Chemistry

The next important agent in chemical investigations is heat. Many chemists have considered heat or caloric as a material substance, capable of entering into combination with all other kinds of matter ; but recent investigations seem to put it and light upon the same ground, and show them to be but certain states of matter, brought about by the undulations of an exceedingly subtile ether, which is considered to fill all the space in the universe not occupied by other matter. Heat appears to be an antagonist principle to attraction ; while tha latter binds together, the former tends to separate, the parts of bodies, first expanding them, then reducing them to the liquid state, and finally to that of gas or vapour. Sometimes it decomposes a body, separating it into its proximate or ultimate elements : so powerful, indeed, is heat, as a repulsive agent, that there can be no doubt that, if the temperature of the globe were sufficiently increased, every iolid mass on its surface would become liquid, and every liquid assume the state of vapour. We have an illustration of this con sequence in the different states of the same body on different parts of the earth's surface. Thus some bodies, as butter, oil, water, &c. which exist only as solids at the poles of the earth, are perfectly liquid at the equator. Ether, which is a liquid in this latitude, can only exist as a vapour in the torrid regions of the earth. Mercury also, which we see always in the liquid state, may, by exposure to the intense cold of a northern sky, be converted into a solid metal like lead. The most immediate and general effect of heat is expansion. If a poker or an iron ball be placed in a fire, it becomes heated, and assumes that state which is called ignition ; at which time its temperature is nearly equal to that of the surrounding fuel. In like manner, if a cup of cold water be placed within a vessel of hot water, the two speedily become of the same temperature, the one giving out caloric, and the other receiving it. If now either of these bodies that have changed their temperature be examined, it will be found to have changed its bulk also. This expansion of bodies by heat, and their consequent contraction by cold, are facts of the highest importance in the arts. The amount of expansion in different substances by a known increase of temperature, has therefore been carefully studied, and a variety of results, more or less accurate, have been obtained by different experimentalists. The property of expansion by heat, and contraction by cold, appears to belong to nearly all bodies upon which experiments have been made; at least in that class of bodies which com pose the mineral kingdom, for among vegetables the effects of heat are so mixed with other effects, that it is difficult to distinguish them. Some minerals, too, as clay, and several metallic substances, have been considered as excep tions to the general law ; but a more minute investigation has shown that they are only apparent. Clay contracts in consequence of the expulsion of mois ture, and the incipient fusion that occurs ; and some of the metals, as iron, bismuth, antimony, in fusion, expand in the act of setting or solidifying, but afterwards contract as they cool The force with which bodies expand is so great as to overcome every resistance that is opposed to it. The general fact of the expansion of solid bodies by heat may be seen by taking a metal cylinder a b that will just fit into a gauge c d, and pass through a circular hole e when cold. When made red hot, it will be found so much enlarged as to be incapable of passing through the hole, or fitting into the gauge, thus proving that it has been enlarged in all its dimensions. The cubic or solid expan sion is always three times greater than the linear. For example, if the expansion of a rod of steel be one-tenth of its length, the whole expansion will be three-tenths of its bulk. The expansion of liquids may be made evident by filling a Florence flask, or any bottle with a narrow neck, with cold water, and then applying heat to it ; the fluid will soon be observed to flow over the mouth of the bottle in consequence of its increased bulk. This fact is also familiarly illustrated by means of an ordinary tea-kettle, which, if it be quite filled with cold water, will be unable to contain the increased bulk occasioned by heat, and will consequently discharge a portion of its contents over the hearth long before the water reaches its boiling point. The expansion of air may be shown by a similar apparatus. If the flask full of air be inverted with its mouth under water, and the flame of a lamp or candle be applied to it, the air will be expanded, and will be seen escaping from the mouth of the flask in large bubbles. If the flame be removed, the air will cool, gradually contract its dimensions, and the water will rush up into the vessel to fill the space vacated by the air. The expansion of solid bodies is ascertained by what is called a pyrometer, for the construction of which see its name. The most general facts connected with expansion are as follows :-1. Nearly all solids, liquids, and gases, are expanded by heat, and contracted by cold ; and of these, the gases are most expansible, and solid bodies least. 2. Different solids and liquids are differently expanded by the same degree of heat, but gases and vapours are equally and equably expanded by equal portions of heat. 3. In the same body, whether solid or liquid, the expansion by a in quantity of heat, is greater at higher than at lower tem peratures ; but In gases and vapours, the expansions are equal, by equal addi tions of heat at all temperatures. 4. The expansion of atmospheric air, gases, and vapours, not in contact with the liquids from which they have been gene rated, is equal to one four hundred and eightieth part of the bulk they occupy at 320 of Fahrenheit's thermometer. The absolute dilatation in length of several of the most generally used substances, is shown in the following table :— . One of the most important applications of the expansibility of liquids, is in the construction of the thermometer, which consists essentially of a closed tube, containing a liquid, the expansion of which can be conveniently observed. To

construct a thermometer, a glass tube of very small bore is usually taken, and a bulb, globular, cylindrical, or conical, is blown at one end, which bulb and part of the tube are then filled with perfectly pare mercury. The tube must then be hermetically sealed by melting the top by means of a blow-pipe flame. Previous to closing the top, however, the mercury must be exposed to heat so as to completely fill the tube; and while the tube is full, the sealing must be effected. When the mercury cools, the surface will subside, and a vacuum will be left in the upper part of the tube. The thermometer is then complete, except its graduation; this is accomplished as follows:—The bulb is to be plunged into snow or ice that is just melting, and a mark made on the tube at the point at which the mercury stands; this is called the freezing point; and in the thermo meters common in this country, called Fahrenheit's (from the inventor's name), it is marked 320. The tube must next be placed in boiling water, or its steam, in such a way that the whole of the mercury may be exposed to heat. When the mercury has risen and is stationary,. another mark must be made i • this ' denotes the boiling point of water, and is marked 2120. The space between the boiling and freezing points must next be divided into 180 equal parts, and the graduation is complete. In the centigrade thermometer used in France, the freezing point is marked and the boiling point the intervals being divided into 100 equal parts or degrees. One degree of Fahrenheit is fore equal to four-ninths of the centigrade ; and one of the centigrade to fourths, or 2i0 Fahrenheit. This is but a brief and general view. For further information, see THER11011111TIR.

It has been already stated that fluidity is the result of heat. Ever solid may be liquefied; and many of them, as well as liquids, may be vaporized at a certain elevation of temperature and there can be little doubt that every known liquid may be solidified if we possessed the means of sufficiently reducing the temperature. Several of the gases, by the united operation of cold and pressure, have already been reduced to the fluid state, and nearly every known liquid may be frozen. The temperature at which a solid body assumes the liquid state is called its fusing or melting point. If the substance be ordinarily in the liquid state, this point will be its freezing, concreting, or congealing point. The concreting or congealing temperatures of various substances will be found in the following table :— The temperature at which bodies melt is fixed, but the congealing point is modified by circumstances. Many liquids may be cooled considerably below their usual freezing points, before they assume the solid state. This is the case with water, which" may be cooled 100, or even 200 below its congealing point without freezing ; but if a small spicula of ice be dropped into it, or the water caused to vibrate, it will instantly solidify. The same phenomena occur with saline solutions. A hot saturated solution of sulphate of soda may be cooled to 50' under i film of oil, and it will remain liquid, and bear to be moved about in the hand without any change ; but if the vessel containing it be placed on a vibrating table, crystallization will instantly take place. One remarkable fact attends the cooling of bodies below their usual freezing point; viz. at the instant solidification occurs, the temperature of the mass rises to the proper freezing point. Thus, if a portion of water be placed in an atmosphere of 210, the liquid will cool and remain fluid at this temperature, till, by touching it with a piece of ice, or causing it to vibrate, we make it freeze, when it instantly rises to 320, or 110 above the surrounding medium. This curious fact was first explained by Dr. Black, and gave rise to the knowledge of the latent heat or caloric of fluidity of bodies. Dr. Black suspended two glass globes, of the same size, near each other; the one filled with ice at 320, the other with water at 330 : in half an hour the water had risen to 400, but it took ten hours and a half to liquefy the ice, and heat the resulting temperature to 400. In this experiment we observe that 70 of beat entered the globe in 1 half-hour, but 21 half-hours were required to melt the ice and raise its temperature to If from the product 7 X 2,1=1470, we subtract 70, which the water was above we have 1400 as the measure of a quantity of heat that has entered the sub stance without being appreciable by the thermometer. Another simple experi ment,• showing the same result, is made by mixing a pound of pulverized ice at 320, with a pound of water heated to 1720 ; the ice is instantly melted, but the temperature is only 320. Here, then, 140* of heat have disappeared with out raising the temperature. From this circumstance, the quantity of heat that always disappears when a body assumes a fluid state, is called latent heat, or caloric of fluidity. The latent heat of different substances varies, as may be seen in the following table by Dr. Irvine : Sulphur 143 68 Zinc 493 Spermaceti 145 Tin 500 Lead 162 Bismuth 550 Bees' wax 175 The quantity of heat that disappears in the liquefaction of solids, explains the origin of that cold which always accompanies solution, and enables us to apply the process of artificial cooling by what are termed freezing mixtures. Snow and salt, when rapidly mixed, dissolve and produce a reduction of temperature equal to 380. The more rapid the liquefaction the greater the cold ; hence, if the snow and salt be placed in a pan over the fire, and a glass tube containing water be immersed in it, the water in a few seconds will be found frozen. The solution of all crystallized salts is attended with a depression of temperature, which in general increases with the solubility of the salt ; the like occurs with certain metals. If a solid amalgam of bismuth be mingled with a solid amalgam of lead, they become fluid, and the thermometer sinks. A variety of experiments have been made on the frigorific effects of different mixtures, some of which are stated in the following table, abridged from Mr. Walker's Experiments on Frigorifsc Mixtures.