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Fig. 26.

rally water) until it ceases to be condensed-that is, until the temperature of the condensing liquid reaches that of the vapour. Fig. 26 shows such an arrangement. The liquid is boiled in the flask F. Its vapour mainly collects and condenses in the bulb B; the remainder condenses in the worm. On finding how much of the liquid in F has been boiled away, or, what is the same, how much vapour has been condensed, and on knowing the weight of the water which, while rising a certain temperature, has withdrawn the latent heat of the liquid vapour and condensed it, we can, as before, deduce the latent heat of the vapour, because the increased heat of the water is exactly F equal to the latent heat of the vapour.

W

B

§ 93. The following are a few instances of the latent heats of

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§ 94. Of all vapours that of water has the greatest latent heat. § 95. Those liquids have the greatest latent heats whose corresponding solids have the highest melting-points.

§ 96. The latent heat of water is of great influence in nature, by equalizing the temperature of the earth, the sea, and the air. For if a portion of the sea be exposed to great cold, it freezes, giving heat out of its great store of latent heat. A continuance of the cold does not depress the temperature of the ice, but causes more to be formed. When the summer sun shines upon the ice, it does not warm it but melts it. Thus, in both cases,

sudden changes of temperature are avoided, and the winds which blow from the polar regions are more uniform in temperature (see § 23). Rivers whose sources are on snow-covered mountains would cause disastrous floods in spring time, if the water at the moment of its formation did not absorb as latent heat a part of that given by the sun; for otherwise the whole of the snow would melt at once on receiving heat, the moment after its temperature rose to 0° C.

CHAPTER X.

LIQUEFACTION.

$97. Liquefaction of solids.—We have hitherto supposed the heat to be applied to the solid to produce liquefaction, and to the liquid to produce vaporization. If the solid be melted by any other means, in liquefying it seeks to obtain heat from surrounding objects. If it cannot do so it refuses to liquefy; so that when a solid liquefies by such means it absorbs heat or "produces cold."

§ 98. Many solids may be liquified by dissolving them in liquids; and some solids when brought together melt in order to mix, provided that their mixture is a liquid. In such cases heat is absorbed by the body undergoing liquefaction, and withdrawn for that purpose, if possible, from surrounding objects.

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$99. Freezing-mixtures" act on this principle. If equal weights of a solid body called nitrate of ammonium (see Part II.) and water, both at 10° C., be mixed, the nitrate of ammonium dissolves and becomes liquid; the mixture sinks in temperature to -15°5 C. If two parts by weight of snow at 0° C. be mixed with one part of common salt at 0° C., a temperature of -20° C. is obtained. Again, mercury dissolves many metals, forming what are called "amalgams :" frequently a considerable fall of temperature attends the solution.

§ 100. Inversely, a liquid may sometimes be converted wholly or partly into a solid without the withdrawal of heat—as when a solid suddenly crystallizes out of a liquid. This is always accompanied by a sudden evolution of latent heat from the liquid.

§ 101. Liquefaction of gases.-When a gas simply dissolves in a liquid without uniting chemically with it (that is, without forming any new compound-Part II.), heat is evolved. Thus, when steam at 100° C. is passed over dry sugar, a syrup is obtained of a temperature above 100° C.

§ 102. When gases unite chemically with one another and form liquids, heat is always evolved; and when a gas unites chemically with a liquid, so as to form a new compound, heat is also evolved. In both coses the heat is due in part to the escape of the latent heat of the gas. Thus heat a given out when a gas (oxygen) unites with another gas (hydrogen) and forms a liquid (water). A gas, ammonia, dissolves in and combines with water, giving out heat.

Liquefaction of gases will be further considered in the next chapter.

CHAPTER XI.

EVAPORATION AND EBULLITION.

§ 103. At temperatures far below their boiling-points, but not at all temperatures, all known liquids in contact with gases or exposed to vacuo are found to give rise to vapour; that is, the surface of the liquid is converted into vapour and spreads out or diffuses into the gas or empty space above it. Such separation of a part of a liquid as a vapour is called evaporation. When the same takes place from a solid, it is called volatilization.

§ 104. The mechanical force which is thus exerted is called the tension or elastic force of the vapour of the body.

§ 105. The tension of the vapour of a body in vacuo may be very conveniently measured by introducing a little of it into the vacuum of the barometer. The mercury is then depressed, and the distance through which it is driven down is a direct measure of the tension of the vapour of the substance.

When a part of a body is thus converted into vapour, the cohesion of the body is overcome.

§ 106. We already know that the particles of a body are separated by heat, the body thereby expanding. When, therefore, the particles of a liquid are further removed from one another by heat, their cohesion as a liquid offers less resistance to the tension of the vapour. Accordingly it is found that the tension of a vapour in contact with the substance which gave rise to it increases with the temperature, and, according to the increased temperature, depresses further the barometric column.

T

Fig. 27.

B

§ 107. To show the increase of tension with temperature, a few drops of the evaporating body (suppose a liquid) (fig. 27) are introduced above the mercury, M, of a barometer, B. The barometer is enclosed in a glass cylinder, C, containing some transparent liquid, which may be heated. Thus, suppose the surrounding liquid is oil, of the temperature 0° C., and the mercury stands c at 760 millims. in the barometer, B. On introducing into the barometic vacuum ice of the temperature -32° C., the mercury sinks 0.310 millim. As the oil is warmed (its temperature being ascertained by the thermometer, T), the mercury in B is forced lower and lower down by the ever increasing tension of the vapour of the ice (which is identical with the vapour of water, and is steam), until the ice melts. Thenceforth the vapour rises from water. The following Table shows in millimetres the elastic force or tension of the vapour at a few different temperatures:

M

Q

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The tension therefore increases much more rapidly than the temperature.

§ 108. At 100° C., that is, when the water begins to boil, the tension of its vapour exactly counterbalances the pressure of the air, and the mercury in the barometer stands at the same level as that outside.

§ 109. The tension of the vapour of different substances at the same temperature is very different. This is seen on introducing different substances at the same temperature into a series of similar barometers (fig. 28).

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