plate vessel, D, furnished with a lid, which acts as a screen, and is further protected from the radiation of the lamp by the tin-plate screen, E; s is a light glass tube for agitating the water. The result obtained has, however, to be corrected by other experiments, for the heat absorbed by the metallic parts of the apparatus, and for that which is lost by radiation during the time that the experiments last; allowance has also to be made for the heat which the condensed liquid has given out after its liquefaction, in cooling down from its boiling point to the temperature of the water used in the condenser."

Alcohol

Ether

Oil of Tur

202-40 364-3

374-9 963·1 173.1 0.8151 90-45 162.8 1684 692.3 94.8 0.7365

pentine 68.73 123-7 128.0 966.9

59. "The numbers contained in the third column indicate the quantities of water in pounds, the temperature of which would be raised 1°F. by condensation into the liquid form of a pound weight of the vapours of each of the liquids mentioned; the liquid condensed being supposed in each case to be at the temperature of its own boiling point. For instance, the conversion of one pound of steam at 212° into water at 212° would raise 966'6 pounds of water from 60° to 61° F. So the condensation of one pound of the vapour of alcohol at 173° into liquid alcohol at 173° would heat 374-9 pounds of water from 60° to 61o.” The numbers in the fourth column are, with the exception of ether, approximatively equal; the same is also true of some other liquids not included in the table; it is. nevertheless, not universally true that equal volumes of vapours of different liquids, under equal pressures, contain equal amounts of latent heat.

60. The heat absorbed in vaporization decreases as the temperature of the vaporizing liquid increases. For instance, a given weight of water at 212° F. requires less

heat for its vaporization than the same quantity at 100°F. requires, therefore the quantity of heat which is rendered latent by the conversion of a liquid into vapour varies with the temperature at which the evaporation takes place, the quantity of heat which becomes latent increasing as the temperature decreases. Watt concluded from his experiments on steam that liquids require for their conversion into vapour the same total quantity of heat at all temperatures, and as the quantity that remained sensible increased, the quantity that became latent decreased in the same proportion. "For instance:A certain weight of steam at 212° F., condensed at 32° F., gives out

180° of sensible heat. $950° of latent heat.

1130°

218°of sensible heat.

912° of latent heat.

1130°

68° of sensible heat.

1062° of latent heat.

1130"*

61. If liquids required for their conversion into vapour the same total quantity of heat, there would be no economy in distilling at one temperature rather than another.

62. Regnault has submitted the subject to a rigorous examination, and his results show that the total amount of heat at all temperatures is not the same, but increases by a constant difference equal to 0.305 for each degree C. He found that the latent heat of steam at 0° C. is 606·5; so that the formula for calculating the total quantity of heat in steam at different temperatures becomes

λ=606·5 +0.305 t,

in which A represents the sum of the latent and sensible

* Miller's " Chemical Physics," 2nd edit.

heat, while 606.5 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.

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.

t

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, s0 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.

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.