334

RELATIVE ABSORBABILITY OF RADIANT HEAT.

[161.

canister of boiling water the thermoscope showed a heat of 100, corresponding exactly with the effect upon it when lamp-black was used.

The following table exhibits some of the results which Melloni obtained by operating in this way:

Relative Absorbability of Different Kinds of Heat.

Lamp-black appears to absorb all the rays that fall upon it, from whatever source they may have originated; and the amount absorbed by metallic surfaces, although smaller is nearly uniform, whatever be the source. It has also been observed, that the less intense the source of heat, the greater usually is the proportion absorbed.

Franklin, nearly a century ago, made the observation, that solar heat is absorbed with greater or less facility according to the colour of the object upon which the rays fall, but that little or no such difference exists with the heat of a lamp or of a candle. He took pieces of cloth, similar in texture and size, but different in colour, and placed them in the sunlight, upon newly-fallen snow, and he found that the snow melted under the pieces of cloth with greater rapidity the darker the tint-the absorption being greatest with the piece of black cloth, then followed the blue, then the green, purple, red, yellow, and white pieces, in the order enumerated. These effects are due, as Tyndall has shown, not to the colour of the object, but to the nature of the material used as the colouring agent.

(162) Transmission of Heat through Screens.—The cause of these remarkable differences will be best understood by a consideration of the phenomena attending the transmission of heat through bodies which allow it to pass unobstructed, as glass allows light to pass. Melloni terms those bodies which thus transmit heat diathermanous, or diathermic (from dia, through, and Oɛpuòs, hot)—those which do not allow this transmission of heat being termed athermanous or adiathermic.

162.]

TRANSPARENT BODIES NOT ALL DIATHERMIC.

335

Bodies that are transparent to light are by no means equally so to radiant heat. This arises from two causes, which require to be carefully distinguished from each other, and which may be separately illustrated by a parallel action on the rays of light. A glass containing pure water absorbs very little light, and transmits almost all that it does not reflect: if the attempt be made to measure its transparency by ascertaining the distance at which a page printed in small type is legible when the vessel of water is interposed, and afterwards, when it is removed, the difference in the two cases is hardly appreciable. If a few drops of a mixture of Indian ink and water be added, the transparency will be diminished, and the characters will be legible at a smaller distance; a further addition of ink will diminish the transparency more and more, until the letters can be no longer discerned. The light that is transmitted, however, although diminished in quantity, possesses the same character as the incident light; and a prismatic analysis shows that both consist of the same colours in the same proportion if in this experiment indigo be substituted for Indian ink, the legibility of the page is diminished to an extent nearly equal; but the prism shows that certain of the rays have been absorbed more completely than others. Similar effects are produced with the rays of heat. There are, however, many substances which are almost perfectly transparent to light-viz., among solids, glass, diamond, Iceland spar, ice, and a great number of crystals; amongst liquids, water, spirit of wine, ether, oil of turpentin, and a multitude of other bodies; and among aëriform bodies, atmospheric air, and the greater number of gases. heat, on the contrary, there is only one known solid that approaches perfect diathermancy, and that is rock salt; many colourless gases possess the property also in a still higher degree; but no liquid has yet been discovered which is free from absorptive action on the thermic rays.

For

The more important parts of the apparatus employed by Melloni in these researches are represented in fig. 132. One of his four principal sources of heatviz., naked flame, ignited platinum, blackened copper, heated to 400° C., or copper heated to 100°, was placed at M, on a movable support, behind the perforated screen, N; the rays being concentrated, when necessary, by the concave mirror, M: they were received at a suitable distance from this upon the thermoscope or thermo-multiplier, T.* If a double screen, s, of polished copper were interposed between the source of heat and the thermoscope, the rays of heat were entirely intercepted. Having placed the thermoscope at such a distance as always to indicate, when the copper screens were removed, a constant elevation of

In these inquiries a peculiar and very delicate thermometric apparatus, termed a thermo-multiplier (317), was generally employed.

336

TRANSMISSION OF HEAT THROUGH SCREENS.

[162.

temperature, a plate of some substance, the diathermancy of which was to be ascertained, was then introduced at P; and on observing the difference of temperature, as marked by the instrument, the portion of heat which the plate transImitted was at once ascertained.

In this manner Melloni found that plates of rock salt of great transparency, varying in thickness from o2mm. to 4 or 5 centimetres, transmitted 92 out of every 100 rays incident upon them, whatever were the source of heat employed; the loss of 8 per cent. being mainly due to a uniform quantity which is reflected at the two surfaces of the plate; rock salt, therefore, is to heat what pure colourless glass is to light. The following experiment shows the independence of diathermancy and transparency:-If a cast-iron ball heated to about 200° C. be placed midway between the blackened bulbs of a thermoscope, each bulb will receive an equal amount of heat, and the liquid will remain stationary; but if a plate of rock salt be interposed between the iron and one of the bulbs, and a plate of glass of equal thickness be placed between the hot ball and the other bulb, it will be found that although both plates are almost equally transparent to light, yet the bulb next the rock salt will rise in temperature much more rapidly than the one next the glass. In liquids, the independence of transparency and diathermancy is still more striking. For example, out of 100 rays that fell from an argand lamp, on each of four liquids equally transparent-viz., water, sulphuric acid, ether, and oil of turpentin, water transmitted only 11, sulphuric acid 17, ether 21, and oil of turpentin 31; while chloride of sulphur, which is of a reddish colour, allowed 63 of the incident rays to pass.

The following table includes some of the results obtained by Melloni, for several solid bodies: in these experiments plates of equal thickness were used in each case. (Thermochrose, 1850,

Part I. 163, 164.)

Diathermancy of Liquids contained in Glass-stratum of liquid

Tyndall (Phil. Trans. 1864, 225) enclosed a certain number of liquids in a cell with sides formed of rock salt, and subjected them to the radiation of a glowing platinum wire. If the amount of heat rays which fell upon the surface of the pile when the empty cell was interposed be called 100, the portion of heat transmitted by the undermentioned liquids was the following:

338

DIATHERMANCY OF GASES AND VAPOURS.

[163.

(163) Diathermancy of Gases and Vapours.-The experiments of Knoblauch have shown that even metallic bodies in very thin films are diathermic, presenting in this respect an analogy with their limited transparency to light in films of similar tenuity. Gold and silver transmit certain of the rays of heat more freely than others, whilst platinum appears to transmit all the rays with nearly equal facility. On the other hand, Tyndall (Phil. Trans. 1861, 1, and 1862, 59) has found that the gases exert different degrees of absorptive action on the rays of heat, and even when colourless that this effect is strongly marked. Coal gas, for example, exerts a much stronger absorptive effect than atmospheric air, and the vapour of ether considerably more than that of the carbonic disulphide. Rays of certain degrees of refrangibility also are more powerfully absorbed by the colourless gases than others. For example, coal gas arrests the heat-rays from a source below a visible red heat, much more perfectly than it absorbs the rays of the lime light after they have traversed a thin layer of water.*

The following table is given by Tyndall as representing the relative absorptive power for heat emanating from a source at 212° F., of various gases at the normal pressure of 30 inches of mercury, when a column of the gas, 4 feet in length, was subjected to experiment:

* In conducting these experiments, an arrangement of apparatus was ultimately adopted by Tyndall, the principle of which will be understood from the following description:-The gases which were to be submitted to experiment were placed in a brass tube four feet long, and polished in the interior; but for particular gases a glass tube was substituted. In either case it was closed airtight at each end by a polished plate of rock-salt, and was connected with an air-pump, so that at pleasure it might be used when exhausted of air, or when filled with different gases in succession. The source of heat employed in most cases was a cube of copper filled with water, which was kept boiling. The face of the cube was turned towards the tube for experiment, and was coated with lamp-black. At the other end of the brass tube, a thermo-electric pile (317) was placed; one face of the pile was directed towards the tube which contained the gas under trial, whilst the other end was directed towards a second cube also containing boiling water. The thermo-electric pile was connected with a very sensitive galvanometer. The experiment was commenced by exhausting the long tube of air, and then adjusting the distance of the second cube of boiling water, interposing or withdrawing a screen until the amount of heat which fell upon the two surfaces of the thermo-electric pile was exactly equal, which was indicated by the needle of the galvanometer standing precisely at zero. The gas for examination, after carefully drying it, was then admitted into the tube; under these circumstances, if the gas thus admitted were capable of absorbing even an exceedingly small portion of radiant heat, the equilibrium of the galvanometer was destroyed, and the needle was deflected to an extent varying with the amount of heat arrested during its passage through the gas.