If a piece of heated metal be placed in the centre of a room, midway between the ceiling and the floor, heat will be disengaged from it equally in all directions, upwards, downwards, horizontally, and obliquely; which may be proved by the melting of a small quantity of tallow placed at certain distances around the metal. This is an instance of radiation. When the bowl of a metal spoon is left, for a few minutes, in a cup of hot tea, the handle of the spoon acquires the same temperature as that of the tea. Here we have an instance of conduction. In one case, the heat separated from the metal will affect the tallow at some distance, passing readily through, or among, the particles of the intervening air. In the other case, the heat first communicating with that part of the spoon in contact with the tea, it is, if we may employ the expression, pushed forward from particle to particle of the metal, along the handle, until it reaches its extremity. As radiation and conduction commonly operate together, they may be considered as different parts, or rather, different forms, of the same process-both equally dependent on that property peculiar to heat, by which it tends to diffuse itself in every direction, and among the particles of every species of matter, whatever may be its form, size, colour, or quality. Thus, if any number of vessels, some constructed of metal, others of wood, others of stone, and others of glass, each vessel containing a liquid of a different kind and at a different temperature, be placed in the same room, the liquids and the vessels containing them will, in a few hours, all arrive at the same temperature, which will be that of the air in the room. would, of course, be the result, with solid or with aëriform bodies, as with liquids. The same Radiation and conduction may be further explained by considering the former as operating at the surfaces of bodies, whilst the latter goes on throughout their interior parts. The rate at which heat is radiated and conducted by any substance, depends very much on the nature of the materials of which that substance is composed. Radiation is also influenced in a remarkable degree by the colours and other conditions of the surfaces of bodies. Those bodies into which heat enters with facility, and among whose particles it is transmitted rapidly, are called good conductors. Those, on the contrary, which offer considerable resistance to the progress of heat among their particles, are termed bad conductors. The latter are frequently denominated non-conductors, a description not philosophically correct; since every substance with which we are acquainted will conduct heat, although in some its transmission is exceedingly slow. Among good conductors the metals are the best; of the gold, silver, platinum and copper are nearly equal. The next in order are iron and zinc, then tin, and the slowest conductor of them all is lead. Wood, stone, and bricks are among the bad conductors: of this class the most perfect are wool, hair, cotton, the fur of animals, the feathers of birds, and especially the down of the swan. Liquids and aëriform bodies, when there is no motion among their particles, are bad conductors of heat. If freedom of motion be established, they become good conductors. The rate at which heat is radiated is dependent, in a remarkable degree,on the colour and other conditions of the surfaces of bodies. If any quantity, say, for instance, a pint, of boiling water be poured into a polished metal tea-pot, and an equal quantity of water, at the same temperature, into a rough black earthenware tea-pot, both the vessels standing in the same room, and at no great distance from each other, the water in the earthenware pot will cool down to the temperature of the surrounding air in less time than that in the metal pot. For a polished metal pot, if we substitute one whose exterior has become rough and tarnished by neglect or ill usage, the water will be found to cool quicker in that than in the other. In addition to the last-mentioned metal pot being rough and discoloured, if it be painted black or some dark colour, the rate of cooling of the contained water will thereby be still further accelerated; but it will be less rapid than in the earthenware pot. Hence we may learn, that a metallic tea-pot is the most useful, as respects keeping the tea hot, but to insure all its advantages it should be kept clean and well polished. The same will apply to tea kettles and various other culinary vessels. Those which are kept clean and bright will retain the heat of water, or other liquids contained in them, much longer than those whose exterior surfaces are rough and discoloured. The circumstances that assist in determining the rate at which heat is disengaged from the surfaces of bodies operate equally favourably upon that which is directed towards those surfaces. Any substance that radiates heat rapidly, will absorb it in the same proportion, provided that in each case the conditions are alike favourable. Those substances, whose surfaces are smooth and bright, and of a light colour, reflect heat; that is, they turn it aside from its straight course, and thus interrupt its progress. Those substances, whose surfaces are rough and dark-coloured, radiate and absorb heat. Hence that substance which reflects heat the most perfectly, is the very worst that can be selected for its radiation or absorption. Water or any other liquid may be made to boil in less time, all other circumstances being the same, in a rough and discoloured metallic vessel, than in one whose outside is perfectly clean and bright. If the metallic and earthenware tea-pots already mentioned, be both filled with cold water, say at the temperature of 45°, and placed in a room whose temperature is 70°, the water in the earthenware pot will acquire the temperature of the air in the room in less time than that in the polished metal pot; proving that the same conditions influence the absorption of heat that, in the first-cited experiments, would be seen to determine its radiation. In the houses of the wealthy, stoves are sometimes employed which are made of polished metal. This is the most injudicious arrangement that could possibly be devised for heating the apartments in which such stoves are fixed. On the same principle, it is improper to surround a fireplace with porcelain tiles; or, if we wish our feet to receive any benefit from a fire, to place in front of it a polished fender. Rough and darkcoloured surfaces are best adapted for domestic stoves. Such stoves are not only the most useful, but the most economical, since, in diffusing heat into the apartment by radiation, the benefits of the ignited fuel in the grate are materially increased. Blacklead, with which stoves are usually polished, could be very well dispensed with, were it not, that in this instance, as in many others, we cheerfully surrender a little scientific propriety rather than part with our early associations and habits of cleanliness. Nothing is more difficult than to form an accurate estimate of the temperature of different substances by means of our ordinary perceptions. If we would avoid frequent mistakes on the subject, we must constantly submit our sensations to the correction of our judgment. Heat and cold, as ordinarily experienced by us, depend on the previous temperature of the particular parts of the body in which these sensations may be induced; and the temperature and rate of conduction possessed by the substance with which such parts may be in contact. On a cold day in winter, if we descend into an underground cellar, or arched vault, the included air will communicate a sensation of warmth. On a warm day in summer, air at the same temperature, in the same cellar or vault, will produce the opposite sensation of cold. In winter the external air being at a lower temperature than that in the vault, we pass from a cold to a warm medium. In summer the air in the vault will be at a lower temperature than that of the external air, and we consequently pass from a cold to a warm medium. Notwithstanding the apparent contradictions in our sensations, it rarely happens that the temperature of the air in a cellar or vault is so high in winter as it is in summer. If we were to judge only by its effects on our body, we should pronounce a different decision. On examining dissimilar substances in the same room, with a view to ascertain their temperatures, if we have no better guide than our sensations, we shall arrive at very incorrect conclusions. Placing the hand successively in contact with a carpet, a table, a marble slab, a polished brass or iron fender, we shall, in the absence of any other information than that derived from our feelings, pronounce the table to be colder than the carpet, the marble slab to be colder than the table, and the fender to be colder than the marble. A thermometer will inform us that the several articles we have enumerated are all at an equal temperature. The different sensations produced by them are, therefore, entirely due to the difference in their rates of conducting heat. Wool is denominated a bad conductor. The heat in the hand placed in contact with a carpet, will pass through, or among, the fibres of the wool, but very slowly. Wood is a bad conductor, but it conducts more rapidly than wool. Compared with the carpet the table will feel cold, because in a given time a greater quantity of heat will pass from the hand to the table than from the hand to the carpet. Marble is classed among bad or imperfect conductors of heat, but it possesses this property in a more eminent degree than either of the beforementioned substances. Metals are good conductors. fender, therefore, will feel colder than the other articles, because, in a given time, it will abstract or carry away from the hand a greater quantity of heat than either the carpet, the table, or the marble slab. The A substance whose surface is smooth or polished, will excite the sensation of cold in a more intense degree than another substance, or a different part of the same substance, at the same temperature, whose surface is rough and irregular. This effect is chiefly mechanical, and it is occasioned by the more perfect contact that takes place between the hand and a smooth surface, than one which is rough and irregular. We may place the hand in contact with a bad conductor of heat without experiencing pain, whilst similar contact with a good conductor, at the same temperature, will inflict a severe wound. In the first instance the heat, moving slowly towards the hand, is easily dissipated; in the second, its motion being rapid, it accumulates, and destroys the parts in its immediate vicinity. For these reasons we perceive the propriety of adapting handles of wood to tea and coffee-pots, box-irons, and many other utensils that are employed at a high temperature. So also, folds of woollen cloth or leather are interposed between the hand and a heated metallic body, for the purpose of intercepting the heat. By constant exposure to the effects of a high temperature, the skin on the inside of the hands will become so thick and insensible as to resist a degree of heat that would scorch to the bone an unpractised hand. Instances are recorded of workmen employed in the smelting of copper, who could dip their hands into the liquid metal without experiencing pain. We knew a female servant who was in the habit of taking vegetables and other articles of food from a saucepan or pot of boiling water, with her hands, instead of using a fork or a ladle. Those persons who are exposed to a high temperature in their ordinary avocations, generally take the precaution to wear woollen clothing. Others, who voluntarily expose themselves to extraordinary degrees of heat, for the purpose of exciting wonder, or gaining a subsistence, are not endowed with any peculiar properties by which they resist its effects. Their secret consists in availing themselves of bad-conducting substances, covering their bodies with woollen garments, shielding their feet by wooden clogs, and carefully avoiding contact with metals, or other conductors of heat. It is possible to remain a short time in a room, constructed for the purpose, whose temperature is sufficiently high to broil a steak. This has been done without any very great inconvenience, by men whose testimony may be implicitly relied on. GREATEST COMMON MEASURE AND LEAST COMMON Find the G. C. M. (1) 700 and 2030. (3) 780 and 4095. (5) 42600 and 666456. Find the L. C. M. MULTIPLE. (1) 10, 25, 40, 24. Reduce to lowest terms. 81 2970 (2) 504 and 1232. (2) 6, 18, 27, 34. (4) 32, 96, 160, 192. (3) 2004. (4) 1879 (5) 1044. (6) 1852. (7) 10290. 4356 1802 1980 Reduce to their least common denominators. 9 4484 5 14410' (2), H. (3), (4) 7,177. 11, 11, 14 11 |