150.]

APPLICATIONS OF CONDUCTIVITY.

319

The rapid change of particles of air which are in contact with the body, by the action of a wind, renders the human frame much less able to bear cold in a windy than in a still atmosphere. Voyagers in the Arctic regions found that, if properly clad, they could endure in a still air a temperature of 48° C. while at -18° C. with a brisk wind, it was impossible to face the breeze with safety. A parallel "case occurs in liquids: the hand may with impunity be kept stationary in water of a temperature so high, that if the hand were in motion severe pain would be occasioned.

Many familiar contrivances for preventing the escape of heat, and for facilitating the employment of hot bodies, depend upon the use of inferior conductors of heat for instance, a layer of charcoal is generally interposed between the case of the furnace and its fire-clay lining, in order to confine the heat. The kettleholder is for this reason used to protect the hand from the heat of the metal; whilst the handles of tea-pots are insulated from the hot metal by non-conducting pieces of ivory. Wicker-work or matting is placed under hot dishes to separate them from the dinner-table by badly conducting substances.

FIG. 125.

Much of the economy of fuel depends upon a judicious application of these principles. An instructive illustration of their importance is exhibited in the manner in which heat may be economized by an appropriate construction of the boiler of a steam-engine. The form which answers this purpose most perfectly is that which is known as the Cornish boiler. Fig. 125 shows a transverse section of this boiler: it consists of two cylinders placed one within the other; between the two is the space for the water; the interior cylinder contains the fire-grate, ash-pit, and the first portion of the flue: the heat, which would otherwise be conducted away by the firebars, and by the masonry of the ash-pit, is thus economized, and the heated products of combustion pass through the boiler for its whole length, which is sometimes as much as 40 or even 60 feet, or from 12 to 18 metres; the hot air then returns along the outside of the

boiler towards the fireplace, and once more passes underneath the boiler before it finally reaches the chimney, c. Loss of heat from the outer surface of the boiler is prevented by covering it with a layer of badly conducting material. In the boiler of the locomotive, where a stronger draught is necessary, the fireplace is surrounded at top and on its two sides by a double casing containing water, and the hot air from the furnace passes through the length of the boiler by a number of small tubes, which open at one end into the fireplace, at the other into the chimney. Loss of heat from the external surface is here also prevented by casing the boiler in some non-conducting material, such as felt, which is usually covered with wood.

(150) Inequality in the Conductivity in different Directions.The researches of De Senarmont (Ann. Chim. Phys. 1847 [3], xxi.

320

VARIATION OF CONDUCTIVITY WITH DIRECTION.

[150.

457, and 1848, xxii. 179) have shown that although the conductivity of solids which are homogeneous throughout, and of crystals which belong to the regular system, is uniform in every direction, yet that in all crystals which do not belong to the regular system the conductivity varies in different directions, according to the relation of the direction to that of the optic axis of the crystal.

FIG. 126.

The fundamental fact is easily demonstrated by taking two 1 slices of quartz, one cut parallel to the axis of the prism, the other cut at right angles to that axis; through the centre of each plate a small conical aperture is drilled for the reception of a silver wire, one end of which can be heated in a flame, and which, by its conductivity, acts as an uniform central source of heat. If, previously to the application of heat, the surfaces of the crystal be coated with bees'-wax, the wax on the plate cut across the axis (fig. 126, 1) will be melted in the form of a circle, of which the wire occupies the centre; 2 whilst on the other plate the wax will be melted in the form of an ellipse, the two diameters of which are as 1000: 1312, the long axis coinciding with the direction of the optic axis of the crystal (fig. 126, 2), showing that the conductivity is greater in this direction than in one at right angles to it: whilst the circular form of the melted wax in the first experiment shows the uniformity with which heat is propagated in all directions around, and perpendicular to the axis of symmetry.

In crystals with two optic axes, the results, although more complicated, present the same intimate connexion with the direction of those lines within the crystal. Bodies which are not crystalline also exhibit an inequality in their conductivity in different directions, when their molecular structure is altered by tension or pressure. Unannealed glass, and plates of glass subjected to compression upon their edges, exhibit these phenomena, the shorter axis of the ellipse being in the line of pressure, or of greatest density.

FIG. 127.

Delarive and Decandolle have shown that similar differences in conductivity occur in wood, which conducts much better with the grain than across it; that is, better in a direction parallel to the fibres, than across them. Tyndall has not only confirmed this fact, but has also proved that heat passes rather more rapidly in a direction from the external surface towards the centre, a b (fig. 127), than it does in a direction parallel with the direcLa tion of the ligneous rings, c d (Phil. Trans. 1853, 217); the direction of greatest conductivity coinciding with the direction of

151.]

CONVECTION OF HEAT.

321

greatest porosity and readiest cleavage. The densest woods are not always the best conductors. American birch, though of very small density, conducts better than oak, which is much denser, and far better than ironwood, which has the unusual density of 1.426.

Convection of Heat.

FIG. 128.

(151) Although the conductivity possessed by liquids and gases is very small, yet they admit of being rapidly heated by a process of circulation or convection, which depends upon the free mobility of the particles that compose them. When heated, each particle expands, and becomes for the time less dense. If the heat be applied at the bottom of a large flask nearly filled with water, into which a little bran has been thrown to enable the eye to follow the motion occasioned, the heated and lighter particles will be seen, by the motion of the bran, to ascend, while their place is supplied by fresh particles from the sides; these in turn come into contact with the heated glass at the bottom, and they again make way for colder portions. An ascending current, as shown in fig. 128, is thus established up the middle, and descending currents flow down the sides of the flask, which are kept cool by the air. Anything that checks this free circulation, and occasions viscosity in the liquid, impedes the distribution of heat. Porridge or starch, during the boiling, requires to be constantly stirred, for the purpose of presenting fresh surfaces to the source of heat, and of preventing the portions in contact with the hot bottom of the vessel from becoming overheated and burned.'

The motion thus communicated by heat to liquids, has been ingeniously applied to the warming of buildings, by the circulation of hot water in pipes. One of the most effective methods will be understood by examining fig. 129, which represents Perkins's arrangement for beating by means of hot water at a high pressure. In its simplest form it consists of a continuous wroughtiron pipe, 1 inch (25 millimetres) in diameter ex

FIG. 129.

322

:

CURRENTS IN GASES-VENTILATION.

[151.

ternally, with a bore half an inch in diameter. The pipe is completely filled with water at F, and closed by a plug. The apparatus is provided with a chamber at E, of about a fifth or a sixth the capacity of the entire tube, to allow for expansion this chamber being left empty. About a sixth of the entire length of the pipe is coiled up at F c, and is heated by a furnace, which is of necessity placed in the basement of the building. At s s, s s, other coils are formed upon the pipe as it passes through the different apartments which are to be heated. The course of the water is indicated by arrows. The hot water, mixed with bubbles of steam, passes off from the upper part of the fire coil, F C, ascends by the pipe a a, to the highest part of the building; it then flows off on either side through the heating coils, s s, s s, and returns by the pipes c c, which unite into one before delivering the cooled water to the bottom of the fire coil, F c. ss, ss, are stop-cocks for arresting the current through any one of the heating coils, s s, s s.

The importance of the exception in the case of water to the regularity of expansion (143), in connexion with these circulatory movements, will now be perceived. During the frosts of winter a rapid process of cooling goes on from the surface of the earth and of our lakes and rivers: the colder water sinks to the bottom, fresh portions being supplied from below, until the whole has reached the temperature of 4o C.; below this point the colder water being the lighter remains at the top, thus protecting the mass beneath from further reduction of temperature by its infe. rior conductivity, and preventing such a reduction at any considerable depth as would be fatal to the animals which it contains. Ice, too, being lighter than water, floats upon the surface, and thus the bottoms of our rivers are protected from the accumulations of frozen water, otherwise inevitable; and which no subsequent summer heat would ever suffice to melt, or even to reach from the surface. In the ocean, where the maximum of density occurs below o° C., the depth is so great that, excepting near the polar circles, the low temperature does not last sufficiently long to reduce the entire mass to a degree injurious to animal life.

(152) Currents in Gases.-Ventilation. The motions produced in gases by the expansive action of heat are still more obvious and extensive than those occasioned by it in liquids. The tapering form of flame is due to an expansion of the air, accompanied by a powerful upward current, produced by the heat with which it is attended. A body of heated air confined in a light envelope possesses considerable ascensional power, and constitutes the ordinary fire-balloon; it was, indeed, by means of such a balloon that the first aëronautic excursion on record took place.

The application of the currents produced in air by differences of temperature to the ventilation of our dwellings is a subject of great practical importance. The draught produced in the chimney is due to the heat derived from the fire, which dilates the air in

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CURRENTS IN GASES-VENTILATION.

323

the flue above it, and renders it less dense: it consequently rises in the shaft, and the longer the chimney the more powerful is the draught. Suppose the temperature in the chimney to be on the average of its length 40° above that of the outer air, which may be assumed to be at 7° C.; the dilatation of air for each 1°C. is of its volume at 7° C., the column of air in the chimney will therefore be dilated, or. A column of such heated air, 8 metres high, which we will assume as the length of the chimney, would therefore only balance a column of 7 metres in height at the temperature of the outer air, and the ascensional force of the heated air would be that due to the difference in weight between the 8 metres of warm air and 8 metres of colder air, or equal to the pressure of a column of the colder air, I metre in height. Air must, however, be supplied to the lower opening, in order to allow the equilibrium to be restored; and if the communication of the apartment with the outer air be insufficient (as when the doors and windows are carefully closed, and listed down, to exclude the draughts of cold air that rush in at every crevice to furnish that required to feed the chimney), air will enter at the top of the chimney; just as when a bottle full of air is plunged with its mouth upwards under water, the water enters at the mouth, whilst the air escapes in gushes or bubbles. The consequence of the entrance of cold air at the top of the chimney will be, that such cold air pours down into the room, and, as a necessary result, the chimney smokes. If the door or the window be opened, however, the annoyance ceases. room properly ventilated, the requisite supply of fresh air will enter freely, without the necessity of setting the door open.

In a

In ventilating a room, it must be remembered that the air which has been used, and which requires renewal, has become heated by respiration and by the burning of lamps or candles; it therefore rises and accumulates in the upper part of the room. This is easily seen by opening the door of a heated apartment, and holding a candle near the upper part of the doorway; if the window be not open, a current will generally be found blowing the flame from the room. Midway down, the flame will be stationary, while near the floor it will be blown strongly into the room. In this experiment the lighter heated air flows out above, while the denser cold air supplies its place, by entering at the lower part of the room. It is for this reason advisable always to make apertures for the escape of heated air near the ceiling; but it must be remembered that no ventilation can be effectual which does not provide for the entrance of fresh air, which may be previously warmed or not, and which is best admitted at the lower part of the room. cases where there is a sufficient height of chimney, a contrivance of Dr. Arnott's is a valuable auxiliary to the ventilation; it consists of a balanced valve, opening into the chimney, whilst any momentary downward draught occasioned by the sudden shutting of the door, or otherwise, causes the valve to close, and thus to prevent the escape of smoke into the room.

In