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 its capacity with water, and placed upon a charcoal furThe tubes of four thermometers, whose bulbs 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. TABLE II.-ELASTIC FORCE OF AQUEOUS VAPOUR IN MILLIMETRES OF MERCURY FOR CENTIGRADE THER 75. The tension of the vapour of other liquids could be determined by the methods which have been given for determining the tension of aqueous vapour. 76. Vapours occupy a much larger volume than the liquids from which they are produced, and equal volumes of different liquids generate very different volumes of vapour. At the same temperature the elastic force of the rapours of different liquids is different,-the more volatile the liquid the greater the tension of its vapour. This is easily proved for elasticities below that of atmospheric air, by filling a number of baro meter tubes (Fig. 8) with mereury, and inverting them in a bath of the same metal. One of the tubes, A, is used as a barometer to measure the pressure of the external air; into B is introduced, by means of a pipette, a few drops of water; into C, a few drops of alcohol; into D, a few drops of bisulphide of carbon; and into E, a few drops of ether: the different liquids will ascend to the top of the mercury and then evaporate. The elastic force of each vapour will depress the column of mercury, but the amount of depression Fig. 8. will vary with the liquid; the vapour of water will cause the least depression, and therefore its elastic force is least; alcohol the next; bisulphide carbon the next; and ether the greatest. The elastic force of the vapours counteracts the pressure of the atmosphere to the extent of the depression they cause in the mercurial column, consequently, it is easy to find the pressure each of these vapours exercises, say, upon every square inch; for, if the atmosphere at the time supports a column of mercury 30 inches in length, its pressure is equal to 15 lbs. on every square inch; if the vapour of water depresses the mercurial column one inch, its pressure must be equal to of 15 lbs.; if the alcohol depresses the mercury 2 inches, and the ether depresses it 20, the pressure they exercise is found in the same way. It has been proved that the elasticity of the vapour emitted from each liquid increases as the temperature rises, until, at the boiling point of each liquid, the elasticity of the vapour is equal to the pressure of the atmosphere. 77. Dalton considered that all vapours have the same elasticity at an equal number of degrees above or below their boiling points; other experimenters have not found |