again when the temperature of the surrounding air was 4.5° above the freezing point. Dr. Black has rendered it probable that the quantity of caloric which disappears during spontaneous evaporation, is as great as that which is necessary to convert water into steam. We have a right then to conclude, that water, when it evaporates spontaneously, is always converted into vapour, and of course that it is only in that state that it enters into the atmo. sphere. In the fourth place, Mr. Dalton has demonstrated, that the water which exists in air, possesses precisely the same degree of elasticity that it does when in the state of a vapour in a vacuum at the same temperature. Hence it follows irresistibly that it exists in air, not in the state of water, but of an elastic fluid or vapour. We are authorised to conclude, then, that the water which exists in the atmosphere is in the state of vapour. This vapour is held in solution by the air precisely as one species of gas is by another. Hence the reason why it is so difficult to separate it, and why it is capable of undergoing a considerable degree of compres sion without assuming the form of a liquid. II. Many attempts has been made to measure the quantity of water contained in air; but Saussure was the first who attained any thing like precision. This ingenious philosopher has shown, in his Hygrometrical Essays, that an English cubic foot of air, when saturated with water, at the temperature of 66°, contains only about eight grains troy of that liquid, or about 1-67th of its weight. But the experiment of Mr. Dalton was susceptible of more precision. As the greatest part of the water of the atmophere is in the state of vapour, the elasticity of which depends upon the temperature, it is obvious that this elasticity, provided it can be ascertained, must measure the quantity of vapour which exists in the atmosphere, the temperature being the same. The elasticity or force of vapour was determined by this ingenious philosopher in the following manner, which had been originally contrived by Le Roy: he took a tall cylindrical glass jar, dry on the outside, and filled it with cold spring water fresh from the well if dew was immediately formed on the outside, he poured the water out, let it stand the while to increase in temperature, dried the outside of the glass well with a linen cloth, and then : poured the water in again. This operation was to be continued till the dew ceased to be formed, and then the temperature of the water was observed; and opposite to it in the Table was found the force of vapour in the atmosphere. This experiment must be conducted in the open are, or at a window; because the air within is generally more humid than that without. Spring water is generally about 50°, and will mostly answer the purpose of the three hottest months in the year: in other seasons an artificial cold mixture is required. From Dalton's experiments it follows that the quantity of va pour in the atmosphere is variable in quantity. In the torrid zone its force varies from 0.6 to one inch of mercury. In Britain it seldom amounts to 0.6, but is often as great as 0.5 during summer. In winter it is often as low as 0-1 of an inch of mer cury t. These facts would enable us to ascertain the absolute quantity of vapour contained in the atmosphere at any given time, provided we were certain that the density and elasticity of vapours follow precisely the same law as that of gases, as is extremely likely to be the case. If so, the vapour will vary from to part of the atmosphere. Dalton supposes that the medium quantity of vapour held in solution at once in the atmosphere may amount to about of its bulk ‡. [Thomson's Chemistry. SECTION IV. Atmospheric Carbonic Acid. THE existence of carbonic acid gas as a constituent part of the atmosphere, was observed by Dr. Black immediately after he had ascertained the nature of that peculiar fluid. If we expose a pure alkali or alkaline earth to the atmosphere, it is gradually converted into a carbonate by the absorption of carbonic acid gas. This fact, which had been long known, rendered the inference, that carbonic acid gas existed in the atmosphere, unavoidable, as soon as the difference between a pure alkali and its corbonate had * Dalton, Manchester Memoirs, V. 547. + Phil. Mag. xxiii. 353. been ascertained to depend upon that acid. Not only alkalies and alkaline earths absorb carbonic acid when exposed to the air, but several of the metallic oxides also. Hence the reason that we so often find the native oxides in the state of carbonates. rust is always saturated with carbonic acid. Thus Carbonic acid gas not only forms a constituent part of the atmosphere near the surface of the earth, but at the greatest heights which the industry of man has been able to penetrate. Saussure found it at the top of Mount Blanc, the highest point of the old continent; a point covered with eternal snow, and not exposed to the influence of vegetables or animals. Lime-water diluted with its own weight of distilled water, formed a pellicle on its sur. face after an hour and three quarters exposure to the open air on that mountain; and slips of paper moistened with pure potash, acquired the property of effervescing with acids after being exposed an hour and a half in the same place*. Now this was the height no less than 15,668 feet above the level of the sea. Humboldt has more lately ascertained the existence of this gas in air brought by Mr. Garnerin from a height not less than 4280 feet above the sur. face of the earth, to which height he had risen in an air-balloont. This fact is a sufficient proof that the presence of carbonic acid in air does not depend upon the vicinity of the earth. The difficulty of separating this gas from air has rendered it difficult to determine with accuracy the relative quantity of it in a given bulk of air. From the experiments of Humboldt, it appears to vary from 0·005 to 0·01. Mr. Dalton's experiments give the quantity much smaller. He found, that if a glass vessel filled with 102,400 grains of rain water be emptied in the open air, 125 grains of lime water be poured in, and the mouth then closed; by sufficient time and agitation, the whole of the lime water is just saturated with the carbonic acid which it finds in the inclosed volume of the air: but 125 measures of lime-water require 70 measures of carbonic acid gas to saturate them. Hence he concludes, that air contains only 1-1400th of its bulk of carbonic acid ‡. From the previous experiments of Mr. Cavendish, however, we * Saussure's Voyages, iv. 199. hil. Mag. xxiii. 354, + Jour. de Phys, xlvii. 202. learn that lime water is not capable of depriving air of the whole of its carbonic acid. A portion still remains, which can only be separated either by milk of lime, or by repeated washings with new doses of lime-water. Hence the quantity of carbonic acid in air must be considerably greater than it was found by Dalton. I do not know exactly the meaning of lime-water being just saturated, unless it signifies that it refuses to absorb any more gas. In that case the whole of the lime is held in solution by the acid. It must be difficult to ascertain the exact point of saturation according to this sense of the word. We may conclude, however, from Dalton's experiment, that the bulk of carbonic acid in air does not much exceed 1-1000th of the atmosphere; but it is liable to variation from different circumstances. Immense quantities of carbonic acid must be constantly mixing with the atmosphere, as it is formed by the respiration of animals, by combustion, and several other processes which are going on continually. The quantity, indeed, which is daily formed by these processes is so great, that at first sight it appears astonishing that the gas does not increase rapidly. The consequence of such an increase would be fatal, as air containing 0.1 of carbonic acid extinguishes light, and is noxious to animals. But we shall find reason afterwards to conclude, that this gas is decomposed by vegetables as rapidly as it is formed. [Thomson's Chemistry. SECTION V. Atmospheric unknown Bodies. FROM the three preceding Sections, we see that the atmosphere consists chiefly of three distinct elastic fluids united together by chemical affinity; namely, air, vapour, and carbonic acid gas; differing in their proportions at different times and in different places; but that the average proportion of each is 98.9 air 1.0 vapour 0.1 carbonic acid 100.0 But besides these bodies, which may be considered as the constituent parts of the atmosphere, the existence of several other bo. dies has been suspected in it. I do not mean in this place to include among those bodies electric matter, or the substance of clouds and fogs, and those other bodies which are considered as the active agents in the phænomena of meteorology, but to confine myself merely to those foreign bodies which have been occasionally found or suspected in air. Concerning these bodies, however, very little satisfactory is known at present, as we are not in possession of instruments sufficiently delicate to ascertain their presence. We can indeed detect several of them actually mixing with air, but what becomes of them afterwards we are unable to say. near 1. Hydrogen gas is said to have been found in air situated ne the crater of volcanoes, and it is very possible that it may exist always in a very small proportion in the atmosphere; but this cannot be ascertained till some method of detecting the presence of hydrogen combined with a great proportion of air be discovered. From the experiments of Gay Lussac and Humboldt, it appears that air does not contain so, much as 3-1000th parts hydrogen. 2. Carbureted hydrogen gas is often emitted by marshes in considerable quantities during hot weather. But its presence has never been detected in air; so that in all probability it is again de. composed by some unknown process. 3. Oxygen gas is emitted by plants during the day. We shall afterwards find reason to conclude that this is in consequence of the property which plants have of absorbing and decomposing carbonic acid gas. Now, as this carbonic acid is formed at the expense of the oxygen of the atmosphere, as this oxygen is again restored to the air by the decomposition of the acid, and as the nature of atmospheric air remains unaltered, it is clear that there must be an equilibrium between these two processes; that is to say, all the carbonic acid formed by com. bustion must be again decomposed, and all the oxygen abstracted must be again restored. The oxygen gas which is thus continually returning to the air, keeps its component parts always at the same ratio. |