CHAPTER XXVII.

MEASUREMENT OF THE MAXIMUM TENSIONS OF VAPOURS.

270. Tension of Aqueous Vapour.-The knowledge of the maximum tension of the vapour of water at various temperatures is important, not only from a theoretical, but also from a practical point of view, inasmuch as this tension is the motive force in the steam-engine. Experiments for the purpose of determining the values of this element have accordingly been undertaken by several experimenters in different countries. The researches conducted by Regnault are especially remarkable for the range of temperature which they embrace, as well as for the number of observations which they include, and the extreme precision of the methods employed. Next to these in importance are the experiments of Magnus in Germany and of Fairbairn and Tate in England.

271. Dalton's Apparatus.-The first investigations in this subject which have any pretensions to accuracy were those of Dalton. The apparatus which he employed is represented in Fig. 259. Two barometric tubes A and B are inverted in the same cistern H; one is an ordinary barometer, the other a vapour-barometer; that is, a barometer in which a few drops of water have been passed up through the mercury. The two tubes, attached to the support CD, are surrounded by a cylindrical glass vessel containing water which can be raised to different temperatures by means of a fire. The first step is to fill the vessel with ice, and then read the difference of level of the mercury in the two tubes. This can be done by separating the fragments of ice. The difference thus observed is the tension of aqueous vapour at zero Centigrade. The ice is then replaced by water, and the action of the fire is so regulated as to give different temperatures, ranging between 0° and 100°C., each of which is preserved constant for a few minutes, the water being at the same time

well stirred by means of the agitator pq, so as to insure uniformity of temperature throughout the whole mass. The difference of level

in the two barometers is read off in each

case; and we have thus the means of constructing, with the aid of graphical or numerical interpolation, a complete. table of vapour-tensions from 0° to 100° C. At or about this latter temperature the mercury in the vapour-barometer falls to the level of the cistern; and the method is therefore inapplicable for higher temperatures. Such a table was constructed by Dalton.

272. Regnault's Modifications.-Dalton's method has several defects. In the first place, it is impossible to insure that the temperature shall be everywhere the same in a column as long as that which is formed by the vapour at 70°, 75°, and higher temperatures. In the second place, there is always a good deal of uncertainty in observing the difference of level through the sides of the cylindrical glass vessel. Regnault employed this method only up to the temperature of 50°C. At this temperature the tension of the vapour is only about 9 centimetres (less than 4 inches) of mercury, and it is thus unnecessary to heat the barometers throughout their entire length. The improved apparatus is represented in Fig. 260. The two barometric tubes, of an interior diameter of 14 millimetres, traverse two holes in the bottom of a metal box. In one of the sides of the box is a large opening closed with plate-glass, through which the necessary observations can be made with great accuracy. On account of the shortness of the liquid column it was very easy, by bringing a spiritlamp within different distances of the box, to maintain for a sufficient time any temperature between 0° and 50° C.

The difference of level between the two mercurial columns should be reduced to 0° C. by the ordinary correction. We should also take into consideration the short column of water which is above the

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mercury in the vapour barometer, and which, by its weight, produces a depression that may evidently be expressed in mercury by dividing the height of the column by 13·59.

To adapt this apparatus to low temperatures, it is modified in the following way. The upper extremity of the vapour barometer tube is drawn out and connected with a small copper tube of three branches, one of which communicates with an air-pump, and another with a glass globe of the capacity of about 500 cubic centimetres. In the interior of this globe is a small bulb of thin glass containing water, from which all the air has been expelled by boil ing. The globe is several times exhausted of air, and after each exhaustion is refilled with air which has been passed over desiccating substances. After the last exhaustion, the tube which establishes communication with the air-pump is hermetically sealed, the box is filled with ice, and the tension at zero of the dry air left behind in

the globe by the air-pump is mea- Fig. 260.-Modified Form of Dalton's Apparatus. sured; it is of course exceedingly

small. Heat is then applied to the globe, the little bulb bursts, and the globe, together with the space above the mercury, is filled with vapour. This form of apparatus can also be employed to measure tensions at temperatures up to 50°, the only difference being that the ice is replaced by water at different temperatures, allowance being made, in each case, for the elastic force of the unexhausted air.

In the case of temperatures below zero, the box is no longer required, and the globe alone is placed in a vessel containing a freezing mixture. The barometric tubes are surrounded by the air of the apartment.

In this case the space occupied by the vapour is at two different

temperatures in different parts, but it is evident that equilibrium can exist only when the tension is the same throughout. But the tension of the vapour in the globe can never exceed the maximum tension for the actual temperature; this must therefore be the tension throughout the entire space, and is consequently that which corresponds to the difference of level observed.

In reality what happens is as follows:-The low temperature of the globe causes some of the vapour to condense; equilibrium is consequently destroyed, a fresh quantity of vapour is produced, enters the globe, and is there condensed, and so on, until the tension is everywhere the same as the maximum tension due to the temperature of the globe. This condensation of vapour in the cold part of the space was utilized by Watt in the steam-engine; it is the principle of the condenser.

Before Regnault, Gay-Lussac had already turned this principle to account in a similar manner for the measurement of low tempera

tures.

By using chloride of calcium mixed with successively increasing quantities of snow or ice, the temperature can be brought as low as -32°C. (-25.6° F.), and it can be shown that the tension of the vapour of water is quite appreciable even at this point.

273. Measurement of Maximum Tensions for Temperatures above 50°. -In investigating the tension of the vapour of water at temperatures above 50°, Regnault made use of the fact that the maximum tension of steam at the boiling-point is equal to the external pressure.

His apparatus consists (Fig. 261) of a copper boiler containing water which can be raised to different temperatures indicated by very delicate thermometers. The vapour produced passes through a tube inclined upwards, which is kept cool by a constant current of water; in this way the experiment can be continued for any length of time, as the vapour formed by ebullition is condensed in the tube, and flows back into the boiler. The tube leads to the lower part of a large reservoir, in which the air can be either rarefied or compressed at will. This reservoir is in communication with a manometer. The apparatus shown in the figure is that employed for pressures not exceeding 5 atmospheres. Much greater pressures, extending to 28 atmospheres, can be attained by simply altering the dimensions of the apparatus without any change in its principle. The manometer employed in this case was the same as that used in testing Boyle's law, consisting of a long column of mercury (§ 121).

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In using this apparatus, the air in the reservoir is first rarefied until the water boils at about 50° C.; the occurrence of ebullition being recognized by its characteristic sound, and by the temperature

remaining invariable. This steadiness of temperature is of great advantage in making the observations, inasmuch as it enables the thermometers to come into perfect equilibrium of temperature with the water. The tension indicated by the manometer during ebullition is exactly that of the vapour produced. By admitting air into the reservoir, the boiling-point is raised by successive steps until it reaches 100°. After this, air must be forced into the reservoir by a compression-pump.

The following is an abstract of the results thus obtained: