What do you call these airs which compose our atmosphere ?* They are called oxygen air and nitrogen air. Are oxygen and nitrogen the only substances which enter into the composition of the atmosphere? No; atmospheric air contains also one part in every thousand‡ of carbonic acid gas, and several adventitious substances. to a careful analysis of the contents of a glass balloon that had been filled with air at the height of 20,000 feet from the earth, it was found similar in every respect to that taken from the surface. Nicholson's Journal, vol. x. 286. * Atmospheric air is a mixture of two distinct and solid substances, viz. oxygen and nitrogen rendered aërial by the expansive power of caloric: it likewise contains a portion of carbonic acid gas. Oxygen and nitrogen, combined in various proportions, form also no less than three other compounds, viz. Nitric acid consisting of about 4 parts oxygen to I of nitrogen. Nitrous gas, of Nitrous oxide, of 3 do. to 1 do. to 2 do. The respirable part of atmospheric air has been called oxygen, on account of its acidifying principle: the other part has been termed azote, from its known quality of killing all animals that are obliged to breathe it, when separated from oxygen. The terms are taken from the Greek language. I have in this work adopted nitrogen in preference to azote, because it is the base of nitric acid, and it agrees in termination with oxygen and hydrogen. Carbonic acid gas and hydrogen gas are as incapable of supporting life as nitrogen; therefore there is no reason why one should be called azote more than the other. It should be remarked, that oxygen denotes the solid base of atmospheric air, and that this requires light as well as caloric in order to convert it into oxygen gas. During combustion, the vital air gives out this light in every direction. The proportion of carbonic acid gas in atmospheric air was formerly calculated at one per cent.; but Mr. Dalton has lately demonstrated that it does not amount to more than one part in a thousand. Manchester Mem. N. S. vol. i. 254. If a pure alkali be exposed to the atmosphere, it will gradually absorb carbonic acid. This is also the case with se What other substances are found in atmospheric air? Besides carbonic acid gas,* it holds a portion of water in solution ;t and sometimes contains hydrogen and carburetted hydrogen gases. What are the sources of these other of these other gases? Carbonic acid gas is constantly formed by the respiration of animals and by combustion; and veral of the metallic oxides. As atmospheric air is always furnished with carbonic acid, no wonder that so large a number of the salts are found in the state of carbonates. Carbonic acid gas is found to exist in the atmosphere not only near the surface of the earth, but at the greatest heights. Saussure found it at the top of Mont Blanc, which is esteemed the highest point of the old continent. However, it is probable that the proportion of carbonic acid is not so large at great heights as it is near the earth; for the unfortunate philosophers who attended La Perouse in his last voyage could not detect it in the atmosphere at the summit of the peak of Teneriffe. See La Perouse's Voyage. * Carbonic acid gas is nearly twice as heavy as common air: hence it was imagined that it must combine chemically with the atmosphere, or it would be found only near the surface of the earth; but Mr. Dalton has proved that when two or more elastic fluids come in contact, they penetrate each other and form an equable mixture; and that therefore a lighter gas cannot float on the surface of a heavier. Upon an average, atmospheric air contains about 1 per cent. of water in a state of elastic vapour. For calculations respecting its pressure on the earth, see Manchester Memoirs, N. S. vol. i. p. 253. It is remarkable, that whenever aqueous vapour is united to atmospheric air an augmentation of volume is the consequence, and damp air is always specifically lighter than dry air. From the frequent decompositions which are taking place upon the surface of the earth, the atmosphere must always contain a portion of hydrogen gas; but we have no ready means of detecting it. The quantity of carbonic acid which is daily formed by these processes is so great, that it must have increased rapidly, hydrogen and carburetted hydrogen gases arise from various sources, particularly from marshes, stagnant pools, &c. all which are prejudicial to the animal creation. If carbonic acid air and carburetted hydrogen air are prejudicial to animal life, how are they corrected in the atmosphere? These airs which would cause the death of any animal obliged to breathe them,* are the proper nutriment of vegetables, and nature has endowed them with organs for their decomposi tion. had not the Almighty provided means for its being as rapidly decomposed. The wisdom and goodness of this appointment must be apparent to those who know that whenever atmospheric air becomes charged with one tenth of this gas, it is unfit for promoting combustion, and is fatal to most animals that are obliged to breathe it. * Every chemist must be aware that a large quantity of carburetted hydrogen gas is perpetually evolved at the surface of the earth; he must also know that this gas is fatal to animal life. I could adduce a melancholy instance of a gentleman who inhaled it by mistake, and died almost immediately in consequence of it. How then has the all-wise Artificer of the world contrived to protect its inhabitants from the baneful influence of that immense quantity with which the atmosphere is perpetually contaminated? The means are as simple as they are important.-Vegetables are so constituted that carbon and hydrogen are the necessary food of plants, and conduce to the support of vegetable life their vegetating organs seize the carbonic acid gas which comes within their reach, and while they appropriate the carbon to themselves, the oxygen is thrown off to renovate the atmosphere by its union with the nitrogen rejected by animal respiration. As all vegetables are in want both of carbon and hydrogen, there can be little doubt but that by their means the atmosphere is divested of carburetted hydrogen gas also. Thus, what is noxious to man is rendered beneficial to vegetables; and the oxygen which vegetables are not in want of, is separated by them in its utmost purity for the use of Is there much difference in the nature of these gases? Yes they are of different and opposite qualities.* What are the properties of oxygen gas? The oxygen gas in atmospheric air is the principle of combustion,† and the vehicle of heat ; atmospheric air, fire would lose its strength, candles would not diffuse such complete light, and animals would with difficulty separate the necessary quantity of the vivifying oxygen. On the other hand, if the atmosphere were more charged with oxygen than nitrogen, animals indeed would acquire a freer respiration; but let us consider the activity which fire would acquire by air of superior purity. We know that, on some occasions, the least spark excites the strongest flame in a combustible body, and which increases so much as to consume it in a few moments: candles then would be no sooner lighted than they would be destroyed, without answering any other purpose than that of dazzling us for a few moments: iron would be calcined, instead of acquiring from the fire that softness necessary for transforming it into various instruments, and which it cannot receive in a more moderate beat. Nothing would be capable of checking the progress of this destructive element, which is nourished by vital air, if this aëriform substance were not abundantly mixed with mephitic air, which serves to restrain it." Anthony de Marti on the Constitution of the Atmosphere. * These gases are of such opposite qualities, that the one is sometimes called vital air; while the other, from its causing the death of those who breathe it, is by the French chemists (as has before been remarked) called azotic gas. The necessity of oxygen for supporting combustion may be shown by the following simple experiment. Pour a little water on a flat dish, place two or three dighted wax tapers of different lengths in the waler, and invert a tall glass jar over them. The flame of the different tapers will soon be seen to grow smaller, and at length will be extinguished in succession. That which is highest will be extinguished first, and the shortest taper the last, owing to the purer air occupying the lower part of the jar. Dr. Higgins having caused a young man to breathe pure oxygen gas for several minutes, his pulse, which was at 64, soon rose to 120 beats in a minute. By abstracting a part of the oxygen from atmospheric air, the pulse may likewise be lowered at pleasure. Pure oxygen gas has been used also with and is absolutely necessary for the support of animal life.* What is the nature of oxygen gas, when in a separate state? Pure oxygen gas has the property of accelerating the circulation of all the animal fluids, and occasions the most rapid combustion of all combustible substances; so that it is the most energetic and powerful agent that we are acquainted with. What is the specific gravity of oxygen gas? Oxygen gas is a little heavier than atmospheric air, and 740 times lighter than water. success in case of suspended animation. See Additional Notes, No. 6 and 8. Water impregnated with oxygen gas has been found a valuable remedy in some particular diseases. By depriving common water of atmospheric air by boiling, and then forcing oxygen gas into it, I have occasioned a large absorption of it, and have found that it retained it better than carbonic acid gas is retained by water. Later experiments by Dr. Henry have determined that water takes up of its volume at 60° whatever be the density of the gas. * It has been proved by Mr. Hassenfratz, that oxygen is necessary to promote the vigour of plants as well as that of animals; and that, to this end, a much larger quantity of oxygen is combined with snow, and in rain water, than in river or spring water. "LEAVES, LUNGS, and GILLS, the vital ether breathe On earth's green surface, or the waves beneath." A collection of experiments on the effects of oxygen on animal and vegetable life may be seen in "Archer's Observations on Oxygen," 8vo, 1798. See Additional Notes, No. 6. At the temperature of 54.50, when the barometer stands at 28 inches, oxygen gas is 12 drachms (or 1 ounce) to each cubical foot: whereas nitrogen gas is only 10 drachms 48 grains to the cubical foot, Oxygen gas is plentifully procured from nitre, or from the black oxide of manganese. Four ounces of nitre melted with a little slacked lime, produced Mr. Ingenhousz 3000 cubic inches of vital air. See Additional Notes, No. 7. |