have been condensed, by cold and pressure, into liquids, and many liquids have been solidified. The most important circumstance relative to liquefaction is the discovery of Dr Black, that a large quantity of heat disappears, or becomes insensible to the thermometer, during the process. If ice be placed above a lamp, it will indicate a gradual rise of temperature-as may be ascertained by placing a thermometer in it-till it arrive at 32° and begin to melt; but no further rise will take place during the process of liquefaction. It will remain stationary at 32°, notwithstanding that it is all along receiving accessions of heat from the lamp, till the process is completed; and all the heat which is added will become insensible. The explanation which Dr Black gave of these phenomena constitutes what is called his doctrine of latent heat. He considered that heat loses its property of acting on the thermometer, in consequence of combining chemically with the solid substance, and that the liquefaction is the result of this combination. As so much heat disappears during liquefaction, it follows that heat must be evolved when a liquid passes into a solid. This may easily be proved. The temperature of water in the act of freezing never falls below 32° F., though it be exposed to an atmosphere in which the thermometer is at zero. It is obvious that the water can preserve its temperature, in a medium so much colder than itself, only by the heat which it loses being instantly supplied; and it is no less clear that the only source of supply is the heat that was insensible in the fluid. Vaporization.-Vaporization is conveniently studied under two heads, Ebullition or Boiling, and Evaporation. In the first, the production of vapour is so rapid that its escape gives rise to a visible commotion in the liquid; in the second it passes off quietly and insensibly. The temperature at which vapour rises with sufficient freedom to cause the phenomena of Ebullition, is called the boiling-point. The heat requisite for this effect varies with the nature of the fluid. Thus, sulphuric ether boils at 96° F., alcohol at 173°, and pure water at 212°; while oil of turpen tine must be raised to 316°, and mercury to 660°, before either shows marks of ebullition. Even the boiling-point of the same liquid is liable to be affected by several circumstances. The nature of the vessel has some influence upon it. Thus, pure water boils precisely at 212° in a metallic vessel, and at 214° in one of glass. It is likewise affected by the presence of foreign particles. But the circumstance which has the greatest influence is variation of pressure. All bodies upon the earth are constantly exposed to the pressure of the atmosphere. Liquids are exposed to this pressure as well as solids, and their tendency to take the form of vapour is very much counteracted by it. In fact, they cannot enter into ebullition at all till their particles have acquired such an elastic force as enables them to overcome the pressure upon their surfaces; that is, till they press against the atmosphere with a force equal to that with which the atmosphere presses against them. The only time at which the pressure of the atmosphere is equal to a weight of 15 pounds on every square inch of surface, is when the barometer stands at 30 inches, and then only does water boil at 212° F. If the pressure be less - that is, if the barometer fall below 30 inches then the boiling-point of water, and of every other liquid, will be lower than usual; or, if the barometer rise above 30 inches, the temperature of ebullition will be proportionally increased. This is the reason why water boils at a lower temperature on the top of a hill than in the valley beneath it; for as the column of air diminishes in length as we ascend, its pressure must likewise suffer a proportional diminution. The ratio between the depression of the boiling-point and the diminution of the atmospherical pressure is so exact, that it has been proposed as a method for determining the heights of mountains an elevation of 530 feet makes a diminution of one - degree of Fahrenheit. The influence of the atmosphere over the point of ebullition is best shown by removing its pressure altogether. Fluids boil in vacuo at a temperature 140 degrees lower than in the open air. Thus water boils at 72° F., alcohol at 33°, and ether at -44°. This that a liquid is not necessarily hot because it boils. proves The formation of vapour is attended, like liquefaction, with a loss of sensible heat. This is proved by the wellknown fact, that the temperature of steam is precisely the same as that of the boiling water from which it rises; so that all the heat which enters into the liquid is employed solely in converting a portion of it into vapour, without affecting the temperature of either. The heat which then becomes latent is again set free when the vapour is condensed into water. Evaporation takes place at common temperatures, as may be proved by exposing water in a shallow vessel to the air for a few days, when it will gradually diminish, and at last disappear entirely. Most fluids, if not all of them, are susceptible of this gradual dissipation; and it may also be observed in some solids, as, for example, in camphor. Those liquids whose boiling-point is lowest, evaporate with the greatest rapidity. The chief circumstances that influence evaporation are extent of surface, and the state of the air as to temperature, dryness, stillness, and density. The effect of heat in promoting it may easily be shown by putting an equal quantity of water into two saucers, one placed in a warm, the other in a cold situation. The former will be quite dry before the latter has suffered an appreciable diminution. When water is covered by a stratum of dry air, the evaporation is rapid even when the temperature is low; whereas it goes on very tardily if the atmosphere contains much vapour, even though the air be very warm. It is far slower in still air than in a current, because the air immediately in contact with the water soon becomes moist, and thus puts a check to it. Pressure, too, has a remarkable influence over it. This is easily proved by placing ether in the vacuum of an air-pump, when vapour rises so abundantly as to produce ebullition.* * As a large quantity of heat passes from a sensible to an insensible state during the formation of vapour, it follows that cold should be generated by evaporation. Water placed under the exhausted receiver of an air-pump evaporates with great rapidity; and so much The presence of watery vapour in the atmosphere is owing to evaporation. The water upon the surface of the earth and the sea is subjected by its means to a natural distillation; the impurities with which the water is impregnated remaining behind, while the pure vapour ascends into the air to give rise to a multitude of meteorological phenomena. And as evaporation goes on to a certain extent, even at low temperatures, it is probable that the atmosphere is never absolutely free of vapour. Abridged from Turner. SPECIMENS OF THE POETRY OF THE SIXTEENTH AND SEVENTEENTH CENTURIES (Chronologically arranged). GIVE me my scallop shell of quiet, My gown of glory (hope's true gage)— SIR WALTER RALEIGH (born 1552). AND is there care in Heaven? and is there love There is; else much more wretched were the case And all his works with mercy doth embrace, That blessed angels he sends to and fro, To serve to wicked man,-to serve his wicked foe! cold is generated as would freeze the water, did the vapour continue to rise for some time with the same velocity. But the vapour itself soon fills the vacuum, and retards the evaporation by pressing upon the surface of the water. This difficulty may be avoided by putting under the receiver a substance, such as sulphuric acid, which has the property of absorbing watery vapour, and consequently of removing it as quickly as it forms. Such is the principle of Sir John Leslie's method for freezing water by its own evaporation. How oft do they their silver bowers leave, To come to succour us that succour want! How oft do they with golden pinions cleave The flitting skies, like flying pursuivant, Against foul fiends to aid us militant! They for us fight, they watch and duly ward, And their bright squadrons round about us plant; And all for love and nothing for reward; Oh, why should heavenly God to man have such regard? EDMUND SPENSER (b. 1553). ALL the world's a stage, And all the men and women merely players: And then, the whining School-boy, with his satchel, Made to his mistress' eye-brow. Then, a Soldier, Seeking the bubble reputation Even in the cannon's mouth. And then, the Justice, In fair round belly, with good capon lined, With eyes severe, and beard of formal cut, And so he plays his part. The sixth age shifts P |