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heated, and thus to act upon the metallic surface. In the observations of Leslie and Melloni, on the contrary, the interposed diathermanous plates were so thick, that only a small portion of the heat absorbed penetrated to the side which was turned towards the thermoscope.

3. With respect to the radiating powers of different substances at the same temperature, Knoblauch confirms the law laid down by Melloni, viz. that the radiating power of a body is influenced by scratching its surface, only in so far as its density and hardness are thereby altered :—also the result previously obtained by both Rumford and Melloni, that: the radiating power increases with the thickness of the radiating film,-a law which furnishes another proof of the correspondence between radiation and absorption. Knoblauch likewise observes that the equality of the radiating and absorbing powers is absolutely true as regards one and the same body; but that with respect to different bodies, it cannot be maintained that a body which at a certain temperature exhibits a higher radiating power than another, necessarily also possesses a greater absorbing power;--for the proportion between the quantities of heat absorbed by two bodies varies with the nature of the calorific rays.—Lastly, it is shown that the radiating power of a body is the same, however different may be the calorific rays by which it is heated.

4. The heat radiated from the most various solid bodies, such as metal, wood, porcelain, leather, cloth, pasteboard, 8c.of different thicknesses and different conditions of surface appears, when tested by all the means at our command, to be of the same nature, in whatever manner it may have been excited.—In the experiments by which this result was obtained, the temperatures of the sources of heat varied from 25° to 90° R.—This result is of some interest with reference to the determination of specific heat; for if the ice in the calorimeter were to absorb the heat radiated from different substances in different degrees, the quantity of ice melted would not be a direct measure of the quantity of heat.

5. Alteration of heat by Irregular Reflection. Melloni has remarked that a white surface reflects, with various degrees of intensity, the heat of a Locatelli's lamp, according as it is used with or without the glass chimney,-also the heat of incandescent platinum, and that of a metal cylinder heated to 400° C.-Metallic plates with rough surfaces are the only bodies which reflect equally the heat from all sources,—whilst lamp-black gives a scarcely perceptible dispersion with any.—The following table contains the results of a number of experiments in relation to this subject.

The source of heat used was an Argand lamp, and the heat, after reflexion from the various substances mentioned at the bead of the table, was made to traverse the several diathermanous media mentioned in the first column,—the object being to determine whether the calorific rays, after diffuse reflection from those various surfaces, would pass through the different media in equal or unequal quantities.

Deflection after interposition and reflection from :

che non con Direct radiation.

Deflection after interposition (without reflection).

Gypsum

White lead.

Peroxide of tin.

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Chalk.

[blocks in formation]

7.63 5.79 4.38 22:25 14.94 11.75

9.00

9.00 9.00 9.21 9.04 9.09 9:04 9:04 9.21 9.29 9.21 9.00 8.33 7:37 7.58 6:58 6:50 6.54

6.46 6:21 6:58 6.50 6.13 6.46

6.46

6.50 6.16 6:13 5.96 5.83
5.71 5.75 5.71 5.71 5:54 5.67 5:46 6.67 5.71 5.75 5.83 6.58

6.13

5.83 4.38 23:06 23:06 23:13 23:00 23:00 22:94 23:13 | 23:00 22:56 | 22:56 23:13 23:13 22:56 21.94 22:25 20:19 20:25 20-25 18:38 | 20:19 20:31 | 19:50 21:31 | 20:31 | 20:25 20:44 20-81 | 19:06 17:44 14:86 16.87 | 16.85 | 16.75 | 15:12 16.88 16.81 | 16.81 18·69 16.88 16.88 | 16.94 18:31 | 17.56 15.81 11:59

Deflection after interposition and reflection from :

Deflection after interposition and reflection from :

Deflection after interposition (without reflection).

che non Direct radiation.

[blocks in formation]

Charcoal.

Brown coal.

9:32 7.66 5.85 22:25 16.75 14.00

9.96 10:14 10.64 9.61 8.79 8:03 9.32 9.50 8.25 9.29 7.86 7.89 8.18 8.07

7.25 6.96 7.75 7:43 6.79 7:11
6.29 6.64 6:57 6.79 6:11 5.36 7.07 6.71 5:94 5.57
23.12 22.69 22.88 | 22:94 21:50 21:12 22:12 22.75 20.88 22:12
20.06 20.94 20 38 19.81 ) 18.62 14.75 20:00 20:06 | 15:12 16.81
16.00 16.88 16.12 16:19 | 15:31 | 12:77 | 17.25 16.69 12.62 | 13.69

9.06 7.88 5.75 22:12 16:50 14:44

8.25 7.50 5:31 21:31 14.69 13.12

Substances interposed.

13°

Red glass..
Blue glass
Alum
Rock-salt.
Calcspar
Gypsum

25°

Substances
interposed.

13°

Red glass..
Blue glass
Alum
Rock-salt.
Calcspar
Gypsum

25°

10.25 7:56 6:06 22:12 19.50 16:56

The rays of heat reflected from certain homogeneous bodies passed in unaltered proportion through the diathermanous media. Such was the case with birchwood, cork, and mahogany ; also with the simple metals and metallic alloys.

Heat is therefore altered by diffuse reflection in very different ways; in a high degree by some bodies, not at all by others. These alterations, in the case of unpolished bodies, are independent of their degree of roughness :—in the case of metallic surfaces, it is even indifferent whether they are used in a state of high specular polish or in any other condition of surface.

By making use of incandescent platinum, the flame of alcohol, and a heated metal cylinder as sources of heat, it was found that: The changes produced in heat by irregular reflection are affected by the nature of the source of heat as well as by the nature of the reflecting surface. And in particular, that the modifications, which are very considerable in the rays of the Argand lamp, are less in those of red-hot platinum, still less in those of the alcohol flame, and in the case of the metal cylinder heated to any temperature between 20° and 90° R, they become absolutely nothing.

It is easily seen how by these modifications the rays of heat reflected from different substances, may to a certain extent, alter their relations one to another. Thus, the heat of an Argand lamp when reflected from carmine, passes through gypsum with less facility than when reflected from white velvet. The rays of incandescent platinum pass equally well through gypsum after reflection from those surfaces; and the heat of an alcohol flame passes through that medium after reflection from carmine better than after diffuse reflection from white velvet.

On repeating the experiments with the four above-mentioned sources of heat with reference to a different object, it was found that surfaces which affect equally the rays from any one source of heat, e. g. of an Argand lamp, likewise modify in an equal degree the rays from any other source. The following lists contain those substances which scatter the rays of heat in such a manner that, as far as regards their passage through red glass, blue glass, alum, rock-salt, calespar, and gypsum, they are not to be distinguished one from the other. The bodies in (1) have likewise this peculiarity,—that the heat irregularly reflected at their surfaces is undistinguishable from non-reflected heat.

(1.) Gold, silver, platinum, mercury, iron, tin, zinc, copper, lead, alloy of lead and tin, brass, German silver, untinned iron plate. (2.) Gypsum, chalk, white lead, white oil-colour, porcelain, linen, white paper, blue paper, white cotton, grey calico, Paris green, green cinnabar, chrome yellow, black lac. (3.) Birch-wood, cork, mahogany, yellow marble. (4.) White satin, black satin, white taffetas, black taffetas. (5.) Blue velvet, black velvet. (6.) Yellow leather, brown morocco. (7.) Light cloth, black cloth. (8.) Blue flock-paper, green flock paper. (9.) White wool, red wool. (10.) Cinnabar, oxide of copper.

Of the following substances, those contained in the same division exhibit a similar but not exactly equal action.

(11.) Carmine, madder, red flock-paper. (12.) White velvet, white wool, green flock-paper. (13.) White lead, Diessbach blue. (14.) Black velvet, green oil-cloth. (15.) Black paper, black glass. (16.) Coal, coke, plumbago. (17.) Lamp-black, animal charcoal.

The following substances, with reference to the dispersion of heat, cannot be included in either of the preceding groups.

(18.) Ultramarine. (19.) Peroxide of tin. (20.) Tannate of per

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oxide of iron. (21.) Indian ink. (22.) Red taffetas. (23.) Green taffetas. (24.) Dark red velvet. (25.) Light red velvet. (26.) Green velvet. (27.) Black morocco. (28.) Brown Manchester. (29.) Mother of pearl. (36.) Ivory. (31.) Charcoal. (32.) Brown coal.

It is also shown that: The changes which heat undergoes by diffuse reflection are wholly due to an elective absorbing power of the reflecting surface for certain of the calorific rays which are sent to it.

İn this respect, the phenomena are perfectly analogous to those which are observed in the diffuse reflection of luminous rays.

With the exception of the metals, which reflect all calorific rays equally well, and of charcoal which absorbs them all, it cannot be said of any substance yet examined, that it reflects heat better or worse upon the whole than another; inasmuch as the proportion varies with each radiation.

6. The preceding results show, with regard to the sources of heat employed, that the diversity of quality in the emitted rays is greatest in the Argand lamp, less in the incandescent platinum, and still less in the alcohol flame, -while in the case of the cylinder heated to 80° R. it vanishes altogether. Generally, the heat emitted from the most various solid bodies between the temperatures of 50° and 90° R (144:59 and 234.5° Fah.) is perfectly homogeneous or monochromatic.

The variety of calorific tints in the heat of incandescent platinum increases with its temperature. It is not however universally true that, of two sources of heat, that which has the higher temperature has also the greater variety of calorific rays; e.g. red-hot platinum emits rays of greater diversity than those emitted by the flame of alcohol. It is likewise remarkable that the variety of calorific rays

emitted from different sources is greater or less, just as those sources contain a greater or less variety of the coloured rays of light: thus, it is evident that the flame of the Argand lamp is richer in this respect than red-hot platinum, and the platinum richer than the alcohol flame. I

The calorific rays of a body heated to dull redness may be polarized like rays of light (P. 164), so that they will or will not be reflected from a second surface and afterwards affect a thermometer, according to the position in which that surface is placed. (Bénard, Gilb. 46, 384.)—Rays of heat may be polarized by transmission through tourmalin or mica, depolarized by doubly refracting crystals, &c. (Forbes, Melloni.)-Baden Powell denies the polarization of heat.

According to Pouillet's approximate estimation, the temperature of the sun is between 1461° and 1761° C, and it sends annually to the earth as much heat as would melt a stratum of ice surrounding the whole earth and 31 mètres in thickness. The temperature of space, according to the same calculation, is about – 142° C; but this likewise imparts yearly to the earth a quantity of heat sufficient to melt a similar stratum of ice of the thickness of 26 mètres.

When heat is restrained in its radiating motion by the adhesive force of liquid and solid bodies it diffuses itself within them slowly and with a creeping motion; it is conducted by them.—According to Fourier's hypothesis, the conduction of heat consists in radiation from one atom to another.

Conducting power of solids. The metals are the best conductors of heat. If the conducting power of gold be assumed = 1000, that of platinum is 981, of silver 973, copper 898, iron 374, zinc 363, tin 304, lead 180, marble 24, porcelain 12, tiles 11.4. (Despretz.) If a number of metallic rods of equal length, breadth, and weight, and covered with wax, be equally heated at one end, the wax on the copper will be melted for a distance of 3.5 inches, on the silver 2.5, and on the platinum and palladium 1 inch. (Wollaston.) Comp. N. W. Fischer (Kasın. Arch. 14, 147; Pogg. 19, 507; 52, 632.)— The passage of heat from one solid body to anotber in close contact with it causes retardation. (Despretz.)—N. W. Fischer's assertion (Pogg. 19, 513) that when water is placed in contact with the heated end of a metallic bar, the heat moves on to the cold end, has been to a certain extent contirmed by Mousson (Bilb. univ. N. S. 12, 418) but contradicted by Schröder (Pogg. 46, 135) and Böttger (Pogg. 50, 60).-Porous bodies are remarkably bad conductors, e. g. those of organic structure, such as wood, wool, feathers, &c.

Senarmont (N. Ann. Chim. Phys. 21, 457) has 'investigated the conduction of heat in crystallized bodies. The mode of experimenting was to heat a cylindrical plate of the crystal in the direction of its axis, and trace the form of the isothermal curves on the two faces by means of melted wax.

A small tube of platinum was inserted through the centre of the plate in the direction of its axis, bent at right angles at the lower extremity and heated by a lamp,—a current of air being at the same time sent through the tube by means of an aspirator. The two bases of the cylinder were covered with wax, which, being melted by the heat, traced out on the surface a curve line whose form was determined by the conducting power of the crystal in different directions.

Plates of homogeneous substances, such as glass and zinc, treated in this manner, gave circles, the centre of which was at the source of heat.

On a plate of calcspar cut perpendicular to the axis of symmetry, the curves were circles with their centres in the axis. On plates parallel to the direction of natural cleavage, the curves were also circles, exhibiting a slight tendency to elongate in the direction of the principal section. On plates parallel to the axis of symmetry, and having their plane perpendicular to one of the faces of the primitive rhombohedron, the curves were ellipses very regular and well defined, and having their longer axes in the direction of the axis of symmetry. The ratio of the axes was 1:118. These experiments show that the axis of symmetry is a direction of greater conductibility.Similar results were obtained with quartz, the ratio of the axes being 1.31.—It may therefore be inferred that: In media constituted like crystals of the rhombohedral system, the conducting power varies in such a manner, that, supposing a centre of heat to exist within them, and the medium to be indefinitely extended in directions, the isothermal surfaces are concentric ellipsoids of revolution round the axis of symmetry, or at least surfaces differing but little therefrom.

On plates of gypsum perpendicular to the crystallographic axis, the curves were ellipses, the ratio of the axes being 1.23. The author was unable to experiment upon plates perpendicular to the direction of easiest cleavage; but there is every probability that the curves in this case would also be elliptical. It may therefore be inferred that: In media constituted like crystals with two optic axes, if we suppose a centre of heat to exist within, and the crystal to be indefinitely extended in all directions, the isothermal surfaces will be ellipsoids with three unequal axes, or curve surfaces differing but little therefrom. It is probable also that the principal axes of these isothermal surfaces coincide with the crystallographic axes, when the latter are likewise axes of symmetry,

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