not an instant of time that heat is not performing some important duty in fulfilment of the Divine purposes. Among all the works of God we know of none on which the evidences of design are more conspicuously inscribed. Whatever may be the nature of heat, be it a peculiar substance or a peculiar property, we know that it exists. To its influence we are indebted for the due performance of all the functions of life-for all that cheers the eye, delights the ear, and gratifies the taste. Nor is it to heat only, but to its being supplied to us in due proportions, that we owe so much. Its excess or its deficiency would be equally fatal to vegetable and animal existence. In one case the earth would become a parched desert, in the other an ice bound plain.

It is important that we should distinguish between heat itself and the sensation of heat. The first is a cause, the second its effect. With a view to prevent mistakes by the frequent interchange of terms meaning sometimes one thing, and at other times another, the term caloric is now extensively employed by scientific writers to denote that condition of bodies by which the sensation of heat is produced, or, in other words, to define the cause of heat as distinct from its effects. Wishing to refrain as much as possible from scientific phraseology, we shall restrict ourselves to the ordinary term (heat), requesting our readers to remember that, unless the contrary is distinctly stated, it always means heat as an element, residing in, or operating upon matter, without any regard to our feelings.

By the continual use of the terms heat and cold, in the affairs of common life, we sometimes employ the latter term, as if it were descriptive of an element or agent, equally energetic in its effects as any other with which we are acquainted, but whose properties are directly the opposite of those possessed by heat. Cold is only the absence of heat. It is easier, (and, because we are accustomed to it,) more natural, to say, "It is cold," than it is to describe that condition by saying, "There is a deficiency of heat." The latter, however, is a correct definition. We know by experi

ence that the gradual abstraction of heat from a body, which at first may possess so much of it as to be unapproachable, induces the sensation we denominate cold. But cold is only a relative term. We know nothing of matter where heat is not present. There is less heat in one substance than in another; but of absolute cold we have no conception.

Temperature is a term that will very often occur whilst treating of the properties of heat. We think it right at once to explain its signification. The temperature of a body means its sensible heat; that is, the heat of which some estimate may be formed by a thermometer, a useful instrument that we have already described.* In comparing two different substances, or two distinct parts of the same substance, if we find the first communicates to the thermometer more than the second, we say the temperature of the former is higher than that of the latter, or that the temperature of the latter is lower than that of the former. Higher and lower, as applied to temperature, are terms that evidently owe their origin to the operation of the thermometer; since the smaller the quantity of sensible heat present in any substance, with which the bulb of a thermometer is placed in contact, the lower will the column of mercury, or other fluid within the tube, descend; the greater the quantity of sensible heat, the higher will it rise. The sensible, or, as it is commonly termed, free heat, thus discoverable in any particular substance by the aid of a thermometer, must be viewed as entirely independent of the heat which permanently resides in that substance, or which may be temporarily combined with it in a latent, that is, a concealed state. We may satisfy ourselves that a vast quantity of heat has entered into some particular substance, but we can neither detect the presence nor estimate the quantity of that which is latent by our ordinary perceptions, nor through the agency of a thermometer.

Heat is communicable from one substance to another by radiation and by conduction. Radiation takes place * See page 144.

between two bodies whose temperatures are unequal at sensible distances. Contact is a condition essential to conduction. If a piece of heated metal be fixed in the centre of a room, midway between the ceiling and the floor, heat will be disengaged from it equally in all directions, upwards, downwards, horizontally, and obliquely, which may be proved by the melting of a small quantity of tallow placed at certain distances around the metal. This is an instance of radiation. When the bowl of a metal spoon is left for a few minutes in a cup of hot tea, the handle of the spoon acquires the same temperature as that of the tea. Here we have an instance of conduction. In one case the heat separated from the metal will affect the tallow at some distance, passing readily through or among the particles of the intervening air; in the other case, the heat, first communicating with that part of the spoon in contact with the tea, is, if we may employ the expression, pushed forward from particle to particle of the metal, until it reaches its extremity.

As radiation and conduction commonly operate together, they may be considered as different parts, or rather, different forms of the same process, both equally dependent on that property, peculiar to heat, by which it tends to diffuse itself in every direction, and among the particles of every species of matter, whatever may be its form, size, colour, or quality.

Thus, if any number of vessels (some constructed of metal, others of wood, others of stone, and others of glass), each vessel containing a liquid of a different kind and at a different temperature, be placed in the same room, the liquids and the vessels containing them will, in a few hours, all arrive at the same temperature, which will be that of the air in the room. The same would, of course, be the result with solid or aëriform bodies as with liquids.

Radiation and conduction may be further explained by considering the former as operating at the surfaces of bodies, whilst the latter goes on throughout their interior parts. The rate, at which heat is radiated and

conducted by any substance, depends very much on the nature of the materials of which that substance is composed. Radiation is also influenced in a remarkable degree by the colours and other conditions of the surfaces of bodies.

Those bodies into which heat enters with facility, and among whose particles it is transmitted rapidly, are called good conductors. Those, on the contrary, which offer considerable resistance to the progress of heat among their particles are termed bad conductors. The latter are frequently denominated non-conductors, a description not philosophically correct, since every substance with which we are acquainted will conduct heat, although in some its transmission is exceedingly slow.

Among good conductors the metals are the best; of these, gold, platinum, silver, and copper are nearly equal. The next in order are iron and zinc, then tin, and the slowest conductor of them all is lead. Wood, stone, and bricks are among the bad conductors of this class; the most perfect are wool, hair, cotton, the fur of animals, the feathers of birds, and especially the down of the swan. Liquids and aëriform bodies, when there is no motion among their particles, are bad conductors of heat. If freedom of motion be established, they become good conductors.

It is attention to this power of conduction which is our guide in the selection of clothing and building materials to suit different climates; bad conductors being selected for cold, and good conductors for warm countries.

MATTER.

By matter we mean the various things on this earth that we see and handle, and which we use for our own comfort. So the food we eat, the liquid we drink, the clothes we wear, and the things from which they are made,

the ground we tread upon, the wood, iron, stone, brick, from which we build ourselves houses, make tools and machinery, and the coal we burn in our fire-places-all these are matter; or, as we call them, when we turn them into use, materials. Thus wood, stone, brick, &c., are building materials; wood, coal, coke, are materials for fuel; gold and silver are materials for exchange or ornament; and cotton, wool, silk, and linen are materials for clothing.

We cannot make matter; it is created for us, either being found in the ground naturally, produced by the ground naturally, or grown upon the bodies of animals, as wool, hair, &c.

And as we cannot make matter, so we cannot destroy it. I daresay you think when the coal is burnt in the grate, or a piece of paper, or an old shirt in the fire, or a candle on the table, it is destroyed. You never see it again, and cannot find it; but it is not destroyed, it is only changed into other matter. You will find some ashes left on the hearth after the burning, and a good deal of soot in the chimney, and some of the heat in the room and sucked into your own body, or into the water in the kettle, or the joint roasted in the oven, while some smoke has gone into the air outside. So the coal has only changed into ashes, soot, heat, and smoke. If I were to burn a candle in a globe containing a little lime water, this lime water at first will be quite clear, but as the candle burn: it would turn muddy or milky, which shews that the candle has put something into the air in the globe that was not there before. It has really put some carbonic gas into the air, and it is this that has made the lime-water look like milk. This carbon is part of the matter of which the candle is composed. If we were to weigh a candle before lighting it, and then weigh all the parts left after burning, viz., the carbon, ash, &c., we should find the weight after the burning greater than it was before. Suppose I weighed the globe and lime-water with the candle in it first, and then lighted the candle, and then weighed it again, when the burning was over, we should find the globe had