but when we plunge it into a cold body, the mercury con tracts, and of course falls into 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: 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° shew 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. Wedgewood's pyrometer 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 intensity of the heat is ascertained, for the clay of which these picces 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. 320. The greatest degrees of heat which are known, have been produced by concentrating the solar rays with a mirror, or lens, or by supplying a blow-pipe with oxygen gas. The greatest degree of cold known to have been produced, has been obtained by mixing snow with certain salts, as 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 stalks of maize, or Indian corn. Upon this bed they place à number of unglazed pans, made of so porous an earth, that the water oozes through their substance. These pans are filled towards the 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 is produced. Experiment. 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. Čaloric is thrown off from bodies in straight lines, in the same manner as light; and is capable of being reflected, and collected into a focus by means of mirrors. If a heated body, as a ball of iron, be placed opposite to a large concave mirror, a thermometer held in the focus of the mirror, will rise immediately. If some ice be put in the place of the ball, the thermometer will fall, seeming to prove that cold is something positive and real, and not merely the absence of heat; but this is explained by philosophers upon the same principle, and is not considered by them as a proof, that cold, like heat, is a distinct substance. SECTION II. MISCELLANEOUS PHENOMENA. Effects of Intense Cold. 321. In Iceland the thermometer frequently falls to zero, which is thirty-two degrees below the freezing point. At Hudson's Bay it has been known to sink even fifty degrees lower. When stones or metals, which have been exposed to such degrees of cold, are touched by the tongue, or the softer parts of the human body, they absorb the heat from those parts with such rapidity, that the flesh becomes instantly frozen and mortified, and the principle of life in them is extinguished. Some French academicians who made a journey to the north of the Baltic, and wintered under the polar circle, found it necessary to use all possible precautions to secure themselves from the dreadful cold which prevailed. They prevented as much as possible the entrance of the external air into their apartments; and if at any time they had occasion to open a window or door, the humidity of their breath, confined in the air of the house, was condensed and frozen into a shower of snow; their lungs, when they ventured to breathe the cold. air, felt as if they were torn asunder; and they often heard the rending of the timber around them, by the expansive power of the frost on the fluid in its pores. In this terrible cold the thermometer fell to 33° below zero. The most intense cold ever known in the neighbourhood of London was on December 25, 1796, when the thermometer indicated 2o below zero. The following narration will show the solidity that water is capable of acquiring when divested of a large portion of its caloric:-During the severe winter of 1740, a palace of ice, fifty-two feet long, sixteen wide, and twenty high, was built at Petersburgh, according to the most elegant rules of art. The river Neva afforded the ice, which was from two to three feet thick, blocks of which were cut, and embellished with various ornaments. When built up, the different parts were coloured by sprinkling them over with water of various tints. cannons, made of and mounted with ice, with wheels of the same matter, were placed before the palace ; and a hempen bullet was driven by one of these cannon, in the presence of the whole court, through a board two inches thick, at the distance of sixty paces. Imperial mistress of the fur clad Russ, Six When thou would'st build ;-no quarry sent its stores And make thy marble of the glassy wave. Silently as a dream the fabric rose; Ice upon ice the well adjusted parts Were soon conjoin'd; nor other cement ask'd Than water interfused to make them one. American Water-burner. COWPER. 322. This apparatus was invented by Mr. Morey of New Hampshire. The construction is very simple: tar is intimately mixed with steam or vapour of water, and made to issue, with a force proportional to the pressure of the steam, from a small orifice, like that in the jet of a blow-pipe, and is there fired. The flame, although the combustible substances issue from so small an orifice, is as large as that of a common smith's forge, and is unaccom panied with smoke; when this flame is directed against the bricks in the back of a fire-place, they soon become heated to redness; if iron or steel filings be thrown into the flame, they burn with a sparkling brilliancy, similar to iron wire in oxygen gas. A few experiments have been made to ascertain the effect of steam on burning bodies, and to learn whether it probably suffered decomposition when issuing mixed with tar from the jet of the Water Burner.' If a jet of steam, issuing from a small aperture, be thrown upon burning coal, its brightness is increased, if it be held at the distance of four or five inches from the pipe through which the stream passes; but, if it be held nearer, the coal is extinguished, a circular black spot first appearing where the steam is thrown upon it. The steam does not appear to be decomposed in this experiment: the increased brightness of the coal is probably occasioned by a current of atmosphe ric air produced by the steam. If the wick of a common oil lamp be raised so as to give off large columns of smoke, and a jet of steam be thrown into the flame, its brightness is a little increased, and no smoke is thrown off. If spirits of turpentine be made to burn on a wick, the light produced is dull and reddish, and a large quantity of |