Bodies Transparent to Heat. 435 the air offers considerable obstruction to the passage of heat-rays from sources of low temperature, the state of the atmosphere as to moisture near the surface of the earth, influences much the temperature existing there. The sun's heat-rays come down readily, not only through the dry atmosphere of great elevations, but also through the humid air below; while the return upward by radiation from the moderately heated earth, is resisted by the presence of moisture, and a useful warmth is retained below. Unimpeded radiation, through perfectly dry clear air during a single night might chill ordinary vegetation very destructively. Dr. Tyndall has directed the attention of physicists to this fact. 626. In our drawing-rooms it is common to have plate-glass firescreens, which, while they allow the light to pass, defend the face from the heat but all persons know that the heat of the sunbeams, as well as their light, enters our green-houses through the glass which covers them. Out Hence, bodies which are transparent to light, or diaphanous,* are not in the same proportion transparent to heat, or diathermanous.† Plates of equal thickness and of equal transparency to light, allow very different quantities of heat to traverse them. of 100 incident rays of heat from an Argand oil-lamp, it was found, in accordance with what is stated above, that while alum allowed only 12 rays to pass, rock-salt was traversed by 92; rock crystal and Iceland spar by 62; and gypsum by 20. Rays of heat may be so concentrated as to ignite a fragment of phosphorus at their focal distance. If a screen of rock-sait is interposed, the phosphorus is equally ignited; but if alum is substituted, this arrests the rays of heat, and the phosphorus does not take fire. Both may be equally transparent to light, but the result shews that one transparent substance is easily traversed by heat, while another obstructs its passage. Rock-salt transmits heat from all sources, and of all degrees of intensity; but of all transparent bodies water is that which admits the smallest number of heat-rays to traverse it. Faraday determined the amount of heat-penetrating power in various kinds of glass, by placing behind equal surfaces, sheets of white blotting-paper soaked in ether. The rapidity of evaporation formed a criterion of their diathermanous properties; and he came to the conclusion that a pale green glass was best fitted for From dia, through, and paivw, to appear. From dia, through, and 0epun, heat-heat penetrating. 436 Radiating Power affected by Surface. conservatories, by reason of its allowing the light to traverse, but to a certain extent arresting the heat-rays. 627. A glass screen interposed between the two concave mirrors in the apparatus above described, destroys almost entirely the effect of the heated body placed in one focus, on the thermometer in the other, and the trifling effect really produced, appears to be owing to the heat that is absorbed by one side of the screen, and then, after passing through it by conduction, is radiated from the other. This conclusion is supported by the fact that screens of metal or of glass, covered with lamp-black, paper, &c., allow transmission nearly in proportion to their several absorbent and radiant powers. The doctrine of radiant heat makes us aware of the importance of having vessels of highly polished metal for containing liquids or other substances which we desire to keep warm; hence tea and coffee-pots, &c., should always be highly polished. Pipes for the conveyance of steam or hot air, if left naked, should be of polished metal so long as they are intended to contain steam; but after arriving at a place where they have to give out their heat, as in the hot-water warming apparatus, their surface should be blackened and made rough. A mirror intended to reflect heat should be of highly polished metal, and such, for an obvious reason, the interior of a screen placed behind roasting meat at a kitchen fire, should be. A fireman's helmet is usually made of highly polished metal. It is of advantage that the bottom of a tea-kettle, or other cooking vessel, be externally black, because the bottom has to absorb heat, but the top should be polished, because it has to confine it. 628. Formation of Dew.—The interesting phenomenon of dew was not well understood until the laws of radiant heat had been investigated. At sunrise, in particular states of the sky, every blade of grass and leaflet is found, not wetted, as if by a shower, but studded with lustrous and transparent globules of water, bending it down by their weight, and falling like pearls when the blade is shaken. These are formed in the course of the night by a gradual and slow deposition on the leaves of vegetables and other bodies, rendered by radiation colder than the air around them, of part of the aqueous vapour which rises invisibly into the air during the heat of the day. In a clear night the objects on the surface of the earth radiate heat to the sky through the air, which impedes but does not altogether prevent radiation, while there is nothing nearer than the stars to return radiation. The substances thus radiating heat consequently soon become colder; and if the air Formation of Dew. 437 around has its usual proportion of moisture, part of this will be deposited on them in the form of dew, exactly as the invisible moisture in the air of a room is deposited on a cold bottle of wine when first brought from a cool cellar and placed on the table. Dew is, therefore, essentially a deposit of condensed aqueous vapour on solids cooled below the temperature of the surrounding atmosphere. Dr. Wells was the first to give a correct explanation of this phenomenon. 629. Spring and autumn are the seasons in which the greatest dif. ference is observed between the temperature of the day and night, and between the temperature of the earth and the air covering it, as the result of radiation. At this time of the year the earth will be sometimes cooled by radiation to 20° below the temperature of the air which covers it. It is this great difference suddenly occurring which leads to the separation of aqueous vapour, sometimes in the form of thick fogs, at others in the shape of dew. In winter, when the thermometer is at or below 32°, the deposited particles of water assume a crystalline arrangement around the twigs of trees, which is well known as hoar-frost. The deposition of dew and hoar frost may be easily imitated. Place in a large glass flask five ounces of strong hydrochloric acid, and add to this eight ounces of powdered sulphate of soda, rapidly mixing and shaking the mixture to promote solution. In a few minutes the air surrounding the flask deposits its moisture, rendering the surface dull, and in a quarter of an hour or less the liquid becomes frozen, and soon forms a thick crust of hard snow on the glass. The beauty of the effect produced by hoar-frost is in the slow formation of perfect hexahedral crystals of snow interleaved with each other, and giving a sparkling lustre to the twigs of trees on which they are deposited. These crystals are in fact frozen dew. (See Illustration, p. 409.) 630. Clouds obstruct radiation from the earth; in other words, they reflect and return the heat radiated; and thus on cloudy nights the deposit of dew does not take place. For a similar reason, cloudy nights in winter are generally warm nights. On the other hand, the effect of clouds in summer, or during the day, is to lower the temperature, because at this time the earth receives more heat from the sun than it radiates, and the access of this heat to the earth is thereby cut off. It is on warm clear nights in the evenings of autumn, that the dew is most abundant. In the tropical climate of India, radiation from the earth during a single night takes place to such a degree as to freeze a thin stratum 438 Deposit of Dew.-The Dew-points. of water placed in a shallow pan and sunk a short distance into the ground. At Calcutta, the temperature of the air rarely falls below 40° on the coldest nights. During the Prince of Wales's recent visit to India it was observed to fall on one night to 29°. Owing to radiation through a clear sky, the water is often frozen under these circumstances. In this way ice is still obtained in some parts of India. Air itself seems to lose little heat by radiation. A thermometer placed upon the earth any time between sunset and sunrise, generally stands considerably lower than another suspended in the air a few feet higher up, owing to the great radiation of heat upwards from it and from the earth, while the surrounding air remains nearly in the same state. During the day, while the sun shines, the earth is much warmer than the air, but during the night it is cooler, as well as all the substances resting upon it. The best radiators, such as loose straw or sticks lying on the ground, the pointed leaves of grass, decayed leaves, &c., the slender twigs of shrubs and trees, receive the deposit most readily from the atmosphere by their being cooled so much below it. Woollen cloth is a much better radiator than polished metal. It will therefore fall by radiation to a much lower temperature, and will have much more dew deposited on it, than the metal. The reason why the dew is deposited so much more copiously upon the soft spongy surface of leaves and flowers, where it is wanted, than on the hard surface of stones and sand, where it would be of no use, is therefore to be ascribed to the difference in their radiating powers. There is no state of the atmosphere in which artificial dew may not be made to form on a body by sufficiently cooling it, and the degree of heat at which the dew begins to appear is called the dew-point, being an important particular in the meteorological report of the day. 631. The proportion of aqueous vapour in air is subject to variation according to temperature. The air is said to be saturated when it contains as much as it can receive at the observed temperature. If cooled below this point, some of the vapour will be separated from it according to circumstances in the form of cloud, fog, or rain. At 520, air retains th of its volume of vapour, and at 32° it retains onlyth of its volume. The more it is saturated the more rapidly is the vapour deposited. Substances protected from radiation by slight coverings placed over them, retain their temperature, or are not cooled to a degree to allow of the deposit of any liquid upon them. Hence they present no appearance of dew. Gardeners thus protect young and tender plants by covering them with glass frames. Radiation-Temperature of Space. 439 632. The temperature of the surface of the earth, depends on the quantity of heat transmitted to it from the sun, through the clear atmosphere, which absorbs but little. The heat thus received is again lost, chiefly by radiation into space, and, to a very small extent, by conduction downwards through the superficial strata. It is from the amount lost by radiation towards the cold, sunless sky, that we learn the temperature of the medium in which our globe is floating. The lowest natural temperature hitherto observed on the earth, was noticed by Erman at Yakutsk in Eastern Siberia. In January, 1829, this philosophical traveller found that the thermometer fell to 72° below the zero of Fahrenheit, or 104° below the freezing point of water! According to the laws which regulate the diffusion of heat, space must be at or below this temperature, and were there not an annual compensation derived from the sun, the surface of the earth, notwithstanding the existence of subterranean heat, would be speedily cooled to a temperature which would lead to the destruction of all animal and vegetable life.* Other very low temperatures on the earth have been noticed. Ross, in one of his northern expeditions, met with a temperature of - 60°; and at Nijne Kolymsk in Siberia, in the winter of 1821, the thermometer is stated to have fallen to - 65°. Sir E. Belcher, while wintering in Wellington Channel in 1854, experienced a temperature of 55°; and in January following, a temperature of 62°. Dr. Kane on one occasion observed that the thermometer fell to · 68° F. Captain Back at Fort Reliance met with a still lower temperature, — 70°, or 102° below freezing. It may be here mentioned that Fourier and Schwanberg have calculated the temperature of space in which our planet moves at from - 58° to — 76° F. "Each particular substance, according to the nature and arrangement of its ultimate particles, takes a certain quantity of heat (said to mark its CAPACITY) to produce in it a given change of temperature or calorific tension." 633. A pound of water, for instance, that its temperature may be * Yakutsk is in latitude 61° 55' North. For two months in every year it has a temperature of - 40° F., hence at this time mercury is a solid metal. A custom is said to have existed formerly of presenting to the governor of the city, on the setting in of the severe cold, an image of a saint in solid mercury, the metal being for this purpose poured into a mould and exposed for a night. |