4 MELTING OF ICE. 1. Melting of Ice: Latent Heat. 4. Pressure of Fluids in every direction. 5. Elastic Force of Steam: High-Pressure Steam. 6. Condensation of Steam. 7. Production of a Vacuum. 8. Mechanical Force produced by the preceding operations. 9. Economical Applications of Heat. 3. Melting of Ice: Latent Heat.—If a mass of ice be introduced into a warm room, and a thermometer be applied to it, the temperature of the ice will continue to rise, till the thermometer stands at 32°; it there remains stationary, although heat is continually entering into the melting ice, as before; and it will remain so, until the whole of the ice is melted. The heat which is absorbed by the ice during the change from the solid to the liquid form, is termed insensible or latent, in consequence of its not affecting the thermometer. The amount of heat which is absorbed and becomes latent, in this process, may be estimated by the time during which the thermometer remains stationary; it will be found to be a hundred and forty times as long as the time required to raise the water, in the liquid state, one degree. The quantity of latent heat therefore, of water, is 140 degrees; or, in other words, the difference between a pound of water at 32°, and a pound of ice at 32°, is, that the former contains, in a latent state, as much more heat than the latter, as would suffice to heat another pound of water a hundred and forty degrees. The heat which is latent in water, is liberated and rendered sensible when the water is reconverted into ice; this may be proved in the following way. Water may be cooled down many degrees below 32° without freezing, provided it be kept perfectly still; it has been cooled as low as 5o. If it be cooled down to this temperature, and a tremulous motion be then communicated to it, congelation is determined, a large portion of it is suddenly converted BOILING OF WATER. 5 into ice, and the latent heat is liberated in such quantity as to raise the temperature of the whole mass suddenly to 32o. 4. Boiling of Water: Latent Heat.-So soon as the entire mass of ice is melted, the thermometer continues to rise until it has reached the temperature of the room. If heat be now applied to the water, by means of a lamp, a thermometer placed in it will gradually rise until it reach the temperature of 212°, and no addition of heat will raise it a degree higher, provided the water be in an open vessel. A new process now begins: bubbles are formed at the lower part of the vessel, rise to the surface, and escape with commotion in the form of Steam. This constitutes ebullition, or the boiling of water. Steam, as it rises from water at 212°, exhibits the same phenomenon as occurs in the conversion of ice into water at 32°: a quantity of heat is absorbed which serves only to change the form of the body, in this case converting water into steam, as in the former it converted ice into water,-in both cases being termed insensible or latent. A much larger quantity of heat is absorbed during the formation of steam from boiling water, than during the melting of ice; its amount may be estimated, as in the former case, by the time during which the thermometer remains stationary at 212°; it will be found to be five and a half times as long as was required to raise the water from the freezing to the boiling point, that is, to raise it 180 degrees. Thus, if the application of heat from a lamp were required for one hour, in order to raise a quantity of water from 32o to 212o, it will require the application of the same heat for five hours and a half, in order to convert the whole of the water into steam. The product of these numbers is 990. The latent heat of steam is therefore estimated, in round numbers, at 1000 degrees; or, in other words, sufficient heat is absorbed, during the formation of steam, to raise the temperature of an equal quantity of water a thousand degrees. When steam is reconverted into water, the latent heat is liberated and becomes sensible. Hence, a gallon of water, in the form of steam, when added to cold water, will impart 6 BOILING POINT OF WATER. more heat to it, than a gallon of water at the same temperature as the steam; one part of water, for instance, at 212o, will raise the temperature of 100 parts of water at 30o, only one degree and a half, whereas one part of water in the form of steam will raise 100 parts of water as high as eleven degrees. 5. When Sea Water is employed, the boiling point varies in consequence of the greater density of saline water than that of pure water, and the force of the steam varies accordingly. Sea water contains 33 per cent. of saline matter, of which common salt, or chloride of sodium, constitutes the largest ingredient. Now, as the saline matters do not rise with the steam, the water in a boiler supplied with sea water becomes gradually more saturated, and after a certain time begins to deposit salt, sometimes to the detriment of the boiler. Considerable difficulty has arisen in applying the steam engine to the purposes of marine transport, from the necessity of supplying the boiler with sea water, instead of fresh water. During the first trip of the City of Edinburgh steam ship to Leith, in 1821, the deposit of salt was so considerable as to require its being cleared out during the passage, while the vessel proceeded under her canvas. This circumstance induced the manufacturers of the machines, to adopt a method of removing the saturated water from the lower part of the boiler, by means of a pump, and subsequently by means of blow-off pipes and cocks, as is now generally adopted. The boiling point of water appears to rise one degree for each addition of 2-6 parts to the proportion of common salt in 100 parts of water, or very nearly so. Thus, the boiling point of sea water, supposed to contain 3:03 per cent, by weight, of saline matters, is 213′2; if the saline matters amount to 6'06, the boiling point is 2144; and so in proportion. When the proportion of saline matter amounts to 36:37 per cent., the solution is saturated, and the boiling point rises as high as 226°. The variations in the force of steam generated from salt and from fresh water, are noticed at page 11. INFLUENCE OF ATMOSPHERIC PRESSURE. 7 6. Influence of Atmospheric Pressure.-The temperature at which steam is formed, depends on the degree of Atmospheric Pressure to which the water from which it is formed is subjected. The Atmosphere is supposed to extend about forty-five miles in height around the earth; and it presses, under ordinary circumstances, with a weight of fifteen pounds on each square inch of the surface of the earth, and of all bodies upon it. We accordingly find that water boils at lower degrees of temperature, as we ascend higher from the surface of the earth, the pressure of the atmosphere being greatest at the level of the sea. An ascent of 530 feet causes the boiling point of water to be lowered one degree of temperature; at an elevation of 2.705 miles from the surface of the sea, the atmosphere loses half its density, or one volume is expanded into two volumes; the density is again halved for every 2.7 miles of additional elevation. When it is said that water boils, or, what is the same thing, that steam is formed, at 212° Fahr., it is always understood that the atmospheric pressure is equivalent to a weight of 15 pounds on every square inch, or to that of 30 inches of mercury, as indicated by the barometer. For every inch by which the barometer varies from this height, the boiling point of water varies 1.76 degree. The following are the variations between the atmospheric pressure as indicated by the inches of mercury in the barometer, and the boiling point of water: When two or more atmospheres are spoken of, the term signifies multiplied pressures of air arising from condensation. If a mercurial column of 30 inches in height presses upon a given surface with the same weight as the atmosphere in its ordinary state, it follows that a 60-inch column is equal to a 8 PRESSURE OF FLUIDS. pressure of two atmospheres, 15 inches to half an atmosphere, and 1 inch to one-thirtieth of the atmospheric pressure. The influence of the atmospheric pressure on the formation of steam may be readily proved by removing the pressure altogether, as by means of an air pump. Liquids boil in a vacuum at a temperature of about 145 degrees below their usual boiling point. If hot water, with a thermometer in it, be placed under the receiver of an air pump, the water boils, and the temperature falls, as the air is being exhausted; as the water receives no heat from without, a portion of its sensible heat is employed in effecting the change of form. Water boils in a good vacuum at 67°. The heat of the hand is sufficient to make water boil in a vacuum, as is exemplified in the common pulse glass. In the absence of an air-pump, the same principle may be illustrated by a simple experiment. Some water is made to boil in a glass flask over a lamp; the flask is then closed with a cork, while the upper part is filled with steam, and it is removed from the lamp. When the boiling has ceased, it may be renewed on plunging the flask into cold water, and the colder the water is, the brisker will be the ebullition. On removing the flask from the cold water, and plunging it into warm water, the boiling again ceases; it may again be renewed on again immersing the flask in cold water. In this experimeut, the boiling ceases on corking the flask, owing to the pressure exerted by the confined steam on the surface of the water; on plunging the flask into cold water this steam is condensed, and the water again boils under the diminished pressure; on immersing the flask in hot water, the steam is no longer condensed, and by its pressure it again prevents the boiling of the water. 7. Pressure of Fluids in every direction.-There is another property by which air, and other elastic fluids, are distinguished, and which is important in the action of the Steam Engine: they transmit pressure equally in every direction. 1. If a vessel, of a cubical foot in capacity, be supposed to be filled with atmospheric air, the elasticity of the particles of the fluid is such, as to press upon every |