heat, while 6065 is the latent heat of the vapour at 0°, and t the given temperature.

63. Equal bulks of different liquids produce very different volumes of vapour. Water furnishes, bulk for bulk, a much larger amount than any other liquid, a cubic inch of water at 212° F. expanding to nearly a cubic foot of steam at 212°, or more accurately, to 1,696 times its volume. The following table shows the volume of vapour which is furnished by a cubic inch of four different liquids, at their respective boiling points. Equal volumes of different vapours, taken at the boiling points of their respective liquids, consequently possess very different weights, as is shown by the last column of the table :

64. "The expansive force of the different vapours obviously depends upon the bulk of vapour produced from an equal bulk of each liquid; and although the latent heat required to convert equal bulks of other liquids into vapour is much less than that required for steam, yet no economy would be experienced, even did they cost no more than water, by substituting these liquids for water, as the materials for generating vapour in the steam engine." -Miller.

39

65. The elastic force of a vapour varies with the condition under which the vapour is formed; if the vapour is produced below the boiling point of the liquid, its elastic force is not equal to the pressure of the atmosphere (48); if the vapour is produced at the boiling point of the liquid, its elastic force is equal to the pressure of the atmosphere (49); if the vapour is produced above the normal boiling point of the liquid, its elastic force exceeds

the normal pressure of the atmosphere. Steam of greater tension than the atmospheric pressure is called high pressure steam. The tension of steam increases very rapidly: at the temperature of 250-52° F. it is equal to two atmospheres (twice the pressure of the atmosphere); at 510 62 F. it is equal to 50 atmospheres.

66. We have now to consider how the tension of vap at different temperatures has been determined.

67. For measuring the elastic force of the vapou water below zero, Gay Lussac made use of two barometer tubes filled with mercury, the open ends dipping under mercury in the same basin. (Fig. 5.) One of the tubes, A, which is straight and perfectly freed from air and moisture, by boiling the mercury in the tube, serves to measure the pressure of the atmosphere. The other tube is bent, 80 that a part of it can be surrounded with a freezing mixture, as represented in the figure. If we now introduce a few drops of water above the mercurial column in the tube, BC, we observe that the level of the mercury in this tube is lower than in the tube, A, by a quantity which varies with the temperature of the freezing mixture.

Fig. 5.

These depressions, which are necessarily due to the tension of the vapour in the tube, B C, show that at very low temperatures there is still aqueous vapour in the air.

68. Only a portion of the vapour in the tube, BC, is exposed to the freezing mixture; but it is an established principle of hygrometry, that when the temperatures of two vessels communicating with each other are unequal, the tension of the vapour is the same in both, and is always that which corresponds to the lowest temperature.

69. Dalton measured the elastic force of the vapour of water from 32° F. to 212° F. by means of the following apparatus. Two barometer tubes, A and B (Fig. 6), are put into the same basin of mercury, which is placed upon a furnace. The barometer, B, is completely freed from air

and moisture; in other words, it is a perfect barometer: the barometer, A, contains a small amount of water above its mercury column. These two barometers are enclosed in a tall glass cylinder filled with water, and a thermometer dips into the water in the centre of the cylinder, which gives the temperature of the liquid. In heating gradually the basin containing the mercury, and consequently the water in the glass cylinder, the water in the tube continues to vaporize, and as the tension of the aqueous vapour augments, the mercury in A falls lower and lower. The depression which is produced in A, below the level in B, for each degree the temperature is increased, is noted upon the scale, E. The apparatus of Dalton can be used so long as the elastic force of the vapour does not exceed the pressure of the atmosphere. When the tension is equal to the atmospheric pressure,

Fig. 6.

the surface of the mercury column will be depressed to the level of the mercury in the basin, and the experiment is at an end.

70. Regnault employed a form of apparatus which has the advantage of indicating all pressures and temperatures, whether above or below the boiling point. "His process consists in boiling water in a vessel under a known pressure, and ascertaining the temperature at which it boils. This method depends upon the principle that when the water boils, the steam it produces will have a pressure precisely equal to that to which the water itself is submitted.

71. "The apparatus consists of a copper boiler (Fig. 7), closed so as to be steam-tight, filled to about a third of

nace.

descend to different depths in it, pass steam-tight through collars in the top. Two of these bulbs are immersed in the upper, and two others in the lower strata of the liquid. The boiler, C, is connected by a tube, A B, with a large glass globe, M, having a capacity of about five gallons, which is filled with air. The tube, A B, is surrounded by a larger tube, D, which is kept filled with cold water, flowing from a cistern, E, and discharged into another, A'. From the upper part of the globe, M, two tubes proceed, one of which communicates with an air gauge, O, and the other, H, is terminated in a connecting pillar, H', which may be attached at pleasure, either to the plate of an air pump, or to that of a condenser, so that the air in M can be made to have any degree of pressure, either above or below that of the atmosphere. The globe, M, is immersed in a reservoir of water, at the temperature of the surrounding air.

72. If it be desired to measure the pressure of the vapour of water corresponding to temperatures below the boiling point, the connector, H', is attached to the plate of an air pump, and the air in M is gradually rarefied, so as to assume a series of decreasing pressures below that of the atmosphere. The thermometer in C, shows the temperatures corresponding to these pressures severally, and the gauge, O, shows the corresponding pressures.

73. If it be desired to ascertain the pressures corresponding to temperatures above the boiling point, the connector, H', is attached to a condenser or a force pump, by means of which the air in M, and in the boiler, C, is submitted to a series of increasing pressures above that of the atmosphere. The corresponding temperatures, as before, at which the water boils in C, are indicated by the thermometers.-Lardner.

74. These researches have resulted in the construction of tables which make known the elastic force of the vapour of water at different temperatures. The following tables give the degree of tension for the various temperatures, at which analyses are likely to be made. The tension of the vapour of water is expressed by the height of a column of mercury counterbalancing it. The first table gives the elastic force in inches of mercury for Fahrenheit's thermometer; the second gives the elastic force in millimetres of mercury for the centigrade thermometer.