dart out sparks and jets of fire with great rapidity and vivacity. The phenomenon was noticed by other observers. 113. The influence of the atmosphere upon mankind may be negatively considered, granting for sake of argument, that in its absence life could be sustained. Upon the sense of hearing, the want of an atmosphere would be tantamount to its occlusion; for as sound is produced by vibrations in the air, none could possibly exist,-the tongue would then be silent, and language unknown,-the organs of smell and vision would no longer serve, or afford us pleasure; for odours would be wanting, and, excepting upon such objects as the sun's rays directly fall, all would be darkness! “darker than the night." CHAPTER VI. 114. Evaporation; dependent on temperature. 115. Latent heat. 116. Dalton's researches; Manoscope. 117. Force of vapour. 118. Dove's theory of the influence of winds upon the tension of vapour in the atmosphere. 119. Humidity. 120. Specific gravity of humid air. 121. Law of vaporic exhalations. 122. General considerations. 123. Hygrometric condition of the atmosphere in elevated regions. 124. Distinction between capacity and quantity; amount of annual evaporation. 125. Cold produced; icy caverns. 126. Dew-point; Mean dryness. 127. How ascertained; range of hygrometer. 114. Upon exposure to the air, a process takes place of conversion of the particle of bodies into the gaseous state. Water at all temperatures assumes the form of vapour. Evaporation proceeds from the snow-clad mountain and the glacier,' as well as from the ocean and the meadow. Evaporation differs from vaporization in the amount of heat required for its production. Water vaporizes when it passes into steam, at a temperature of 212° F. under ordinary pressure; below that temperature, it evaporates, passing into the ambient air in insensible moisture, where it is retained till a diminution of temperature renders it apparent,-for Dalton found that the The evaporation of ice may be experimentally proved. Under the receiver of an air-pump place a saucer containing sulphuric acid, which has a strong affinity for water, and above, a cup partly filled with water. Exhaust the air and evaporation takes place. The acid, by absorbing much of the aqueous vapour, maintains the capacity of the air for moisture, and more of the water is evaporated. The cold becomes great, and the water freezes. If a piece of lint has been put round the water-cup, it will be seen to become covered with an icy crust. The attached barometer now indicates a pressure of less than an inch of mercury. In course of a few minutes the coat of ice disappears, and as the rarefaction is increased, the ice within the cup vanishes. G amount and tension of vapour in the atmosphere is independent of the presence of the air, and wholly regulated by caloric.' Hence arise clouds, mists, and other aqueous meteors, when the thermometer falls in a humid atmosphere. In the state of vapour, the moisture exists normally in the form of hollow vesicles, frequently mingled, however, with globules filled with water. 2 115. The first effect which follows the application of caloric is the expansion of the body heated, and this progresses so long as the cause continues; the fluid then assumes the gaseous form, ceasing to manifest to the thermometer its farther ingress, i. e. the heat becomes latent or insensible: Upon withdrawal of the caloric, the body returns to its pristine condition. 116. Our own Dalton,-the father of the atomic theory, whose memory will be as imperishable as the beautiful system he has propounded,-devoted much of his time to meteorological research. Besides other truths, he demonstrated that the elasticity of water at the boiling point under mean pressure, and that of our atmosphere at the same pressure, are equal; that aqueous vapour possesses exactly the same repulsive force in the atmosphere which it assumes in a vacuum, the temperatures being the same; and that as its elasticity depends upon the increase of heat, so it is possible to ascertain the amount of vapour floating in the air at any temperature, by measuring the elasticity of the aerial fluid. For this purpose, an instrument called the Manoscope3 has been constructed. According to Gay Lussac, 1000 volumes of air at the freezing point, become 1375 when raised from 32° to 212° F.; consequently, upon each, there is an increase of bulk of 0.375 for 108°, and 0.002083, or 1-480th of its volume, for every degree of Fahrenheit's thermometer. This beautifully simple law applies to all gases: the converse is likewise true; conse It is important to bear this constantly in remembrance, for the expression air saturated with moisture conveys a false impression, and should be discarded. Watt estimated the volume of water, and the same quantity converted into vapour at the ordinary atmospheric pressure, as 1 to 1800.-Ph. Tr. 1784, p. 335. 3 Manometer of Hooke, similar to the Sympiesometer of Adie,--Forbes, Ed. Jour. of Sc. v. x. 334; Ib. N. S. vol. iv. 91, 329. 2 quently, for the diminution of each degree of Fahrenheit, there will be a condensation of volume amounting to the 1-480th of the mass, if we adopt the numbers of Gay Lussac. Magnus, however, has fixed the rate of expansion at 1-459th of the volume at zero; Regnault, at 1-458th at 0°; Rudberg, at 1-481st at 0°; and Dulong and Petit, at 1-448th at 0°,which is the same with that of Gay Lussac. 117. Dalton reasoned upon evaporation as we do on the inertia of matter, and by experimental research discovered in it a conformity with the laws of mechanics. He introduced water into the closed end of a barometer, and applied heat, watching the depression of the mercury from the expansive force of the fluid. Seeing that the atmosphere offers resistance to the diffusion of moisture, till this force is overcome by that of vaporic expansion, Dalton found that the force, or tension, of aqueous vapour,'-i. e. its power of supporting a column of mercury, at the temperature of 25° F., equals 0.156 inch.; at 32°.2; at 35°.221; at 38°.245; at 40° =.263; at 42°.283; at 44° =.305; at 46°.327; at 48° = .351; at 50°=.375; at 52°=.401 ; at 54° = .429; at 56°=.458; at 58.49; at 60°.524; at 62° = .56; at 64°.597; at 66.635; at 68°.676; at 70.721; at 73.796; at 76° =.88; at 80°-1.0; at 84°1.114; at 90=1.36; at 95° = 1.58; at 100=1.86; at 125° 3.79; at 1456.53; at 180° 15.15; and at 212° it amounts to 30 inches, or one atmosphere the grains of water evaporated in the same time are in a similar ratio. = Let 10 cub. in. of air at 45° be raised to the temperature of 60°, then the volume of the same air under the increased heat will equal 10 × (60 + 448÷45 + 448) 10.304 cub. in. Hence the formula, t' + 448 where v represents the gas or air in its original volume, v' its altered capacity; 480-32. t = the original temp.; t' the increased temperature; and the constant 448= 2 Poggendorf's Annalen. lv. 1. 3 Ann. de Chim. et de Phys. 3d ser. iv. 5, and v. 52. Dalton, Manch. Mem. vol. v. p. 559; Ure,-Ph. Tr. 1818; Ann. de Ch. 1830. xliii. 74, where the experiments of the French Commission composed of MM. Arago, Dulong, Ampère, De Prony, and Girard are recorded; Bulletin Univ. xii.; Brande's Journ, N. S. viii. 191 ; &c. In applying this table of vaporic force, let us suppose that the atmospheric temperature is 70°, that the dew-point, a term to be presently explained, is found to be 50°, and the barometer 30 in., then the moisture floating in the atmosphere will be equal to the force of vapour at that temperature divided by the barometric pressure, i. e. in this instance 0.375÷30, or 1-80th of its volume,-100 parts of such an atmosphere consisting of 98.75 of dry air, and 1.25 of the vapour of water in an expanded condition. In this case, 29.625 inches of the mercury in the barometer are supported by the dry air, and the remaining fraction 0.375 inch by the aqueous vapour-(vide 120). Again, let us suppose the dew-point to be 40° and the barometer 30 in., then the humidity will be 0.263 divided by 30, or about 1-114th of its volume. Let us illustrate this law by another example; if the dew-point be 58°, the vaporic force will be 0.49, and if the barometer stands at 30.5 inches, then the humidity will be a little more than 1-62d of the volume of the air. 118. The tension of vapour, according to Professor Dove, reaches the maximum when the wind is south-west, and declines to the minimum when north-east winds are blowing; it falls on the western, and rises on the opposite side of the windrose. This arises from the repressing influence of currents, cold, dense, and dry, upon the warm humid ones acting on the western side of the compass: the converse is the case on the eastern side. 'The S. W. is the equatorial current which has descended through the lower current to the surface of the earth; the N. E. is the polar current prevailing undisturbed." 119. The diffusion of aqueous vapour in the atmosphere is what is meant by its humidity. Various circumstances lead to alterations in this respect, as temperature and the physical features of the locality. The presence of water blended with the air may be experimentally illustrated. Let that fluid be exposed to the air in a closed vessel; speedily a portion will have risen in invisible vapour, and that in proportion to the temperature; now introduce one or other of those salts which easily dissolve or deliquesce on exposure to moisture,—e.g. acetate, nitrate, or hydriodate of potass, or iodide of iron,—and |