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Papin's Digester.

229

heated in open vessels. Only under these conditions can ebullition take place; for, in a closed vessel, since the vapours cannot escape into the atmosphere, their elastic force and their density continually increase, but that peculiarly rapid disengagement which constitutes boiling is impossible. There is, moreover, this difference between heating in an open and in a closed vessel; that, in the former case, the temperature can never exceed that of ebullition, while in a closed vessel it may be raised, so to speak, to an indefinite extent. Thus we have seen (233) that, in an open vessel, water cannot be heated beyond 100° C., all the heat imparted to it being absorbed by the vapours disengaged. But as this disengagement of vapour

cannot take place in a closed vessel, water and the vapour may be raised to a far higher temperature than 100°. Yet this is not unattended with danger, from the very high tension which the vapour then assumes.

Figure 185 represents the apparatus used in physical lectures for the purpose of heating water in a closed vessel beyond 100 degrees. It is known as Papin's Digester. It consists of a cylindrical bronze vessel, M (fig. 185), provided with

Fig. 185.

the cover and the vessel. At the bottom of a cylindrical cavity, which traverses a cylinder and tubulure, the cover is perforated by a small orifice in which there is a rod, n. This rod presses against a lever, ab, movable at a, and the pressure may be regulated by means of a weight, p, movable on this lever. The lever is so weighted, that when the tension in the interior is equal to six atmospheres, for example, the valve rises and the vapour escapes. The destruction of the apparatus is thus avoided, and the

mechanism, which will be described in speaking of the steam engine, has hence received the name of safety valve. The digester is filled about two-thirds with water, and is heated on a furnace. The water may thus be raised to a temperature far above 100°, and the tension of the vapour increased to several atmospheres, according to the weight on the lever.

The apparatus has received the name digester, from a Latin word signifying to dissolve, for the high temperature which water can acquire greatly increases its solvent power. Thus it is used to

Fig. 186.

extract from bones the substance known as glue, which could not be accomplished at 100°.

From the enormous elastic force which vapour may acquire in a closed vessel, it will be understood how important it is not to close tightly the vessel, in which water is contained for

domestic purposes. Thus the hot water-bottles for heating the feet of invalids should be uncorked before being placed near the fire; for it might burst, or at any rate the cork might be driven out, and a more or less serious accident be caused. In like manner, when a locomotive stops, the steam must be allowed to escape; for otherwise, as it is continually being formed in the boiler without any being consumed in working the engine, it would ultimately acquire such an elastic force that an explosion would ensue.

236. Measurement of the elastic force of aqueous vapour. The important applications which have been made of the elastic force of aqueous vapour, have led philosophers to measure with care the intensity of this force at various temperatures.

Dalton first measured the elastic force of aqueous vapour for

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Elastic Force of Aqueous Vapour.

231

temperatures between o° and 100°, by means of the apparatus represented in fig. 186. Two barometer tubes, A and B, are filled with mercury, and inverted in an iron bath full of mercury, and placed on a furnace. The tube, A, is an ordinary barometer tube, freed from air and moisture; but into the tube, B, is introduced a small quantity of water. The tubes are supported in a cylindrical vessel full of water, the temperature of which is indicated by the thermometer t. The bath being gradually heated, the water in the cylinder becomes heated too : the water which is in the tube B vaporises, and in proportion as the tension of its vapour increases, the mercury sinks. The depressions of the mercury corresponding to each degree of the thermometer, are indicated on the scale. Thus if, when the thermometer is at 70°, the mercury is 233 millimeters lower in the tube B than in the tube A, this shows that at 70° the tension of aqueous vapour is 233 millimeters; which amounts to saying that it exercises on the sides of the vessel which contains it a pressure equal to the weight of a column of mercury 233 milli. meters in height.

By noting in the above manner the depression in the barometer, B, as compared with A, Dalton determined the elastic force of aqueous vapour from 0 to 100°. He found it to be 760 millimeters, or 29.92 inches; that is to say, an atmosphere.

Dulong and Arago determined the elastic force of aqueous vapour above 100° up to 24 pressures of 24 atmospheres. More recently Regnault measured the elastic force of aqueous vapour both above and below 100°; and from the researches of this experimenter the following table has been taken, in which the elastic forces at various temperatures are respectively measured by the height in millimeters of the column of mercury which they can balance.

This table shows that the elastic force of aqueous vapour increases far more rapidly than the temperature. Thus at 50° the tension is only 91.9 millimeters; while at 100° degrees, that is to say, double the temperature, the tension is eight times as great.

237. Latent heat of vapour.-In speaking of ebullition we have seen that, from the moment a liquid begins to boil, its temperature ceases to rise whatever be the intensity of the source of heat. It follows that a considerable quantity of heat becomes absorbed in ebullition, the only effect of which is to transform the body from the liquid to the gaseous condition. And conversely, when a saturated vapour passes into the state of liquid, it gives out an amount of heat.

These phenomena were first observed by Black, and he described them by saying that, during vaporisation, a quantity of sensible heat became latent, and that the latent heat again became free during condensation. The quantity of heat which a liquid must absorb in passing from the liquid to the gaseous state, and which it gives out in passing from the state of vapour to that of liquid, is spoken of as the latent heat of evaporation.

The analogy of these phenomena to those of fusion will be at once seen. The modes of determining them need not be described; but the following results which have been obtained for the latent heats of evaporation of a few liquids may be here given:—

The meaning of these numbers is, in the case of water, for instance, that it requires as much heat to convert a pound of water from the state of liquid at the boiling point to that of vapour at the same temperature, as would raise a pound of water through 540 degrees, or 540 pounds of water through one degree; or that the conversion of one pound of vapour of alcohol at 78° into liquid alcohol of the same temperature would heat 208 pounds of water through one degree.

238. Cold due to evaporation.—Whatever be the temperature at which a vapour is produced, an absorption of heat always takes place. If, therefore, a liquid evaporates, and does not receive from without a quantity of heat equal to that which is expended in producing the vapour, its temperature sinks, and the cooling is greater in proportion as the evaporation is more rapid.

-239]

Water frozen in a Vacuum.

233

This may become a source of very great cooling. Thus if a few drops of ether be placed in the hand, and this be agitated to accelerate the evaporation, great cold is experienced. With liquids which are less volatile than ether, like alcohol and water, the same phenomenon is produced, but the cooling is less marked.

On coming out of a bath, and more especially in the open air and with some wind, a very sharp cold is experienced, due to the vapour formed on the surface of the body. Moist linen is cold and injurious, because it withdraws from the body the heat necessary for evaporation.

The cooling effect produced by a wind or draught does not necessarily arise from the wind being cooler, for it may, as shown by the thermometer, be actually warmer; but arises from the rapid evaporation it causes from the surface of the skin. We have the feeling of oppression, even at moderate temperatures, when we are in an atmosphere saturated by moisture in which no evaporation takes place.

The cooling produced by the use of fans is due to the increased evaporation they produce. The freshness occasioned by watering the streets is also an effect of evaporation.

The cold produced by evaporation is used in hot climates to cool water by means of alcarrazas. These are porous earthen vessels, through which water percolates, so that

on the outside there is a continual evaporation, which is accelerated when the vessels are placed in a current of air. For the same reason wine is cooled by wrapping the bottles in wet cloths and placing them in a draught.

239. Water and mercury frozen in a vacuum. From the great quantity of heat which disappears when a liquid is converted into vapour it will be seen that by accelerating the evaporation we have a means of producing intense cold. We have seen that liquids vaporise more rapidly the lower the pressure. Hence, if a vessel containing water be placed in Fig. 187. a space from which the air is exhausted, it should cool very rapidly. Leslie succeeded in freezing water by means of rapid evaporation. Under the receiver of the air-pump is placed a vessel containing