100° F. (38° C.), so long as any ice or tallow remains unmelted, but when the melting is complete it will commence rising. The degree of heat at which a body melts is called its melting point. Every substance has its own melting point, sometimes above and sometimes below the freezing point; for example, lead melts at 620° F. (327° C.), silver at above 1800° F. (1000° C.), solid mercury at 38° F. (—39° C.). If the two vessels containing the melted ice and the melted tallow are placed in the cold, it will be observed that the tallow soon hardens at about 95° F. (35° C.), but the water does not until the mercury has fallen to 32° F. (0° C.). Thus the congelation of fluids takes place at about the same temperature as that at which they pass from the solid to the fluid state. Many substances, charcoal for instance, have never yet been melted, and others have never been frozen, as, for instance, alcohol; but it is very probable that, when some method of producing still greater degrees of cold and heat is discovered, we shall succeed in rendering all solid bodies liquid, and all liquids solid. Latent Heat.-Experiment 3.-Put two vessels of equal size on the top of a warm oven, one of them containing a pound of snow at 32° F. (0° C.), and the other a pound of water at 32° F. (0° C.); when the snow is melted, remove them both. By the touch merely it will be perceived that the snow-water is still cold, while the water in the other vessel has become warm; and the thermometer will indicate that in the former the temperature is at 32° F. (0° C.), in the latter at 174° F. (79° C.). As both vessels have received equal degrees of heat, and when placed upon the oven were of the same temperature; the question suggests itself, What has become of the 142° of heat imparted to the vessel filled with snow? The reply is, This heat has been absorbed by the snow, thus converting it into a fluid,-melting it. Experiment 4.-Put one pound of snow at 32° F. (0° C.) into the vessel containing the water heated at 174° F. (79° C.), and then examine it with the thermometer; as soon as all the snow has disappeared, the quicksilver will fall to the freezing point. Consequently, the snow has taken from the hot water 142° F. (79° C.) of heat, and has thus become liquid. The heat which is lost in the melting of ice and other solids is employed in overcoming the cohesion of the solid. The force of cohesion is set free in the manner described in the previous chapter, and when the liquid again becomes solid the heat is reproduced; the exertion of the cohesive force being attended with the evolution of heat. Heat lost during liquefaction is often called latent heat, because it was formerly supposed to lie hidden, or stored up in the liquid. In a limited sense the term may still be retained, for although we now know that heat is nothing but motion, there can be no doubt that the molecules (particles) of the liquid are in incessant motion, which motion they lose to a great extent when the substance becomes solid. (2) From Liquid to Gas.-Boiling of Liquids.—Most liquids, when heated to a certain temperature, different for every liquid, boil and become converted into vapour or gas. It must be understood that there is no real difference between the condition of a gas and of a true vapour, except in regard to the temperature at which they are formed. Many of the so-called gases can by great cold or pressure be converted into liquids; and, on the other hand, when a liquid is converted by heat into vapour, the vapour has all the properties of a true gas. In fact, some substances are known which are liquid in cold weather and gaseous in hot. Gases which become liquid at ordinary temperatures are generally called vapours. The phenomena of boiling are most conveniently studied in the case of water. Fig. 15. 212 Experiment 1.-Two-thirds fill a flask with spring water, and heat it gently over a lamp (Fig. 15). In a short time numerous little bubbles will appear on the walls of the flask, which will gradually increase in size, and rise towards the surface. These bubbles consist of the gases of air, which are expanded by the heat and expelled from the water. All spring-water contains gases in solution, and to these is chiefly due its refreshing taste, which is not found in boiled water or in that which has been standing for some time. Afterwards, when the water has become quite hot, larger bubbles appear on the hotter part of the flask, which also ascending become smaller, and entirely disappear before reaching the surface of the water; they consist of gaseous water (steam), which condenses as it comes in contact with the cooler liquid above. The collapsing of the particles of water at the places where these steam-bubbles disappear occcasions that peculiar noise which precedes boiling, and which is commonly called the singing of the water. When the whole mass of water is heated to 212° F. (100° C.), these bubbles no longer condense, but rise to the surface, where, surrounded by a thin film of water, they remain quiescent for a few seconds, and then, their watery mantle again sinking, they finally burst. This is the boiling of water. It boils at 212° F. (100° C.); other liquids boil more readily-alcohol, for instance, at 176° F. (80° C.); others again more difficultly-mercury, for instance, at 662° F. (350° C.). Latent Heat of Steam.-The space above the boiling water in the interior of the flask appears empty, but it is in fact filled with aeriform water, which has displaced the air that was in it. This aeriform water is called steam. It is almost 1700 times lighter than water, because a measure of water yields nearly 1700 measures of steam at 212° F. (100° C.). Within the flask the steam is transparent and invisible, but in the open air it ascends in the form of white clouds, which greatly increase if cold air is blown into the flask by means of a glass tube; for on cooling the transparency of the vapour is disturbed, on account of the formation of drops of water, so small and light as to float in the air. Clouds also consist of this partly-condensed vapour. As the condensation increases, the drops become so large and heavy, that they descend as rain. A thermometer immersed in boiling water indicates 212° F. (100° C.); if placed in the steam immediately above, it shows the same; and this temperature will not rise higher, however long the boiling be continued, or however strongly the flame of the lamp be urged. This is similar to what occurs in the melting of snow; heat disappears, and its disappearance proceeds from the same cause in both cases; steam requires heat for its production, and this heat disappears, or becomes latent during its formation, and reappears, or becomes sensible during its condensation. We have, in fact, to study the latent heat of steam as well as the latent heat of water. Experiment 2.-Adapt the shorter limb of a bent glass tube, by means of a perforated cork, to the neck of a flask, and pass the longer limb to the bottom of a beaker-glass or common tumbler. Pour into each of these two vessels five ounces of ice-cold water, and the gradually heat until it boils. Note flask. The steam Fig. 16. LA formed in the flask has no other outlet than through the tube into the water, where it condenses, until the contents of the second glass reach the temperature of 212° F. (100° C.), and boil. = Both of the vessels must now be weighed; and it will be found that the flask weighs one ounce less, and the beakerglass one ounce more, than before; consequently, one ounce has passed from the former as steam, and has been condensed again in the latter; and yet this ounce of steam has raised five ounces of water from 32° to 212° F. (0° to 100° C.), that is, through 180° F. (100° C.). Now five ounces raised 180° is the same as one ounce raised 180 x 5 900°, so that the ounce of steam in condensing gave out 900° F. (500° C.) of heat. This is expressed by saying that the latent heat of steam is 900° F. (500° C.). More exact experiments give as the true number 967° F. (537° C.). The property of steam to give out a large quantity of heat during condensation peculiarly adapts it for the heating of other bodies, the burning of them being thus guarded against, as the heat of steam in open vessels can never exceed 212° F. (100° C.). Apothecaries avail themselves of steam in the preparation of infusions, decoctions, and extracts; it serves for many of the processes of cookery, and for the distillation of spirits and other liquids; it is employed in dyeing and bleaching establishments, and is often resorted to for heating apartments, buildings, laundries, &c. All gases give out heat when they condense into liquids, but the amount of heat so evolved is only known in a few cases. Steam gives more than any other known gas. Evaporation. Aqueous Vapour.-Water exposed in a vessel to the open air disappears in summer more rapidly than in winter; the heat of the air converts it into gas-it evaporates. The same happens as in evaporation over the fire, only in the former case evaporation takes place without any visible motion of the water, owing to its becoming aeriform, not throughout the whole mass at once, but upon the surface only. The vapour rises in an invisible form in the air. Warm air, indeed, takes up more of it than cold, but a fixed quantity of it only for each temperature. Thus one hundred measures of air at 32° F. (0° C.) absorb two-thirds of a measure of vapour; at 50° F. (10° C.), one measure and a quarter; at 68° F. (20° C.), two and an eighth measures, &c. If the air has not absorbed all the vapour which it can, it eagerly takes up more, as, for example, when one hundred measures of air at 68° F. (20° C.) contain only one or one and a half measures of vapour; it is then called dry air, and wet articles are soon dried in it by rapid evaporation. But if it be already saturated with vapour it is called moist air; and damp articles cannot be dried in it, or at least but slowly. If yet more vapour be added to this saturated atmosphere, or if it be cooled, then the excess separates in visible particles, called mist or fog when they lie near the surface of the earth, and clouds when they float in the higher regions of the atmosphere. The white smoke which in winter is seen rising from the chimneys, the visibility of the breath in frosty weather, and the smoking of rivers in winter and after a storm, are phenomena of the same kind. If the cooling of the air is occasioned by a cold solid body, the vapour is then condensed in small drops of water, as may be observed on the outside of a cold glass when brought into a warm room, and the deposit of moisture on the inside of our window-panes, when cooled by the external cold air. The temperature at which this occurs is called the dew-point, |