen;—which substances, when in the proportions found in such bodies, have but slight attraction for each other, and all of which, except the carbon, usually exist as airs. Their connection, therefore, is easily subverted, and particularly by a slight change of temperature, which either so weakens their mutual hold as to allow new arrangements to be formed, or altogether disengages the more volatile of them. -Arnott.

Why is iron preserved from rust by dipping it when at a dull red heat, into water, and then into linseed oil?

Because the first part of the process frees it from the scales and extraneous matter, and the remaining heat disposes it to receive the oil, which forms a varnish, and filling up all the minute interstices of the surface, prevents any rust.

Why do clocks and watches vary in their rate of going? Because of the expansion and contraction of the metals of which they are constructed. Thus, in regulating the length of the second's pendulum, an exact acquaintance with the dilatation of metals, is essen

tial; for when the bob is let down a hundredth part of an inch, the clock loses ten seconds in twenty-four hours; hence, a thousandth part of an inch will cause it to lose one second per day, and a change of temperature equal to 30° of Fahrenheit will alter its length about one five thousandth part, and occasion an error in the rate of going, of eight seconds per day. Variations of temperature also occasion variations in the balance-wheels of watches, which are obviated by various compensating apparatus.

Why does a compensation balance prevent the effects of expansion or contraction?

Because it consists of interrupted concentric rings of different metals, joined together, so that the expansion of one counteracts the expansion of the other.

Why do the iron hoops of brewers' casks bind with such great force?

Because those which are at first made too small to fit, are heated until they are sufficiently enlarged; they are then driven on, and suddenly cooled, by throwing water upon them; the contraction of the iron which ensues on cooling, bringing the parts of the vessels in closer contact than they could easily be brought by other means, and fixes the hoops firmly round them.

Why does hay, if stacked when damp, take fire?

Because the moisture elevates the temperature sufficiently to produce putrefaction, and the ensuing chemical action causes sufficient heat to continue the process; the quantity of matter being also great, the heat is proportional.

Why are concave mirrors employed as burningglasses?

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Because they collect the heat of the sun's rays the whole of its surface to a single point, thus accumulating a very great degree of heat, for the combustion and fusion of various natural substances, that