bottles in our bed-rooms, by which they are broken; the paving-stones in the street are frequently displaced bythe water freezing beneath them; the waterpipes are also sometimes burst during severe frost; in alpine countries trees are often split by the freezing of the sap during the cold of winter. The force with which water expands was also exemplified by the Florentine academicians, who filled with water a small brass globe of sufficient strength to endure a force of 27,270lbs. without bursting; and yet so great is the expansive force of water, that it was burst during the congelation of water in it; the same experiment was made by Major Williams at Quebec in different manner; he filled a bomb-shell with water and placed in the opening of it an iron plug of 3lbs. weight, and on congealing the water, the plug was projected from the shell the distance of 415 feet; which experiments are quite conclusive as to immense force with which water expands during its congelation; we also see a reason why the ground appears soft and spongy after severe frost has been followed by a thaw, because during the frost the particles of water contained in the earth are frozen and expanded with great force, thereby increasing the distance between the particles of clay.

Having given a short view of some of the appearances attendant on the passage of heat through bodies, we shall now proceed to a different part of our subject, and consider the phenomena attendant on the reflection and radiation of heat. There are few of our readers who have not seen the tin reflectors which are in common use in most kitchens, and which serve the double purpose of protecting the heat from the current of cold air which is flowing towards the fire, and of reflecting the heat of the fire, which would be lost by passing into the room, upon the meat. Now we have taken this as the most familiar example that presents itself of the reflection of heat, and to explain it more fully let us suppose that heat consists of a number of small spherical particles emitted from a heated body, each of these issuing with an almost infinite velocity strikes any substance placed before it, into which it either enters and combines, or is thrown off or reflected, exactly in a similar manner to what takes place when

we throw any spherical body, as a marble or a grain of shot against an even surface; if it be thrown obliquely against that surface, it will rebound in an oblique direction, the reverse of that by which it came, if it be thrown perpendicularly, it will rebound to that point from which it has been thrown→→→ Now heat is found to obey all the laws of light, we have observed above that it is reflected from polished surfaces as light is, and it is also found that heat is refracted though in a less degree than light is. Now that heat is reflected from polished surfaces, we can easily prove by placing before a hot fire a polished me. tallic surface, as a silver tea-pot, or a sheet of tin, and a piece of wood; and we shall find, that when the wood has become very hot, the polished metal will have hardly acquired any degree of heat, which arises from the polished metal's having reflected or turned away all the heat which fell upon it, while the piece of wood has absorbed this heat. Now if we place before the fire a number of substances with their surfaces of various colours, or in different states, we shall find that these will absorb a different quantity during the same space of time. That which has its surface polished will absorb little or no heat, a white surface will absorb less than coloured surfaces, and a black will absorb the greatest quantity of heat. This experiment was first made by placing on the surface of snow exposed to the sun pieces of linen cloth of different colours; it was found after the lapse of a short space of time, that the snow had hardly melted under the white piece of cloth, but that under the black it had melted so much that the cloth had sunk several inches in the snow; the same experiment may be more easily tried by taking several metallic plates, and by having one po lished and the others variously coloured and by coating the uncoloured sides with wax, and holding before a fire the coloured sides; it will be found that the wax will melt immediately from the plate with the blackened surface, next from the plate with the reddened surface, and lastly from that with the white surface; from the plate the surface of which is polished, it will not melt for a very considerable time, owing to the perfect reflecting nature of the surface.

When we sit opposite a fire, or hold our hands near a heated body, either

above, below, or at the side of it, we perceive the sensation of heat. It is evident this heat is given off in all directions, and from its being emitted in rays or right lines, it is termed radient heat; and as the phenomena attendant on it are of continual occurrence, and of great importance as the causes of many interesting natural appearances when taken in connection with what we have just stated relative to the reflection and absorption of heat, we shall enter as minutely as our limits permit us into this part of our subject. We have mentioned above, in speaking of the different powers of different surfaces to absorb heat, that polished surfaces possess this power in the least degree, that blackened surfaces are the best absorbers of heat, and that roughened and coloured surfaces posses this quality in intermediate degrees. Now this assertion is equally true with respect to their powers of giving off or radiating heat; polished surfaces are the worst, and blackened surfaces the best radiators of heat. The truth of this assertion was fully proved by the late Sir John Leslie, in his researches relative to the nature of heat, by a series of the most admirably contrived experiments, of which we shall give a short sketch. He provided a number of vessels of the same materials, equal in size, and similar in every respect, but that the surfaces of each were in different states; the surface of one being blackened, of another, polished, of a third roughened, and of a fourth, whitened into each of these was poured an equal quantity of boiling water, and they were all placed in similar situations for some time; and on examination of the water in these vessels it was found that the water in the blackened vessel had cooled much more than the whitened, the whitened had cooled more than the roughened, and the latter more than the polished; which proved that this surface was the worst, and the blackened surface the best radiator of heat. He also contrived to shew the actual heat given off from each surface, by taking a large cubical vessel with four sides of equal size, the surface of each of these being in different states, viz:-polished, roughened, whitened, and blackened; this vessel was placed before a metallic concave mirror, so arranged as to reflect all the heat which fell upon it on a de

cate instrument constructed by him, for shewing small differences of temperature, and which he named a differential thermometer. Now when this vessel was filled with hot water and the polished side was placed opposite the mirror, it was found that the quantity of heat given off was very small, but that when the blackened surface was placed opposite the mirror, there was a considerable encrease of heat in the focus of the mirror; the rough and white surfaces gave off less than the black, and more than the polished surface; these experiments satisfactorily proving that they have different powers of giving off or radi. ating different quantities of heat.

Having given the above short sketch of this branch of our subject, which is necessarily in many respects imperfect, we shall now extract from Dr. Lardner's Treatise, an account of some familiar facts, which are easily explicable by reference to the theories of radiation and absorption.

"Vessels intended to contain a liquid at a higher temperature than the surrounding medium, and to keep that liquid as long as possible at the highest temperature, should be constructed of materials which are the worst radiators of heat. Thus, tea-urns, and tea-pots, are best adapted for their purpose when constructed of polished metal, and worst when constructed of black porcelain. A black porcelain tea-pot is the worst conceivable material for that vessel, for both its material and colour are good radiators of heat, and the liquid contained in it cools with the greatest possible rapidity. On the other hand, a bright metal tea-pot is best adapted for the pupose, because it is the worst radiator of heat, and, therefore, cools as slowly as possible. A polished silver or brass tea-urn is better adapted to retain the heat of the water than one of a dull brown colour, such as is most commonly used.

"A tin kettle retains the heat of water boiled in it more effectually if it be kept clean and polished than if it be allowed to collect the smoke and soot, to which it is exposed from the action of the fire. When coated with this, its surface becomes rough and black, and is a powerful radiator of heat.

"A set of polished fire-irons may remain for a long time in front of a hot

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fire without receiving from it any increase of temperature beyond that of the chamber, because the heat radiated by the fire is all reflected by the polished surface of the irons, and none of it is absorbed; but, if a set of rough, un polished irons were similarly placed they would speedily become hot, so that they could not be used without inconvenience. The polish of fire-irons is, therefore, not merely a matter of ornament, but of use and convenience. The rough, unpolished poker, sometimes used in a kitchen, soon becomes so hot that it cannot be held without pain.

"A close stove, intended to warm an apartment, should not have a polished surface, for in that case it is one of the worst radiators of heat, and nothing could be contrived more unfit for the purpose to which it is applied. On the other hand, a rough, unpolished surface of cast iron is favourable to radiation, and a fire in such a stove will always produce a more powerful effect.

"A metal helmet and cuirass, worn by some of our regiments of cavalry, is a cooler dress than might be at first imagined. The polished metal being nearly a perfect reflector of heat, throws off the rays of the sun, and is incapable being raised to an inconvenient temperature. Its temperature is much less increased by the influence of the sun $than that of common clothing.

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"The polished surfaces of different parts of the steam engine, especially of the cylinder, is not matter of mere ornament, but of essential utility. A rough metal surface would be a much better radiator of heat than the polished surface, and if rust were collected on it, its radiating power would be still further increased, and the steam contained in it would be more exposed to condensation by loss of heat.

"It may be frequently observed, that a deposition of moisture has taken place on the interior surface of the panes of glass of a chamber window on a morning which succeeds a cold night. The temperature of the external air during the night being colder than the atmosphere of the chamber, it communicates its temperature to the external surface of the glass, and this is transmitted to the interior surface, which is exposed to the atmosphere of the room. This atmosphere is always more or less charged with vapour, and the cold of the internal surface of the glass, VOL. I.

acting on the air in contact with it, reduces its temperature below the point of saturation, and a condensation of vapour takes place on the surface of the panes, which is observed by a copious deposition of moisture in the morning. If the temperature of the external air be at or below the freezing point, this deposition will form a rough coating of ice on the pane. Let a small piece of tin foil be fixed on a part of the exte rior surface of one pane of the window in the evening, and let another piece of tin foil be fixed on a part of the interior surface of another pane. In the morning it will be found that that part of the interior surface which is opposite to the external foil will be nearly free from ice, while every other part of the same pane will be thickly covered with it. On the contrary, it will be found that the surface of the internal tin foil will be more thickly covered with ice than any other part of the glass. These effects are easily explained by the principle of radiation. When the tin foil is placed on the exterior surface it reflects the heat which strikes on the exterior surface, and protects that part of the glass which is covered from its action. The heat radiated from the objects in the room striking on the surface of the glass, penetrates it, and encountering the tin foil attached to the exterior surface, is reflected by it through the dimensions of the glass, and its escape into the external atmosphere is intercepted; the portion of the glass, therefore, covered by the tin foil, is, in this case, subject to the action of the heat radiated from the chamber, but protected from the action of the external heat. The temperature of that part of the glass is therefore less depressed by the effects of the external atmosphere than the temperature of those parts which are not covered by the tin foil. Now, glass being, as will appear hereafter, a bad conductor of heat, the temperature of that part opposite to the tin foil does not immediately affect the remainder of the pane, and, consequently, we find that while the remainder of the interior surface of pane is thickly covered with ice, the portion opposite the tin foil is comparatively free from it. On the contrary, when the tin foil is placed on the internal surface, it reflects powerfully the heat radiated from the objects in the room, while it admits through the di

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