Of Materials used in Building. 405 probably also for heat. This is remarkably the case with copper. Steel containing carbon is a worse conductor than iron. The conducting power of minerals or earthy substances is much less than that of metals-e.g., marble, 236; porcelain, 122; fire-clay, I'14. The following articles used in building have a relative conducting power expressed by the following figures, compared with slate (100) : These results were obtained by Mr. Hutchinson in experiments on the building stones employed for the Houses of Parliament. The warmest substances, or those which resist most the passage of heat, are the lowest in the scale. Marble is a better conductor than plaster or cement. A marble column feels much colder than one made of artificial scagliola. Sulphur is a very bad conductor of heat. If a roll of sulphur is held over a candle it cracks immediately. Even the warmth of the hand wiil cause it to crack. The heat is not conducted off, but concentrated at the part heated, and the solid gives way, owing to sudden expansion. 593. Solids are usually divided into conductors and non-conductors; but all bodies conduct heat more or less, hence the latter term is incorrect. As the above table shews, the various substances differ only in degree. Of all earthy substances, asbestos is probably the one which most retards the passage of heat, or possesses the least conductivity. The writer has tested the properties of this substance by holding for a short time a red-hot poker in his hand covered by a thick asbestos glove. Its fibrous and porous structure prevents the ready transmission of heat (Art.”58, p. 23). A mass of red-hot coke soon cools and becomes black on the exterior, although a red heat is still maintained in the interior. An equal mass of red-hot iron remains visibly red-hot for a much onger time, and it is long before it appears black on the exterior. The coke is a bad conductor and retains the heat, while the iron is a good conductor and conveys heat rapidly through its sub stance 406 Bad Conductors of Heat. The lava of volcanoes is a very bad conductor of heat. It rapidly cools and becomes black on the rugged surface, but at two or three feet below it retains a red heat for weeks, months, and even years, according to the thickness of the current. The writer observed, when ascending Vesuvius in 1829, that a stratum of lava which had been thrown out in an eruption a year before still retained a great degree of heat below. When on the brink of the crater, he also noticed that the fused lava thrown out in huge flakes from the inner cone to the height of at least 600 feet, appeared quite black as it reached the atmosphere, but on falling on the floor of the crater the lava broke into fragments, and showed a red heat in the interior. It was then so soft that coins could be thrust into it. 594. Porous wood-charcoal is a bad conductor, but the dense form of native carbon known as diamond, readily conducts heat. By the application of a diamond to his lip, a jeweller is able to distinguish the diamond from a paste imitation by the sense of coolness which it imparts, owing to its higher conducting power. It would be a mistake to suppose that the denser the metal, the more readily it conducts heat. Thus platinum is much denser than silver, but it has only one-twelfth of the conducting power. Copper is superior to gold in conductivity. It requires the closest contact of atoms to manifest this property. If a bar of copper is cut through, it ceases to conduct, or at any rate heat is transmitted only slowly, although the cut ends are in the closest contact. Finely divided metals, as iron filings or spongy platinum, have but little conducting power compared with that of the solid metal. Finely divided or porous mineral substances, for a similar reason, oppose the ready transmission of heat. Red-hot cannon balls may be safely carried on sand. 595. Count Rumford adopted the following method in order to ascertain the relative degrees in which furs, feathers, and other organic materials used for clothing, conduct heat, or, which is the same thing, resist its passage. He covered the ball and stem of a thermometer with a certain thickness of the substance to be tried, by placing it within a larger bulb of glass, and then filling the surrounding interval between the two with the substance; and after heating this apparatus to a given degree by dipping it in liquid of the desired temperature, he surrounded it by ice, and marked the comparative times required to cool the thermometer a certain number of degrees. The figures following the names of some of the substances in the subjoined list, mark the number of seconds This table may be read thus: the greater the number of seconds required to cool down to the standard, the worse the conducting power of the substance, and the greater the amount of heat retained. Owing to its lightness and the large amount of air which it locks up, the down of the eider-duck is one of the worst conductors known, and is, therefore, best adapted for retaining or preventing the access of heat. 596. In reference to air, if its particles are not allowed to move about among themselves, so as to carry heat from one part to another, by what has been called convection, it conducts (in the manner of solids) so slowly, that Count Rumford doubted whether it conducted at all. It is probably the worst conductor known, that is, it is the substance which, when at rest, impedes the passage of heat most. To this fact seems to be owing, in a considerable degree, the remarkable non-conducting quality of porous or spongy substances, as fur, feathers, loose filamentous matter, powders, &c., which have much air in their structure, often adherent with a force which immersion in water, or even their being placed in the vacuum of an air-pump, can scarcely overcome. Double windows are very effectual in retaining heat by reason of the non-conducting stratum of air between them. In heating an apartment with warm air, this should be always let in from the lowest part. If it enters above, it simply floats on the stratum of cold air below, without warming the room generally. 597. While contemplating the facts set forth in the above table, one cannot but reflect how admirably adapted to their purposes the substances are which nature furnishes as clothing for the inferior animals, and which man afterwards accommodates with such curious art to his peculiar wants. These animals required to be protected against the chills of night and the keen blasts of winter; and some of them which dwell among enduring ice could not have 408 Protection of Animals from Cold. lived at all but for a garment which should shut up within them nearly all the heat which their vital functions produced. Those textures, or coverings, which are known under the name of fur and feather, perfectly protect the wearers. These textures grow from the bodies of the animals in quantity exactly suited to the climate and season, and are reproduced when, by use or wear, they become too thin. In warm climates the hairy coat of quadrupeds is comparatively thin as of the elephant, the monkey, the tropical sheep, &c. It is seen to thicken as the temperature is lower, furnishing the abundant fleeces of the temperate zones; and towards the poles it is externally shaggy and coarse, and internally shorter and finer, as in the Arctic bear, &c. In amphibious animals, which have to resist the cold of water as well as of air, the fur acquires a peculiar character, as in the otter and beaver. Birds, from having very warm blood, require for the preservation of heat a warm covering, and in order that they may pass easily through the air this covering should be at the same time light, strong, and smooth. These objects are secured by the marvellous structure and arrangement of feathers. Feathers, like fur, appear in kind and quantity suited to particular climates and seasons. The birds of cold regions have plumage almost as bulky as their bodies; and those of them which live much in the water have, additionally, both a defence of oil on the surface of the feathers, and the interstices of the ordinary plumage filled up by the still more delicate structure called down, particularly on the breast, which in swimming first meets and divides the cold wave. There are animals with warm blood which yet live immersed in water, as the whale, seal, walrus, &c.; and neither hair nor feathers, however oiled, would have been a fit covering for them; but they are furnished with an equal protection in the vast mass of fat or blubber which surrounds their bodies, giving them buoyancy and completely retaining internal heat. In reference to the vegetable kingdom, it may be observed that the bark of trees is also a structure slowly permeable to heat, and therefore it serves to retain the temperature which is necessary to vegetable life. 598. While we admire what is thus provided for preserving heat in animals and vegetables, we must not omit to notice the important protection of snow and ice, as winter clothing for the fields and gardens, for the lakes and rivers. Ice is at all times lighter than water, and floats on the surface. Its specific gravity has been found to be 918 at 32°, and '920 at 0°. It is, therefore, rather more than th Influence of Snow and Ice. 409 lighter than its bulk of water.* Ice is a very slow conductor of heat, and defends the water underneath from the cold air above, preserving it liquid and as a fit dwelling for the finny tribes. By this arrangement the extreme of cold is not only thus prevented below, but a moderate temperature sufficient for the life of fishes is preserved throughout the water. In the formation of ice, therefore, nature, by a remarkable exception to the general law of crystallization, has secured a winter garb or protection for the inhabited lakes and rivers as effectual as for terrestrial animals by the periodical thickening of their wool or fur. Snow, which may be called the pure white fleece of the earth, is a structure which resists the passage of heat nearly as much as feathers. It consists of fine crystallized spicula closely entangled and locking up, as they fall to the earth, a large quantity of air as well as many impurities contained in the atmosphere. A common form of snow-crystal is given in the annexed engraving (fig. 162). These crystals are generally in the shape of hexahedral plates, the spicula crossing each other at an angle of 60°. It has been found by direct experiment that sixty cubic inches of compressed snow will yield only eight cubic inches of water in the liquid state. Snow, of course, has to defend substances only from degrees of cold below 32°, or the freezing point; but it does this most effectually by preserving the roots, and seeds, and tender plants during the severity of winter. Under deep snow, while the thermometer in the air may be far below zero, the temperature of the ground rarely remains below the freezing point; and this temperature, to persons who have to bear sharper cold in the outer air, is mild and even agreeable. It is much higher than what often prevails for long periods in the atmosphere of the north of continental Europe. The Laplander, who, during his winter, lives chiefly under ground, is glad to have additionally overhead a thick covering of snow. Among the hills of the north and west, even of Britain, during the storms of winter, a covering or shelter formed of snow Fig 162. *This will serve to convey a notion of the enormous masses of the iccbergs found floating in the sea. One measured by Dr. Hayes in Melville Bay, was 315 feet in height and three-quarters of a mile in length. There was twelve times as much ice below as above the level of the sea. Its estimated weight was two thousand millions of tons. The influence of these vast masses of solid water at 32°, in lowering the temperature of the air and sea for a great distance around them may be readily understood. |