105. Cooling Effects of Evaporation.-As evaporation consumes heat, it is a cooling process. We experience this in the cold sensation of evaporating a few drops of ether from the hand. As the perspiration evaporates from the skin, it becomes a powerful cooling agency and regulator of bodily temperature, while the vapor which escapes from the breath, by its absorption of heat, exerts a cooling effect within the body. 106. The Cryophorus, or Frost-Bearer, is an instrument which strikingly illustrates this principle. It consists of a tube with a glass bulb at each extremity, FIG. 43. ་ The Cryophorus. The one of which contains a little water. Air The 107. Dew-Point.-The air always contains moisture, the amount of which varies with the temperature. power of the air to absorb moisture is called its capacity for absorption. When it contains as much as it is capable of holding at a given temperature, it is said to be saturated, and any lowering of the temperature condenses it in the form of clouds, mist, fogs, dew, etc. The degree of temperature at which the moisture is condensed is called the dew-point. If the temperature of the air has to fall but a few degrees before moisture is deposited, the dew. point is said to be high, and there is much moisture in the air; while, if the temperature must fall far, the dew-point is low, and the air contains less moisture. It is obvious, therefore, that, by finding these two points of tempera ture, one can easily obtain the amount of atmospheric humidity. FIG. 44. 108. The Hygrometer.-This is an instrument for measuring atmospheric moisture. Daniell's hygrometer, Fig. 44, is constructed on the principle of the cryophorus. The long limb ends in a glass bulb b half filled with ether, into which dips a small thermometer. The bulb a on the short limb is empty and covered with muslin. The temperature of the air is shown by another thermometer, c, affixed to the stand of the instrument. When an observation is to be made, a little ether is poured upon the muslin, and, as it evaporates, the temperature of the other bulb becomes reduced. When it is sufficiently cold to condense the moisture of the air, it will be covered with dew. The thermometer in the tube b shows at what temperature this deposition takes place, and, of course, gives the dew-point. The amount of moisture in the air of our artificiallyheated rooms is a matter of great importance to health, and the hygrometer is very valuable in enabling us to determine it. Daniell's Hygrometer. 109. Volume and Density of Vapor.-Equal bulks of different liquids generate unequal volumes of vapor. Water yields a larger amount than any other liquid. While a cubic inch of water gives 1,694 inches of vapor, a cubic inch of alcohol yields 528, one of ether 298, and of oil of turpentine 193. But, the less the volume of vapor, of course the greater its density. The density of vapor is increased, either by cold or pressure. The point at which its temperature cannot be further lowered, without returning to the liquid state, is called its maximum density. 110. Elastic Force of Vapors.-All vapors are elastic, and have a tendency to diffuse themselves through space, exerting more or less force against any obstacle that resists their expansion. This expansive force of vapors is called their elastic force or tension. The expansive force of heat, acting through the vapor of water, is the impelling power of the steam-engine. FIG. 45. 111. Distillation consists in vaporizing a liquid by heat in one vessel, and condensing it by cold in another, Fig. 45. The object may be either to separate a liquid from nonvolatile substances dissolved in it, as in distilling water, to purify it from foreign ingredients, or to separate two liquids which evaporate at different temperatures, as alcohol and water. In the latter case, the heat is carried just high enough to vaporize the most volatile liquid. The product of the process is called the distillate. When solids are vaporized, the process is termed sublimation, and the condensed vapor a sublimate. Distillation. FIG. 46. 112. Condensation of Gases.-When a gas loses heat enough to change it to the liquid or solid state, it is said to be condensed. Under the joint effect of pressure and extreme cold, many gases once considered permanent have been reduced to liquids, and some even to the solid condition. Dr. Faraday effected this by a very simple method. He placed the materials from which the gas was to be generated in one end of a glass tube bent in the middle, which was then hermetically sealed, Fig. 46. The expanding gas confined in so small a space exerted a tremendous pressure, the force of which condensed a portion of it into a liquid in the other end of the tube, which was immersed in a Condensation Tube. freezing mixture to facilitate the process. By this method, and at a temperature of −166°, he succeeded in liquefying carbonic dioxide, chlorine, ammonia, and several other gases, More recently M. Natterer, of Vienna, applied a cold of -220° F., and a pressure of 3,000 atmospheres; but some of the gases, as oxygen, hydrogen, nitrogen, refused to liquefy, even under this tremendous force. 113. Heat and Chemical Action.-Besides these physical changes and transformations, heat is also employed to effect innumerable chemical changes. By means of lamps, baths, and furnaces, the chemist is able to subject bodies to all degrees of temperature, and to promote and modify their action on each other. By its repellant action upon the constituent parts of matter, heat overcomes chemical attraction, destroys compounds, and brings new affinities. into play with the production of new substances. At a sufficiently high temperature, indeed, we can conceive the repulsion to be so great that all affinities are suspended, and the chemical elements are dissociated. 4. The Nature of Heat. 114. Heat and Cold.-The difference between heat and cold is merely one of degree, and we must be careful not to misinterpret their impressions upon ourselves. If we plunge one hand in ice-water and the other in hot water, and then transfer both to water intermediately warm, it will seem hot to the one and cold to the other. Indeed, if we trusted our ordinary sensations, we should believe in two opposite principles of heat and cold, a doctrine which was long advocated until it was found that these are merely relative, and that cold is but the absence of heat. Intense heat and intense cold produce the same sensations; frozen mercury blisters the flesh like hot iron. Putting aside, then, our sensations, what is it that we know concerning the nature of heat? 115. The Caloric Hypothesis.-In the foregoing brief statement of the actions and effects of heat, we have confined ourselves to facts which can be shown by experiment, and have spoken of heat merely as a force, or agent. But how are its effects produced? It has long been regarded as a kind of matter—a subtile fluid-an imponderable element, whose entrance into our bodies produces warmth, and its escape cold. This fluid caloric was supposed to be stored up in the interstices of bodies, some holding more than others, according to their various capacities. It was assumed to have an attraction for the particles of matter, and to combine with them, while its own particles are self-repulsive, and thus cause the atoms with which they unite to repel each other. This notion of the materiality of heat is now generally abandoned in the scientific world. 116. Grounds of a New Theory.-Facts were pointed out in the last century by Count Rumford which were wholly inconsistent with the caloric hypothesis, and many other facts of similar import have been since observed. If an iron bar is struck upon an anvil, its temperature is raised, and, if it continues to be hammered, it may be made red hot. Two sticks may be rubbed together till they take fire; and water may be agitated by friction till it boils. It is a general law, that arrested mechanical motion produces heat, and that the amount of heat produced depends not at all upon the capacities of bodies for heat, but upon the amount of mechanical force expended. Prof. Rood made the following beautiful experiment: A ball weighing a pound is allowed to fall from a height of one or two inches on a thermo-electric couple made by soldering together German-silver and iron. The heat thus developed generates a current of electricity which is measured by a galvanometer. The amount of heat generated was found to be directly proportional to the distance through which the ball fell. It has been demonstrated |