Atmosphere. Volta's eudiometer. De Marti's riment, we found it pretty exact, provided the diameter of the glass vessel employed be not less than three or four inches. In narrow vessels it is very inaccu rate. The second kind of eudiometer was first proposed by Volta, and hence is usually known by the name of Volta's eudiometer. It consists in mixing 100 parts of the air to be analysed with 100 parts of hydrogen gas in a graduated tube, and passing an electric spark through the mixture. A detonation takes place; the whole of the oxygen, and part of the hydrogen, being converted into water. This method is very easy, and is susceptible of great precision. From numerous and decisive experiments, it follows, that one part of oxygen combines with two parts of hydrogen, when the experiment is made in this way. It appears, too, that the whole oxygen disappears, provided the quantity of hydrogen present be sufficient. We have only to mix 100 parts of air and 100 parts of hydrogen gas together, detonate the mixture, and observe the diminution of bulk. The third part of that diminution indicates the quantity of oxygen present. Suppose, with the preceding proportions, that the diminution of bulk amounts to 63, the third part of that number, or 21, indicates the quantity of oxygen gas in 100 parts of air. He must be a careless experimenter, that, with this eudiometer, commits an error of 1 per cent. We therefore consider it as one of the best means of determining the proportion of oxygen gas present in any gaseous mixture. It does not answer quite so well when we use it to ascertain the purity of oxygen gas, or of a gas composed chiefly of oxygen; because, in that case, the diminution of bulk in the gaseous mixture is so sudden and so great, that a vacuum is formed, and the water over which the experiment is made lets go a portion of air, which mixes with the residue, and makes it appear greater than it otherwise would be, and, of course, diminishes the proportion of oxygen which the gas really contained. This error is diminished if the water has been recently boiled. We cannot state the amount of this error; though we have convinced ourselves that it often exceeds 2 per cent. Another method of analysing air, is to expose and Davy's. 100 measures of it to a solution of sulphuret of lime, or of sulphate of iron saturated with nitrous gas. The first of these liquids was recommended by De Marti, the second by Mr Davy. They both answer very well; they gradually absorb the whole of the oxygen, and leave the azote: Hence the diminution of bulk gives the oxygen in the air examined. The absorption may be made in a graduated tube, or in a eudiometrical instrument contrived for the purpose by Mr Pepys, and described in the Philosophical Transactions for 1808. about 3 per cent. of its bulk; but if you bring it near the fire, the phosphorus soon absorbs the whole oxygen. When the thermometer stands at 70°, the absorption is completed in a few hours. You know the completion by the phosphorus ceasing to smoke. The oxygen, by this process, is removed; but as the azote dissolves a portion of the phosphorus, its bulk is a little greater than it ought to be. According to Berthollet, to obtain the true bulk of the residuary azote, you must diminish it by d part. This method is not so convenient as the preceding, at least in this country, because it is so tedious; but it is sufficiently accurate. : Atmo sphere. The third constituent of the atmosphere is car- Proportion bonic acid gas. Its presence in the atmosphere of carbonic was recognised as soon as Dr Black had ascertained acid in the the cause of the difference between mild and caustic atmosphere alkalies For it was known, that a caustic alkali soon becomes mild by exposure to the air. Dr Black ascertained, that the mildness is owing to the absorption of carbonic acid. From the observations of Saussure we learn, that this gas exists in the atmosphere on the summit of Mount Blanc, which is nearly 16,000 feet above the level of the sea; for limewater soon deposited its lime in the state of carbonate, when exposed upon the summit of that mountain, (Saussure's Voyages, iv. 199.) Humboldt found it in a quantity of air brought down by Garnerin from a height of 4280 feet, to which he had ascended in an air balloon, (Jour. de Phys. xlvii. 202.) It appears, therefore, to constitute a part of every portion of the atmosphere to which we have access. As this acid gas is produced in great quantities by combustion, respiration, fermentation, and many other of the most common processes of nature, one would be disposed to believe, at first view, that its quantity must be constantly increasing. But this does not ap pear to be the case, it must therefore be decomposed and separated from air as fast as it is formed. It is of so deleterious a nature, that, if it were to accumulate to any extent, it would render air incapable of supporting life. A candle will not burn in air contaminated with one-ninth of carbonic acid gas. The quantity of this gas in air is small. Many attempts have been made to ascertain it; but the process is so difficult, that absolute precision cannot be looked for. It was long believed that the carbonic acid present in the atmosphere amounted to one per cent. Humboldt made many experiments on the subject, and concluded from them, that its bulk varied from one per cent. to half a per cent. But this determination is certainly excessive. According to the experiments of Mr Dalton, a quantity of air, equal in bulk to 102,400 grains of water, contains a quantity of carbonic acid just capable of saturating 125 grains of lime-water: 70 measures of carbonic acid gas would produce the same effect: Hence he concludes, that the atmosphere contains th part of its bulk of carbonic acid gas, (Phil. Mag. xxiii. 354.) This quantity we consider as rather below the truth. Mr Cavendish has shewn, that lime-water is not capable of depriving air completely of carbonic acid gas: Hence a portion would still remain in Mr Dalton's experiment. Perhaps we shall not err far sphere. if we state the bulk of carbonic acid gas in the atmosphere at th part. vapour. Proportion The 4th constituent of the atmosphere is water of aqueous in the state of vapour. That water forms a constituent part of the atmosphere, has been known in all ages, and indeed is demonstrated by the rain and dew which is continually falling, and by the great quantity of moisture which sulphuric acid, potash, and other bodies, absorb when exposed to the atmosphere. The quantity of moisture in the atmosphere has been observed to vary greatly at different times, and various instruments have been invented to mea. sure that quantity. These instruments are called hygrometers. The most ingenious of them are those of Leslie, Saussure, and De Luc. Unknown bodies. It was at first supposed, that the water in the atmosphere was still in the state of water, and that it was held in solution in air precisely as salts are dissolved in water. But it has been at last established by satisfactory experiments, that the water in the atmosphere is in the state of vapour. To De Luc, Saussure, and Dalton, we are chiefly indebted for these experiments. As to the quantity of water which exists in the atmosphere, it depends upon a variety of circumstances, the investigation of which belongs to that branch of science called METEOROLOGY; to which, therefore, we refer. Saussure found that a cubic foot of air, saturated with moisture at 66°, contains about 8 grains troy of water, or th of its weight. Supposing air always saturated with moisture, the quantity always increases with the temperature, because the elasticity of aqueous vapour increases with the temperature. Hence, in cold weather, the quantity of vapour in air is always small; whereas, in warm weather, it is often considerable. In the torrid zone the aqueous vapour in the atmosphere is capable of supporting from 0.6 to 1 inch of mercury. In Britain it is hardly ever capable of supporting 0.6 inch of mercury; but in summer it is often capable of supporting 0.5 inch, while in winter it often does not exceed 0.1 inch. From these facts it follows, that the weight of water present in the atmosphere varies from to of the whole. Mr Dalton supposes, that the medium quantity of vapour held in solution at once in the atmosphere, may amount to th of its bulk. These four bodies, oxygen, azote, carbonic acid, and vapour, are the only known constituents of the atmosphere. It cannot be doubted, that other bodies are occasionally present in it. The dreadful effects of marshy situations upon the health of the inhabitants, and the fatal rapidity with which certain diseases are propagated, cannot well be accounted for, without supposing that certain substances which duce a deleterious effect on the animal economy, are occasionally present in the atmosphere. But hitherto no method has been discovered of ascertaining the presence of these bodies, and subjecting them to examination. They are too subtile for our apparatus, and altogether escape the cognizance of our senses. It has been ascertained, however, that certain acid fumes, as those of the muriatic acid, nitric acid, and above all, of the oxymuriatic acid, have the property of destroying these miasmata, or at least of preventing pro Having considered in the preceding paragraphs the dilatation of atmospherical air by heat, and its chemi- Physical cal composition, we shall now proceed to give a brief properties and general view of its physical properties, reserving of the atmosphere. the full discussion of the subject to the article PNEUMATICS. That atmospherical air is a heavy, compressible, and elastic substance, may be proved by many simple and direct experiments. A bladder filled with air is heavier than when it is in its flaccid state. When subjected to compression, it may be made to occupy a smaller space; and when that pressure is removed, its elasticity enables it to resume its original size. Since the air is heavy, the lower strata of the atmosphere are compressed by the weight of the superincumbent mass. The lowest stratum supporting Gradation the weight of almost the whole atmosphere, will be in the denmore dense than the rest; and the superior strata will sity of the atmosphere gradually become more rare, in proportion to the weight which they sustain. The air in the higher regions, therefore, will be extremely rare, on account of its elasticity, which is not checked by any superincumbent pressure. If the air were perfectly elastic, it is obvious that there would be no limit to its expansion, and that the whole atmosphere would be dissipated through infinite space. The elasticity of the atmosphere must consequently diminish in a greater ratio than the weight which compresses it, and there must be a certain state of rarity at which its elasticity ceases. Upon the supposition that the rarity of the air is reciprocally proportional to its superincumbent weight, it may be demonstrated, that if the heights in the atmosphere be taken in arithmetical progression, the rarity of the air at these heights will be in geome trical progression, or, what is the same thing, the altitudes in the atmosphere are as the square roots of the corresponding rarities. Hence we have a method of measuring differences of altitude, by ascertaining with the barometer the rarity of the air at two places whose vertical distance is required. A full account of this method will be found under the article BARO METER. The weight and pressure of the atmosphere may be Weight of ascertained by very simple experiments. If we im- the atmo merse in water a glass tube open at both ends, the sphere. water included in the tube will be on the same level with the fluid which surrounds it. When we apply our mouth to the upper end of the tube, and draw out the air, the included water instantly ascends till the weight of the elevated column added to the elasticity of the remaining air, exactly balances the pressure of the atmosphere on the surrounding fluid. If we now take a long tube, 40 feet long for example, shut at one end, and having filled it with water, plunge the open end into a vessel of water, the fluid will then descend in the tube till the weight of the column exactly equals the pressure of the atmosphere; for the air is now excluded from the upper part of the tube, and the weight of the column of fluid is the only force which is left to balance the weight of the atmospherical column. By making this experiment, it will be found that the water stands at from 34 to Atmosphere. Variations sure of the atmo 35 feet above the general level of the surrounding From the changes which take place in the atmoin the pres- sphere, its pressure is liable to very considerable variations. The column of quicksilver which we have shown to be a measure of that pressure, varies from 28 to 31 inches. The cause of these changes, which are yet but imperfectly known, will be considered under the article METEOROLOGY. sphere. Height of the atmosphere. Refraction of the at If the atmosphere were of uniform density, it would When a ray of light enters the atmosphere, it is bent from its course by the same cause which reInosphere. fracts the rays of light when they pass through any dense medium, such as glass or water. The refraction sustained by light at its first entrance into the atmosphere must be very small, from the extreme rarity of the air. The deviation, however, will gradually increase as it penetrates the denser strata, and the ray will describe a path increasing in curvature as it approaches the earth. From this property of the atmosphere, the apparent altitude of the sun, moon, and stars, is greater than their real elevation, and they appear to be raised above the horizon when they are actually below it. The refraction of the atmosphere near the earth's surface is liable to very considerable anomalies. A very extraordinary phenomenon arising from this cause has been described by Mr Vince. The castle of Dover, concealed by the hill which lies between it and Ramsgate, appeared, on the 6th of August 1806, as if it had been brought over and placed on the side of the hill next to Rams-. gate. This phenomenon must have arisen from some variation of density in the intermediate air. Pheno- Atmo sphere. atmo But while the solar rays traverse the earth's at- Modifica mosphere, they suffer another change from the re- tions of sisting medium which they encounter. When the light by the sun, or any of the heavenly bodies, are considerably sphere. elevated above the horizon, their light is transmitted to the earth without any perceptible change; but when these bodies are near the horizon, their light must pass through a long tract of air, and is considerably modified before it reaches the eye of the observer. The momentum of the red, or greatest refrangible rays, being greater than the momentum of the violet, or least refrangible rays, the former will force their way through the resisting medium, while the latter will be either reflected or absorbed. A white beam of light, therefore, will be deprived of a portion of its blue rays by its horizontal passage through the atmosphere, and the resulting colour will be either orange or red, accordthat rays ing to the quantity of the least refrangible have been stopt in their course. Hence the rich and brilliant hue with which nature is gilded by the setting sun; hence the glowing red which tinges the morning and evening clouds; and hence the sober purple of twilight which they assume when their ruddy glare is tempered by the reflected azure of the sky. We have already seen, that the red rays penetrate Reflective through the atmosphere, while the blue rays, less power of able to surmount the resistance which they meet, are the atmo reflected or absorbed in their passage. It is to this sphere. cause that we must ascribe the colour of the sky, and the bright azure which tinges the mountains of the distant landscape. As we ascend in the atmosphere, the deepness of the blue tinge gradually dies away; and to the aeronaut who has soared above the denser strata, or to the traveller who has ascended the Alps or the Andes, the sky appears of a deep black, while the blue rays find a ready passage through the attenuated strata of the atmosphere. It is owing to the same cause that the diver, at the bottom of the sea, is surrounded with the red light which has pierced through the superincumbent fluid, and that the blue rays are reflected from the surface of the ocean. Were it not for the reflecting power of the air, and of the clouds which float in the lower regions of the atmosphere, we should be involved in total darkness by the setting of the sun, and by every cloud that passes over his disc. It is to the multiplied reflections which the light of the sun suffers in the atmosphere, that we are indebted for the light of day, when the earth is enveloped with impenetrable clouds. From the same cause arises the sober hue of the morning and evening twilight, which increases as we recede from the equator, till it blesses with perpetual day the inhabitants of the polar regions. If the earth were at rest, and not influenced by Figure of any other body of the system, its own figure, and the atmo that of its atmosphere, would be exactly spherical. sphere. In consequence of the diurnal motion of the earth, however, the figure of its atmosphere must be sphe Atmosphere. roidal like itself. "All the atmospheric strata," says La Place," should take after a time the rotatory motion, common to the body which they sur round. For the friction of these strata against each other, and against the surface of the body, should accelerate the slowest motions, and retard the most rapid, till a perfect equality is established among them. In these changes, and generally in all those which the atmosphere undergoes, the sum of the products of the particles of the body, and of its atmosphere, multiplied respectively by the area which their radii vectores projected on the plane of the equator describe round their common centre of gravity, are always equal in equal times. Supposing then, that by any cause whatever, the atmosphere should contract itself, or that a part should condense itself on the surface of the body, the rotatory motion of the body, and of its atmosphere, would be accelerated, because the radii vectores of the area, described by the particles of the primitive atmosphere becoming smaller, the sum of the product of all the particles, by the corresponding area, could not remain the same, unless the velocity of rotation augments. At its surface the atmosphere is only retained by its weight, and the form of this surface is such, that the force which results from the centrifugal and attractive forces of the body, is perpendicular to it. The atmosphere is flattened towards the poles, and distended at its equator; but this ellipticity has limits, and in the case where it is the greatest, the proportion of the axis of the pole and the equator is as two to three. The atmosphere can only extend itself at the equator, to that point where the centrifugal force exactly balances the force of gravity; for it is evident, that beyond this limit the fluid would dissipate itself. Relative to the sun, this point is distant from its centre by the length of the radius of the orbit of a planet, the period of whose revolution is equal to that of the sun's rotation. The sun's atmosphere then does not extend so far as Mercury, and consequently does not produce the zodiacal light, which appears to extend even to the terrestrial orbit. Besides, this atmosphere, the axis of whose poles should be at least two-thirds of that of the equator, is very far from having the lenticular form which observation assigns to the zodiacal light. The point where the centrifugal force balances gravity, is so much nearer to the body, in proportion as its rotatory motion is more rapid. Supposing that the atmosphere extends itself as far as this limit, and that afterwards it contracts and condenses itself from the effect of cold at the surface of the body, the rotatory motion would become more and more rapid, and the farthest limit of the atmosphere would approach continually to its centre: it will then abandon successively in the plane of its equator, fluid zones, which will continue to circulate round the body, because their centrifugal force is equal to their gravity. But this equality not existing relative to those particles of the atmosphere, distant from the equator, they will continue to adhere to it. It is probable that the rings of Saturn are similar zones, abandoned by its atmosphere. VOL. III. PART I. Atmo Atnah. If other bodies circulate round that which has been considered, or if it circulates itself round another spheres body, the limit of its atmosphere is that point where its centrifugal force, plus the attraction of the extraneous bodies, balances exactly its gravity. Thus the limit of the moon's atmosphere, is the point where the centrifugal force due to its rotatory motion, plus the attractive force of the earth, is in equilibrium with the attraction of this satellite. The mass of the moon being of that of the earth, this point is distant from the centre of the moon, about the ninth part of the distance from the moon to the earth. If, at this distance, the primitive atmosphere of the moon had not been deprived of its elasticity, it would have been carried towards the earth, which might have retained it. This is perhaps the cause why this atmosphere is so little perceptible." The earth's atmosphere must experience similar Oscillations oscillations to those of the ocean, from the action of of the atthe sun and moon. In an atmosphere, however, like mosphere. ours, which is so much agitated by other causes, the winds and variations in the barometer, which, arising from the same cause, have the same periods as the tides, must be very inconsiderable. The change in the altitude of the mercury in the barometer is only about of an inch at the equator, where it is a maximum; though it is not improbable, that the oscillations of the atmosphere, like those of the ocean, may be increased by local circumstances. "If we consider all the causes," says La Place," which disturb the equilibrium of the atmosphere; its great mobility arising from its fluidity and elasticity; the influence of heat and cold on its elasticity; the great mass of vapour that it alternately absorbs and deposes; and lastly, the changes which the rotation of the earth produces in the relative velocities of its particles, which for this reason are displaced in the direction of the meridians; we should not be surprised at the inconstancy and variety of its motions, which it would be very difficult to subject to any fixed and certain laws." See ANEMOMETER, ASTRONOMY, BAROMETER, CHEMISTRY, CLIMATE, METEOROLOGY, PNEUMATICS, and THERMOMETER. (0) ATMOSPHERES of the Sun, Moon, and Planets. See ASTRONOMY. ATMOSPHERES of Electrical Bodies. See E LECTRICITY. ATMOSPHERICAL CLOCK, the name of a machine proposed by Dr Brewster for measuring the mean temperature of the atmosphere during any given interval. This machine records every variation of temperature that takes place during a given period, and the result indicated on the dial-plate is the exact average of all the heights of the mercury in the thermometer. The variations of heat and cold affect the pendulum, which may be either of the tubular or gridiron kind; and which is so constructed as to render sensible in the motion of the clock the alternate contractions and dilatations which it undergoes. This instrument shall be fully described in a subsequent part of the work. (0) ATNAH, a tribe of Indians, who inhabit that part of the north-west of America, which lies in W. Long. 122°, and N. Lat. 52°. Their language, according to Mr Mackenzie, has no affinity with any Н See Macken Atomical other with which he was acquainted. ATOMICAL PHILOSOPHY, that doctrine which professes to explain the origin of all things, by a combination of atoms. The philosophers, who adopted this doctrine, may be divided into two classes; the theistical, and the atheistical. The first are those who adopted the ancient doctrine concerning atoms, said to have been first taught by Moschus the Phoenician, who, according to Strabo, lived before the Trojan war. This philosopher taught, that all bodies were composed of atoms, uniform in substance, impenetrable, indivisible, eternal; that the different forms and qualities of matter, arose solely from different combinations of these ultimate atoms; in the same manner as all the words of a language are formed by different combinations of the letters of the alphabet. The same body, for instance, becomes hard, or soft, or fluid, not from any alteration in its substance, but merely from a different arrangement of its constituent atoms. In this way they account for all the primary qualities of matter. And with regard to the secondary qualities, such as heat, cold, sweet, bitter, &c. as these are altogether distinct from the figure, situation, and motion of the insensible atoms, they held, that they must be nothing but sensations or passions excited in the mind, though they are commonly mistaken for qualities in bodies without us. Now all this is not only perfectly harmless, but also very ingenious, with the exception of the eternity of atoms; an error, which it was not to be supposed that any of the ancient philosophers should avoid, who all maintained the eternity of matter in some form or other. With this exception, the doctrine is very little different from that which has been received and improved in modern times, under the name of the Corpuscular Philosophy. For Sir Isaac Newton affirms, that matter was at first created in solid, hard, impenetrable, moveable particles; and that out of these result the various forms and qualities of body. Indeed, no doctrine can be more consistent with pure theism, than that of the ancient atomists for, whilst they denied to the atoms sensation and innate motion, (an error adopted by the later atomists,) there was an absolute necessity for some intelligent power to arrange them into form, so as to produce that order and regularity, which we perceive in the universe. : Some have attempted to give eclat to this philosophy, by making Moschus, the reputed author of it, the same as Moses. This, however, is very impro-. bable. Moses, in his cosmogony, certainly teaches nothing concerning atoms; and there is no evidence nor probability, that he ever wrote or taught any thing on the subject beyond the concise and simple account contained in the Scriptures. It has even been doubted whether the doctrine is entitled to such high antiquity, as has been ascribed to it. And many have maintained, that it was first broached by Leucippus, Democritus, or Protagoras, many ages after the æra of Moschus. Such as wish to see this point cleared up, may consult Cudworth's Intellectual System, where the antiquity of the doctrine is ably main- Atomica! tained; and where it is traced with infinite learning, Philosophy. though with little method, through all its changes and ramifications, till it ended in absolute atheism. We now proceed to consider the philosophy of the later atomists, which was decidedly atheistical. The author of this system is generally allowed to have been Leucippus, who is said to have been a disciple of Zeno the Eleatic philosopher, who flourished about the 84th Olympiad. According to Zeno, there is only one being, and that being is God. This appears, as far as it can be understood, to be nothing else than the Pantheistic doctrine, so commonly adopted by the ancient philosophers. But the pupil departed so far from the tenets of his master in this respect, that he introduced a system, which excluded the agency of Deity altogether, and professed to account for the production of all natural bodies from physical causes. All this is effected by giving to atoms an internal principle of motion, and making them dance together, till at last they produce a world. Observe then the steps, by which this important process is completed. The universe, which is infinite, is in part a plenum, and in part a vacuum. The plenum contains innumerable corpuscles or atoms of various figures, which, falling into the vacuum, struck against each other; and hence arose a variety of curvilinear motions, which continued, till at length atoms of similar forms met together, and bodies were produced. The primary atoms being specifically of equal weight, and not being able, on account of their multitude, to move in circles, the smaller rose to the exterior parts of the vacuum, whilst the larger, entangling themselves, formed a spherical shell, which revolved about its centre, and which included within it all kinds of bodies. This central mass was gradually increased by a perpetual accession of particles from the surrounding shell, till at last the earth was formed. In the mean time, the spherical shell was continually supplied with new bodies, which in its revolution is gathered up from without. Of the particles thus collected in the spherical shell, some, in their combination, formed humid masses, which by their circular motion gradually became dry, and were at length ignited, and became stars. The sun was formed in the same manner, in the exterior surface of the shell; and the moon in its interior surface. In this manner the world was formed." Enfield's Hist.of Phil. Democritus adopted the atomical doctrine as newmodelled by Leucippus; and, by the help of a little more ingenuity, extended its reputation, without correcting any of its absurdities. Both these philosophers had retained the gods in their systems, from a regard to their own safety, and in compliance with popular prejudices. But Protagoras, a little bolder, and a little honester, than his predecessors, hesitated not to speak freely on the subject, to deduce from the doctrine its legitimate consequences, and thus fairly to explode the gods from the universe. For this instance of his zeal, he was banished from Athens; and may claim the honour of being the protomartyr of atheism." At length appeared Epicurus, who so far outdid the labours of all who had gone before him, in en |