« PreviousContinue »
will also boil with much less heat on lofty mountains than in the valleys.
Bodies differ very much with respect to the facility with which heat passes through them. Those which transmit caloric easily are called conductors of caloric; and, according to the power of doing so, they are termed good or bad conductors. Those which do not transmit heat at all, or with great difficulty, are called non-conductors. However, it should be observed, that, perhaps, no substances are absolutely non-conductors; but liquids and gases admit the passage of caloric through them with such great difficulty, that, for practical purposes, this division is found useful. Liquids carry heat chiefly by transportation ; the part of the liquid that is heated rises to the surface, and gives place to another which is warmed in its turn, and so on until the whole has been heated.
Many very important applications of this principle have been made by Count Rumford to æconomical purposes.
He showed that a stratum of confined air was one of the best modes of preventing the escape of heat.
The best conductors of heat are metals, and the best non-conductors are fluids and porous substances. Charcoal is an excellent non-conductor. Heat
may be excited by mere friction; and this, probably, was the earliest mode of obtaining it to procure fire. It is still practised among uncivilized nations. For this purpose they take two pieces of dry wood, one about eight or nine inches long, and the other piece quite flat. They cut a blunt point upon the first, and, pressing it upon the other, they whirl it round very quickly, holding it
between both their hands, as we do a chocolate mill. In a few minutes the wood takes fire. If the irons of the axle of a coach-wheel be left without grease or oil, they will become so hot as to set fire to the wheels; and accidents of this kind sometimes happen.
It is no uncommon practice in the country, for a blacksmith to hammer a piece of iron till it becomes red hot, as a substitute for a tinder-box. The heat excited by the boring of a cannon is sufficient to cause water to boil.
Heat is also produced by collision; when a piece of hardened steel is struck with a flint, some particles of the metal are broken off, and so violent is the heat produced by the stroke, that they are rendered red hot, and melted. If the fragments of steel be caught upon a piece of white paper and examined with a microscope, they will be found to be spherules, and highly polished, showing that they had been fluid.
No heat seems to follow from the percussion of liquids in soft bodies,
The instruments for measuring heat by the expansion of bodies are, thermometers for fluids, and pyrometers for solids.
A thermometer is a hollow tube of glass, hermetically sealed, and blown at one end into the shape of a hollow globe, or bulb. The bulb and part of the tube are filled with mercury, which is the only fluid that expands equally. When we immerse the bulb of the thermometer in a hot fluid, the mercury expands, and, of course, rises in the tube; but when we plunge it into a cold body, the mercury contracts, and, of course, falls in the tube. The rising of the mercury, therefore, indicates an increase of heat; its falling, a diminution of heat.
To facilitate the observation, the tube is divided into a number of equal parts, called degrees, or there is a divided scale attached to it.
This scale is graduated in different manners by different nations : Fahrenheit's scale is that always used in this country.
The standard points are obtained by freezing and boiling water, degrees of heat which are constantly the same in nature. The heat at which the mercury stands, when immersed in each, being marked, the distance between them is divided into 180 parts, and 32 parts of the same size are continued downwards, so that 32° shows the heat of freezing water, and 212° that of boiling water. Water cannot be made hotter than this in open vessels, because it then becomes converted into steam, or aqueous gas.
The mercurial thermometer, it is evident, cannot measure degrees of heat above that of boiling mercury, nor below that of freezing mercury; the former is 600°, and the latter 40° below 0 of Fahrenheit's scale.
For greater degrees of cold, thermometers of spirits of wine, or essential oil, are used ; and to measure those higher degrees of heat to which the thermometer cannot be applied, pyrometers are employed. An instrument of this kind was invented by the late Mr. Wedgewood. It consists of two pieces of brass, fixed so as to form an angle, having the legs divided into equal parts. Pieces of baked clay are prepared for this scale, so as to fit the brass at a certain place. If then the piece of clay be exposed to the heat required to be examined, it will contract in its dimensions, and, when again applied to the brass scale, it will be seen how much it has contracted. By this the in
tensity of the heat is ascertained, for the clay of which these pieces are prepared, has the property of contracting regularly, according to the degree of heat.
This is an exception to the general law of bodies expanding by heat; the expansion of melted metal in the act of cooling is another, as likewise the expansion of water in the act of freezing.
The greatest degrees of heat which can be raised have been produced by concentrating the solar rays with a mirror or lens, or by supplying a blowpipe with oxygen gas; or, what is still more powerful, by a mixture of oxygen and hydrogen. This last method has been but lately employed, and produces a far greater degree of heat than any other. . The mixture is itself of an explosive nature, and, therefore, without proper precaution, exceedingly dangerous. The greatest degree of cold known to have been produced has been obtained by mixing snow with certain salts. The best salt for this purpose is muriat of lime. If this be mixed with dry light snow, and stirred well together, the cold produced will be so intense as to freeze mercury in a few minutes. Salt and snow also produce a great degree of cold.
Evaporation, likewise, produces cold. The method of making ice artificially in the East Indies depends upon this principle. The ice-makers at Benares dig pits in large open plains, the bottom of which they strew with sugar-canes, or dried stems of maize, or Indian corn. Upon this bed they place a number of unglazed pans, made of so porous an earth that the water oozes through their substance. These pans are filled towards evening, in the winter season, with water which has been boiled, and are left in that situation till morning,
when more or less ice is found in them, according to the temperature of the air ; there being more formed in dry and warm weather than in cloudy weather, though it may be colder to the human body.
Every thing in this operation is calculated to produce cold by evaporation; the beds on which the pans are placed suffer the air to have a free passage to their bottoms, and the pans, constantly oozing out water to their external surface, are cooled by the evaporation of it.
In Spain, they use a kind of earthen jars called buxaros, the earth of which is so porous, being only half baked, that the outside is kept moist by the water which filters through it; and, though placed in the sun, the water in the jar becomes as cold as ice.
It is a common practice in China, to cool wine, or other liquors, by wrapping a wet cloth round the bottle, and hanging it up in the sun. The water in the cloth evaporates, and thus cold produced.
Ice may be produced, at any time, by the evaporation of ether. Take a thin glass tube, four or five inches long, and about two or three eighths of an inch in diameter, and a two-ounce bottle of ether, having a tube drawn to a point, fitted to its neck. Pour some water into the glass tube, and let a stream of ether fall upon that part of it containing the water, which, by that means, will be converted into ice in a few minutes. If a thin spiral wire be introduced into the tube before the water is poured in, the ice will adhere to it, and may be drawn out.