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as lampblack. It applies to powerful absorbents like cloth, cotton, wool, and other organic substances when exposed to luminous heat. Accordingly, the most suitable coloured clothing for summer is just that which experience has taught us to use, namely, white, for it absorbs less of the sun's rays than black clothing, and hence feels cooler.

The polished fire-irons before a fire are cold, whilst the black. fender is often unbearably hot. If, on the contrary, a liquid is to be kept hot as long as possible, it must be placed in a brightly polished metallic vessel, for then, the emissive power being less, the cooling is slower. It is for this reason advantageous that the steam pipes, etc., of locomotives should be kept bright.

Snow is a powerful reflector, and, therefore, neither absorbs nor emits much heat. It is owing to its small emissive power that it protects from cold the ground and the plants which it covers; and owing to its small absorbing power it melts but slowly during a thaw. A branch of a tree, a bar of metal, a stone in the midst of a mass of snow, accelerate the fusion by the heat they absorb, and which they radiate about them.

In the Alps, the mountaineers accelerate the fusion of the snow by covering it with earth, which increases the absorbing power.

Metallic cooking vessels should be black and rough on the outside, for then their absorbing power is greater and they become heated more rapidly. Their bright and polished surface is purchased at the expense of combustible. This is what is seen in vessels of silver and of white porcelain. In common unglazed earthenware liquids are more rapidly heated, but also more rapidly cooled. It is observed that the ripening of grape and other fruits is accelerated when they are placed in contact with a black wall (mortar mixed with lampblack). This arises from the fact, that from the great emissive power of the wall, as well as from its great absorbing power, it becomes more highly heated under the influence of the sun, and gives up more to the fruit.

CHAPTER IV.

CONDUCTING POWER OF BODIES.

207. Conductivity of solids.—In the phenomena of radiation which have been considered, heat is transmitted from one body to another through space, without raising the temperature of the spaces

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Conducting Power.

203

through which it passes. It may also be propagated through the mass of a body by an internal radiation from molecule to molecule. This internal propagation in the mass of a body is called conductivity; and good conductors are those bodies which readily transmit heat in their mass, while those through which it passes with difficulty are called bad conductors.

Organic substances conduct heat badly. De la Rive and De Candolle have shown that woods conduct better in the direction of their fibres than in a transverse direction; and have remarked upon the influence which this feeble conducting power, in a transverse direction, exerts in preserving a tree from sudden changes of temperature, enabling it to resist alike a sudden abstraction of heat from within, and the sudden accession of heat from without. Tyndall

[graphic][merged small]

has also shown that this tendency is aided by the low conducting power of the bark, which is in all cases less than that of the wood. Cotton, wool, straw, bran, etc., are all bad conductors.

In order to compare the conducting power or conductivity of different solids, Ingenhousz constructed the apparatus which bears his name, and which is represented in fig. 175. It is a metallic trough, in which, by means of tubulures and corks, are fixed rods of the same dimensions, but of different materials; for instance, iron, copper, wood, glass. These rods extend to a slight distance in the trough, and the parts outside are coated with wax, which melts at 61°. The box being filled with boiling water, it is observed that the wax melts to a certain distance on the metallic rods, while on the others there is no trace of fusion. The conducting power is

evidently greater in proportion as the wax has fused to a greater distance. The experiment is sometimes modified by attaching glass balls or marbles to the ends of the rods by means of wax. As the wax melts, the balls drop off, and this in the order of their respective conductivities. By these and other experiments it has been ascertained that metals are the best conductors, then marble, porcelain, brick, wood, glass.

208. Conducting power of liquids. Manner in which they are heated.—Liquids, with the exception of mercury, which is a metal, are all bad conductors of heat. They conduct so imperfectly that Rumford assumed water to be entirely destitute of conducting power. But its conductivity, though small, has been established beyond doubt, as well as that of other liquids, by the most accurate experiments.

From their small conducting power, liquids are not heated in the same manner as solids. If heat be applied to a solid, whether on the top, the bottom, or the sides, it is transmitted from layer to layer, and the whole mass becomes heated. This is not the case

Fig. 176.

with a liquid; if it is heated at the top, the heat is only propagated with extreme slowness, and it cannot be completely heated. But if it be heated at the bottom, the temperature of the liquid rapidly rises. This, however, is not in virtue of conductivity, but by ascending and descending currents, which, in virtue of the mobility of the molecules, are produced throughout the whole mass of liquid.

The existence of these currents may be demonstrated by placing in the water a powder of approximately the same density, for instance, oak sawdust, and then gently heating this at the bottom. As the lower layers become heated they expand, while the upper layers, which are colder and therefore denser, sink and take the place of the first; these in their turn become heated, rise, and so on, until the entire mass is heated. These

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Conductivity of Gases.

currents are evident from the shavings which are seen slowly in the centre, and to redescend near the edges.

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to rise

209. Conductivity of gases. Gases are extremely bad conductors of heat; but this cannot be easily demonstrated by experiment, owing to the extreme mobility of their particles. For so soon as they are heated in any part of their mass, expansions and currents are produced, in virtue of which the heated parts mingle with the cold ones; hence a general elevation of temperature, which we might be tempted to consider as due to conductivity. When, however, gases are hindered in their motion, their conductivity seems extremely small, as the following examples show. 210. Applications.-The greater or less conductivity of bodies meets with numerous applications. If a liquid is to be kept warm for a long time, it is placed in a vessel and packed round with nonconducting substances, such as shavings, straw, bruised charcoal. For this purpose water pipes and pumps are wrapped in straw at the approach of frost. The same means are used to hinder a body from becoming heated. Ice is transported in summer by packing it in bran, or folding it in flannel.

Double walls constructed of thick planks having between them any finely divided materials such as shavings, sawdust, dry leaves, etc., retain heat extremely well; and are likewise advantageous in hot countries, for they prevent its access. During the night the windows are opened, while during the day they are kept close. If a layer of asbestos, a very fibrous substance, is placed on the hand, a red-hot iron ball can be held without inconvenience. Red-hot cannon balls can be wheeled to the gun's mouth in wooden barrows partially filled with sand. Lava has been known to flow over a layer of ashes underneath which was a bed of ice, and the non-conducting power of the ashes has prevented the ice from fusion.

The clothes which we wear are not warm in themselves; they only hinder the body from losing heat, in consequence of their spongy texture and the air they enclose. The warmth of bed-covers and of counterpanes is explained in a similar manner. Double windows are frequently used in cold climates to keep a room warm --they do this by the non-conducting layer of air interposed between them. It is for the same reason that two shirts are warmer than one of the same material, but of double the thickness. Hence too the warmth of furs, eider down, etc.

That water boils more rapidly in a metallic vessel than in one of porcelain of the same thickness; that a burning piece of wood can

be held close to the burning part with the naked hand, while a piece of iron heated at one end can only be held at a great distance, are easily explained by reference to their various conductivities.

The sensation of heat or cold which we feel when in contact with certain bodies is materially influenced by their conductivity. If their temperature is lower than ours, they appear colder than they really are, because from their conductivity heat passes away from us. If, on the contrary, their temperature is higher than that of our body, they appear warmer from the heat which they give up at different parts of their mass. Hence it is clear why carpets, for example, are warmer than wooden floors, and why the latter are warmer than stone floors.

CHAPTER V.

MEASUREMENT OF THE EXPANSION OF SOLIDS, LIQUIDS, AND GASES.

211. Expansion of solids.-The expansion of bodies by heat being a general effect which exerts its influence on all bodies, and is continually changing their volume, it will be readily understood that the determination of the amount of this expansion is a problem of great importance, both in its purely scientific, as well as in its practical, aspects. We shall first describe the method of determining the expansion of solids. We have already seen that the expansion of solids may be either as regards the length or the volume. Hence the investigation of the expansion of solids may be divided into two parts, the first relating to linear, and the second to cubical expansion.

Linear expansion. In order to compare with each other the expansion of bodies, the elongation is taken which the unit of length undergoes when it is heated from zero to 1 degree, and this elongation is called the coefficient of linear expansions. The coefficients of a great number of substances were accurately determined towards the end of the last century by Lavoisier and Laplace. They took a bar of the substance to be determined, placed it in melting ice, and then accurately determined its length. Having placed it then in a bath of boiling water, they again measured its length. They then observed an elongation, which represented the total expansion for an increase of temperature of 100 degrees. This, divided by

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