GALLERY OF NATURE AND ART PART I. NATURE. BOOK II. GEOLOGY. [CONTINUED.] CHAP. XXXIV. ATMOSPHERICAL DEPARTMENT. We now advance to the atmospherical phænomena of the science of Geology, in the extensive sense in which we have em ployed this term; and shall proceed to a brief survey of whatever is most curious or worthy of observation in the composition of the Atmosphere; the variation of Climate; the extremes of Heat and Cold; the existence of Electricity and electric Powers, as Thunder and Lightning, Aurora Borealis, Water-spouts; Falling-stars, and Ignes Fatui; Echoes, Wind, Hurricanes, and Storms; the origin of Mist, Dew, Sleet, Snow, Mirages, or Fata Morgana, Meteoric, Stones, and various other appearances connected with or dependent upon the preceding; and which collectively constitute the Science of Meteorology. VOL, IV. B CHAP. XXXV. GENERAL NATURE OF THE ATMOSPHERE. THE atmosphere is that invisible elastic fluid which surrounds the earth to an unknown height, and incloses it on all sides. It was thus denominated by the Greeks in consequence of the vavours which are continually mixing with it, or combined in it*, In contemplating the nature of the atmosphere there are two points of considerable importance to be attended to, respecting which therefore we shall offer a summary of the best established facts and opinions of the present day; and these are the materials that enter into its composition, and the changes to which it is liable. SECTION I. Composition of the Atmosphere. NEITHER the properties nor the composition of the atmosphere seems to have occupied much of the attention of the ancients. Aristotle considered it as one of the four elements, situated between the regions of water and fire, and mingled with two exhalations, the dry and moist; the first of which occasioned thunder, lightning, and wind; while the second produced rain, snow, and hail. The ancients, in general, seem to have considered the blue colours of the sky as essential to the atmosphere; and several of their philosophers believed that it was the constituent principle of other bodies, or at least that air and other bodies are mutually convertible into each other. Thus Lucretius: Semper enim quodcunque fluit de rebus, id omne • From ăτμ, a vapour, and cpaîpa, a sphere. Lib. v. 274. All that pours profuse From things, perpetual, the vast ocean joins Paid not, thus ballancing the loss sustain'd, All into air would dissipate and die. Hence, born from things, to things air still returns GOOD. But these opinions continued in the state of vague conjectures, till the matter was explained by the sagacity of Hales, and of those philosophers who followed his illustrious career. It was not till the time of Bacon, who first taught mankind to investigate natural phenomena, that the atmosphere began to be investigated with precision. Galileo introduced the study by pointing out its weight; a subject which was soon after investigated completely by Torricelli, Paschal, &c. Its density and elasticity were ascertained by Boyle and the Florence Academicians. Ma. riotte measured its dilatability; Hooke, Newton, Boyle, Derham, pointed out its relation to light, to sound, and to electri city. Newton explained the effect produced upon it by moisture; from which Halley attempted to explain the changes in its weight indicated by the barometer. But a complete enumeration of the discoveries made upon the atmosphere in general belongs to pneumatics; a science which treats professedly of the mechanical properties of air. The knowledge of the component parts of the atmosphere did not keep pace with the investigation of its mechanical properties. The opinions of the earlier chemists concerning it are too vague and absurd to merit any particular notice. Boyle, however, and his contemporaries, put it beyond doubt that the atmosphere contained two distinct substances. 1. An elastic fluid distinguished by the name of air. 2. Water in a state of vapour. Besides these two bodies, it was supposed that the atmosphere contained a great variety of other substances, which were continually mix. ing with it from the earth, and which often altered its properties, and rendered it noxious or fatal. Since the discovery of carbonic acid gas by Dr. Black, it has been ascertained that this elastic fluid always constitutes a part of the atmosphere.] The constituent parts of the atmosphere therefore are, These shall form the subject of the four following Sections. [Thomson's Chemistry. SECTION 11. Atmospheric Air. THE word AIR seems to have been used at first to denote the atmosphere' in general; but philosophers afterwards restricted it to the elastic fluid, which constitutes the greatest and the most important part of the atmosphere, excluding the water and the other foreign bodies which are occasionally found mixed with it. For many years all permanently elastic fluids were considered as air, from whatever combinations they were extricated, and supposed to possess exactly the same properties with the air of the atmosphere. It is true, indeed, that Van Helmont suspected that elastic fluids possessed different properties; and that Boyle ascertained that all elastic fluids are not capable of supporting combus. tion like air. But it was not till the discoveries of Cavendish and Priestley had demonstrated the peculiar properties of a variety of elastic fluids, that philosophers became sensible that there existed various species of them. In consequence of this discovery, the word air became generic, and was applied by Priestley, and the British and Swedish philosophers in general, to all permanently elastic fluids, while the air of the atmosphere was distinguished by the epithets of common or atmospheric air: but Macquer thought proper to apply the term gas, first employed by Van Helmont, to all permanently elastic fluids except common air, and to confine the term air to this last fluid. This innovation was scarcely necessary; but as it has now been generally adopted, it will be proper to follow it. By the word air, then, in this Section, is meant only common air, or the fluid which forms by far the greatest part of the atmosphere. The foreign bodies which are mixed or united with air in the atmosphere are so minute in quantity compared to it, that they have no very sensible influence on its properties. We may therefore consider atmospheric air, when in its usual state of dryness, as sufficiently pure for examination. 1. Air is an elastic fluid, invisible indeed, but easily recoguized by its properties. Its specific gravity, according to the experiments of Sir George Shuckburgh, when the barometer is at 30 inches, and the thermometer between 50° and 60°, is usually reckoned 1.000: It is 816 times lighter than water. One hundred cubic inches of air weigh 31 grains troy. But as air is an elastic fluid, and compressed at the surface of the earth by the whole weight of the incumbent atmosphere, its density diminishes according to its height above the surface of the From the experiments of Paschal, Deluc, General Roy, &c. it has been ascertained, that the density diminishes in the ratio of the compression. Consequently the density decreases in a geometrical progression, while the heights increase in an arithme. tical progression. Buguer had suspected, from his observations made on the Andes, that at considerable heights the density of the air is no longer proportional to the compressing force; but the experi ments of Saussure junior, made upon Mount Rose, have demon. strated the contrary +. 2. Although the sky is well known to have a blue colour, yet it cannot be doubted that air itself is altogether colourless and invisible. The blue colour of the sky is occasioned by the vapours which are always mixed with air, and which have the property of reflecting the blue rays more copiously than any other. This has been proved by the experiments which Saussure made with his cyanometer at different heights above the surface of the earth. This instrument consisted of a circular band of paper, divided into fifty-one parts, each of which was painted with a different shade of blue; beginning with the deepest mixed with black, to the lightest mixed with white. He found that the colour of the sky always corresponded with the deepest shade of blue the higher the observer is placed above the surface; consequently, at a cer tain height, the blue will disappear altogether, and the sky appear black; that is to say, will reflect no light at all. The colour becomes always lighter in proportion to the vapours mixed with the air. Hence it is evidently owing to them. * Mem. Par. 1753. p. 515. |