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heat. Sensible heat is that which is appreciable to the touch and the thermometer. Latent heat does not affect either of these, but exists combined with bodies, keeping them in their normal or natural state of existence. This will be best illustrated by an example or two. If you mix one pound of ice, the temperature of which is 32° of the thermometer, and one pound of water at 172°, every particle of ice will be dissolved, and you will have two pounds of water, the temperature of which, instead of being a mean between 32° and 172°, will only be 32°. What, then, has become of the 140° of sensible heat existing in the one pound of water? It has all become latent in the one pound of ice, now converted into one pound of water, and may be derived from it, by causing it to become solid again: 140° is, therefore, the latent heat of water, or its heat of liquefaction; that is, water must have this combined with it, to preserve it in a liquid form; and the whole of this heat disappears in melting ice, before its sensible temperature is raised a single degree. In the same way, though steam and boiling water both indicate the same temperature, namely, 212°, yet it can be proved that the steam contains 980° more latent heat than water: in other words, this quantity of sensible heat disappears, or becomes latent, before water is converted into steam; and all this may be derived from it, by making it pass from the form of vapour into that of a liquid. For if we convert one pound of water into steam, and then re-condense it, by passing it through ice, we shall find seven pounds of ice dissolved. As before stated, 140° is the heat of liquefaction of water, or, in other words, its latent heat; and 7 times 140° gives us 980°, which is the latent heat of steam. It is the latent heat which enters into the composition of bodies that causes the variations in their forms or modes of existence, of which we have already spoken; that is, it is the quantity of heat which exists in combination with a body, that determines whether it is a solid, liquid, or gas. In this way heat is the cause of fluidity, both with reference to liquids and aeriform fluids or gases.

The term specific heat refers to the capacities of bodies for

heat; in other words, to the quantity of heat which bodies require to raise their sensible temperatures. This quantity varies considerably in different bodies. Thus, if you take one pound of quicksilver and one pound of water, each at 40°, and introduce the same quantity of sensible heat into them, for every degree that the temperature of the water is raised, that of the quicksilver will be raised 23°; and in cooling down the same bodies, a given quantity of water will melt twenty-three times as much ice as the same quantity of quicksilver, or give out twenty-three times as much heat. The specific heat, therefore of quicksilver, or its capacity for heat, as compared with that of water, is as 23 to 1. That of other bodies may be ascertained in the same way, and be expressed by the same standard.

The principal SOURCES OF HEAT are the sun, and mechanical and chemical condensation. Several others might be named; but these are the most important. I need not say anything about the sun as a source of heat; but there are several interesting points connected with the other sources which I have named. It is a law in physics, that as the density of a body becomes increased, that is, as its bulk diminishes, its capacity for caloric diminishes also; and thus heat previously latent becomes sensible. Thus there is condensation in the experiment already mentioned of liquefying ice, by passing steam through it: the molecules, which were widely separated in the steam, owing to there being a large quantity of caloric combined with them, become pressed into a more compact form when the steam is condensed into a liquid; a large portion of the heat which exists as an atmosphere or halo around the particles being in fact pressed out, as the atoms are brought into closer contact, and this serves to liquefy the ice. The converse of this is also true as a body changes its state for one of less density; that is, as its bulk increases, its capacity for caloric also increases; heat previously sensible becomes latent in it; and when this heat is abstracted from surrounding bodies, cold is produced. In this way we account for the heat evolved when the steam is condensed into

water. In the experiment just referred to, it enters into the ice, and increases its bulk or density by liquefying it. The perspiration is an admirable provision for cooling the body in hot weather, and it produces its effects on the same principle. Ordinarily it escapes as an insensible vapour; but it reaches the skin in the form of a liquid, and, in assuming an aeriform condition, it abstracts heat from the body, producing a grateful sense of coolness, and thus counteracting the effects of external heat.

Several familiar examples may be named of heat being evolved as the result of mechanical condensation. The blacksmith lights his fire, or at any rate used to do, by hammering a piece of soft iron, until it becomes nearly red hot: he puts this amongst his coals, and ignites these with a few blasts from his bellows: the particles of iron are mechanically condensed by the hammering, and a portion of heat is thus squeezed out from between the particles. In the old flint and steel, heat was in the same way evolved from the metal by percussion. The Indian lights his fire by rubbing a hard and a soft piece of wood together; and forests have been set on fire by the friction of dry branches on each other, produced by the wind: here also the friction produces condensation, the particles are heaped closer together, and heat existing in the interstices between them is squeezed out in sufficient quantity to ignite the whole.

I may mention also an example of the evolution of heat through chemical action. If a few drops of cold water be poured upon a piece of newly-burnt lime, a portion of the water enters into chemical combination with the lime, forming what is called a hydrate. In doing so, it changes its state suddenly from a liquid to a solid; and thus, on the principle already explained, its capacity for caloric is diminished: it parts, therefore, with its latent heat; that is, its heat of liquefaction; and this combines with another portion of the water, and becomes latent in it, as it converts it into steam. Heat is also evolved when oil of vitriol and cold water are mixed together; but it is a disputed point whether this is the result of mechanical or of chemical condensation.

Combustion is the evolution of heat, through the rapid chemical union of a supporter of combustion with a combustible body. There are five supporters of combustion; namely, oxygen, chlorine, and the vapours of iodine, bromine, and fluorine. Of these, the first is by far the most important, owing to its constant presence and interference, as a constituent of the atmosphere. When oxygen combines slowly with any other body, as in the rusting of iron, and tarnishing of lead exposed to the atmosphere, there is heat evolved in a very gradual manner; but it is dissipated, and never accumulates. Here we say that oxidation takes place. But when this combination takes place rapidly, as when iron is exposed to oxygen in a state of ignition, that is, red hot, it is said to be burned, and to suffer combustion; the product, namely, oxide of iron, being the same in both cases,-only in the latter its formation is attended with a sensible evolution of heat. The affinity of oxygen for combustible bodies is greatly promoted by heating the latter; but when the combustible body is once inflamed, it maintains itself sufficiently hot to continue burning till it is entirely consumed. If the heat evolved be in sufficient quantity, it renders the body luminous or incandescent; that is, it emits light, as well as heat. The combustible constituents of wood, coal, oils, tallow, &c., which enter into combination with oxygen, are the same, namely, carbon and hydrogen, which in combining with oxygen, at a high temperature, produce carbonic acid and water; and these, being volatile, disappear, forming part of the heated aerial column that rises from the burning body. No loss whatever of ponderable matter occurs: nothing is annihilated. If the gaseous products given off, and the ashes remaining, be collected, they will be found to have exactly the same weight as the oxygen and the combustible body which have disappeared. We may see the effects of heat in disposing oxygen to unite with a combustible body, when a supply of coals is laid on a fire: until they become sufficiently ignited, combustion only goes on to an imperfect degree; the whole of the combustible ingredient of the coal is not consumed, part passing off as smoke. If you promote

a more liberal supply of oxygen, by stirring the fire, or, better still, by using the bellows, the whole is soon ignited, and the smoke as soon diminishes. When this takes place, the whole is rendered luminous; that is, light is emitted as well as heat. If I blow out the candle, combustion goes on for a few minutes, to a slight degree, and the whole of the carbon from the tallow is not consumed, but passes off as smoke: but on applying a light, the chemical union of the oxygen and carbon is more rapid; the whole of the latter is consumed, and the combustible body becomes luminous. You may see that the smoke is unconsumed carbon, by applying the light an inch or two above the wick, when the flame will descend to the wick. The ascent of the melted tallow along the wick affords, by the way, a good example of what I before spoke of as capillary attraction.

(To be continued.)

SAXON HUSBANDS AND BRITISH WIVES.

THE Saxons had never been refined by peaceful approximation to the Roman frontier. No Missionary had set his foot among their forests or on their coasts. They were Pagan pirates. They invaded Britain by detachments, and under different independent Chiefs. They never landed in such imposing force as to awe the invaded into bloodless submission, but merely in sufficient numbers to fight their way; to conquer indeed, but only to conquer inch by inch. Their savage Paganism inflamed them with peculiar frenzy against all that the Christianised Britons held most sacred; each side upbraided the other with perfidy and fraud; no possible bond of fair union existed between them; and, probably, in no conquest were the victors more ruthless to the vanquished than in the desperate and chequered struggle by which the Saxons won their slow way over this island.

Led by this historical circumstantial evidence, and by the great fact of our language being essentially Ger

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