E V A P O 11 A T 1 N 731 aome obtained by Magnus, will be found in the accompany ing table. The pressures are measured in millimetres uf Table of Pressure of Aqueous Vapour. Temperature in Degrees Centigrade. Pressure in Millimetres of Mercury. Temperature in Degrees Centigrade. Pressure in Millimetres of Mercury. Magnus. Regnault. Magnus. Regnault -32 310 105 907-157 906-41 -30 365 110 1077-261 107537 -25 553 115 1272-986 1269-41 -20 "910 841 us 1403915 1399-02 -15 1-403 1-2S4 120 1491-28 -10 2-109 1-963 125 1743-88 - 5 3-115 3-004 130 2030-23
4-525 4-000 135 2:353-73 5 6-471 6534 140 2717-63 10 9-126 9-165 145 3125-55 15 12-677 12-699 150 3581-23 20 17-396 17-391 155 4088-56 25 23-582 23 -J MO 160 4651-62 30 31-602 31-548 165 5274-54 35 41-893 41-827 170 5961 66 40 54-969 54-9U6 175 6717-43 45 71-427 71-391 180 7546-39 50 91-965 91-965 185 8453-.>3 55 117-:i78 117-478 190 9442-70 60 118-579 148-791 195 10519-63 65 186-601 186-915 200 11688-96 70 232-606 2,13-093 205 12955-66 75 287-893 288-517 210 14324-80 80 353-926 354-643 215 15801-33 85 432-295 433-441 220 17390-36 90 524-775 525-450 225 19097-04 95 633-305 633-778 230 20926-40 100 760-000 760-000 mercury at C. 60 metres above the level of the sea in the latitude of Paris. An account of Regnault s researches on this subject will be found in the Memoires de Vlnstitut, tome xxi., the Nouvelles Annales de Chimie, xi. 334, and xiii. 396, and in the first volume of the publications of the Cavendish Society. The researches of Magnus, who arrived independently at nearly the same results as Regnault, were published in Poggcndorff s Annalen, Ixi. 225. Regnault also determined the density of aqueous vapour in air and in vacuo for temperatures between C. and 25 C., and concluded that when the pressure is not very great nor the air nearly saturated (for when it is nearly saturated there is probably deposition of moisture upon the glass vessels), the density may be calculated from the known density of steam at the boiling-point and under ordinary atmospheric pressure by supposing it to obey " the gaseous laws." According to Regnault the mass of a litre of dry air at C., and under a pressure of 760 millimetres of mercury, is 1/2931S7 grammes, and the density of steam, compared with air at the same pressure and temperature as unity, is 6235. Hence, by help of the table of pressures, the amount of aqueous vapour in any given volume can be determined when we know the dew- point and the temperature of the air. If P denote the pressure of vapour at the dew-point in millimetres of mercury, the mass of vapour in a litre of air at t" C. will be P 273 1-293187 x _- - x - - grammes. i uU 2 1 3 + t A curve which represents the relation between the pres sure and volume of the unit mass of steam in contact with water as the temperature changes is called the steam line, and the corresponding curve for aqueous vapour in contact with ice is called the hoar-frost line. Since water can be cooled below the freezing-point without solidifying, it is possible to obtain data for drawing the steam line correspond ing to a range of temperature below C. This Regnault did, and his results showed that the steam line so continued does not coincide with the hoar-frost line, but that the two intersect very obliquely just above the freezing-point. Regnault supposed that this must be due to errors of measurement, and drew his steam line so as to coincide with the hoar-frost line ; but it has since been shown from theoretical considerations, by James Thomson, that such a difference must exist, and that the point of intersection of the two curves corresponds to a particular relation between the pressure, volume, and temperature for which ice, water, and steam can all exist together in equilibrium, no other gas or vapour being present in the inclosure. On ex amining Regnault s results, the intersection of the curves was found to be distinctly indicated by them. At this point the steam line, ice line, and hoar-frost line inter sect, and it has therefore been called the triple point. The corresponding temperature is a little above. - 007 C. The number of units of heat absorbed by the unit of mass of a substance, in passing from the solid or liquid into the gaseous condition, without change of temperature, is called the latent heat of vaporization. According to Andrews, the latent heat of steam at 100 C. is 535 9, or a gramme of water in being converted into steam at 100 C. would ab sorb sufficient heat to raise 535 9 grammes from 0* to 1 C. Soon after Dr Black enunciated his theory of latent heat, James Watt examined the latent heat of steam produced at different temperatures, and concluded that, when added to the amount of heat required to raise the unit of mass of water from C. to the temperature at which the steam is formed, the result, often called the total heat of steam, is the same for all temperatures. This statement is known as Watt s law, but is far from true, for Regnault has shown experimentally that when steam is produced at a temperature of f C. its total heat is represented by 606 5 + 305 within the limits of error of his ex periments. Putting t equal to 100, this formula gives for the total heat of steam at 100 C. the value 637, and its latent heat is therefore about 536, since about 101 units of heat are required to raise the unit mass of water from C. to 100 C. At C. the latent heat of steam is 606 5. The latent heat of steam is greater than that of any other known vapour. According to Favre and Silbermann, the latent heat of the vapours of alcohol and ether are 208 31 and 9 I ll respectively; and according to Andrews, they are 202 4 and 90 45 respectively. In consequence of the great amount of heat absorbed in evaporating, volatile liquids are frequently employed for the purpose of producing cold. The cryophorus of Wol- laston consists of a glass tube with a bulb at each end, one of which is partially filled with water. The air is removed by boiling the water and sealing the tube when full of steam. On turning all the water into one bulb, and placing the other in a mixture of pounded ice and salt, the pressure of vapour will be diminished by condensation taking place in the cold bulb, and this allows such rapid evaporation to take place in the other bulb that the water remaining in it becomes readily frozen. Gay Lussac showed that water placed in a vacuum at 8 C., or in per fectly dry air at 2 C., may be frozen by evaporation. The action of Carre s freezing-machine depends upon the heat absorbed by the rapid evaporation of ammonia, which has been liquefied by pressure. Solid carbonic anhydride dissolved in ether will produce by evaporation in vacuo a temperature of about- 110 C. , and Natterer, by means of a mixture of liquid nitrous oxide and bisulphide of carbon evaporating in vacuo, obtained a temperature which he estimated at - 140 C. When a vapour passes into the liquid or solid state a quantity of heat is produced equal to that absorbed in evaporating at the same temperature. Thus, if a gramme of steam be made to pass into 5 33 grammes of water at 0, it will raise the temperature of the water almost to 100 C., and if steam at 100 C. be blown into a saturated solution of common salt, the temperature will rise to 109" C. before the steam will pass freely through it. In 1822 Cagniard de la Tour inclosed a quantity of
alcohol in a strong tube, so as to occuoy about two-fifths of