but still within the southern lower ranges of the Atlas system, lies a long valley without any water-course, which seems to extend up to the foot of the Greater Atlas. The greatest part of this valley is entirely unknown to us. Shaw has given some information on the eastern portion of it, called Wad-reag, in which Tuggurt and twentyfour other villages are situated, and of another branch of it, in which the town of Wurglah is found. No river traverses this country; but by digging wells to the depth of a hundred and sometimes two hundred fathoms, a plentiful stream is always found. Through different layers of sand and gravel a flaky stone-like slate is reached by the workmen, under which the sea underground, as it is called, lies concealed. No sooner is this stone broken through, than it is followed by a great rush of water. These seem to be such wells as are described by Olympiodorus. [See ARTESIAN WELLS.] The further continuation of this valley to the west up to the Atlas is entirely unknown to us, and its existence is only proved by the caravans, which depart from Fez and Marocco for Mecca, and choose this country for the usual road of their journeys; from which we may infer that no ranges of considerable height are encountered in these parts. Wurglah, Fiz Fighig, and Aksabi Surefa are named as the principal stations of the caravans in this valley. (Shaw's Travels; Jackson's Account of Marocco, and Account of Timbuctoo and Housa; Lieut. Washington, in the Journal of the Geographical Society, i. &c.) The name Atlas first appears in the writings of the early Greeks, who were acquainted with the general fact of the existence of a mountainous region in the north-west_portion of the African continent. But the Atlas of Herodotus (iv. 184) is rather a single mountain than a mass of mountains: it is of contracted dimensions, and circular; and said to be so high that it is not possible to see its summits, for the clouds never leave them either in winter or summer: the natives say this mountain is the pillar of heaven.' In these western regions the fables of the Greeks placed Atlas, the brother of Prometheus, bearing the heavens on his shoulders. (Esch. Prom. 348.) From the name of this mountain-region came the name of the adjoining or Atlantic Ocean. The native name of these mountains, according to Pliny (v. i.) and Strabo, was Duris: the reader may see Shaw's speculations on this name in his Travels. It does not appear that the antient geographers had a very complete knowledge of the Atlas; but still the Romans probably knew more about it than we yet do, having colonized many parts of the country which these mountains and their branches occupy. As far as we can collect, it was only the highest and western part, in the kingdom of Marocco, to which they applied the term Atlas; and they do not seem to have extended the name to the high lands to the east so far as we now do. The consul Suetonius Paulinus, who was contemporary with Pliny, was the first Roman commander who crossed the Atlas. His report of their great height agreed with all that had up to that time been said of them; he found the lower parts of the range covered with thick forests of lofty trees, and the summits with deep snow in the midst of summer. The offset (pózovc) of the Greater Atlas has been described as terminating at Ceuta, the Septem Fratres, or Seven Brothers, of Pliny and Strabo. The Greek geographer seems to make the Atlas mountains commence at Cotes, now Cape Spartel, and continue along the Atlantic side of the continent. (Compare Strabo, p. 825, and Pliny, v. i.) Pliny says that the Greeks gave the name of Ampelusia, the Vine Tract, to the headland which we now call Cape Spartel. Strabo gives no name to the mountain-range stretching eastward and in the interior from Cotes to the Syrtes; but he describes it, together with the ranges parallel to it, as inhabited first by the Maurusii or Moors, and in the interior by the Gætuli. ATLAS, the first vertebra of the neck, so named because it sustains the globe of the head. It differs in several important circumstances from all the other vertebræ that ster into the composition of the spinal column; because it has distinct and peculiar offices to perform. It has to support the head, and to allow it the power of exercising two different kinds of motion, viz., a motion forwards and backwards, or that of flexion and extension; and a rotatory motion, or the power of describing a certain portion of a circle, as it does when it turns from side to side. These motions are accomplished by the peculiar mode in which the head is connected to the atlas, and the atlas to the second vertebra of the neck, the vertebra dentata or axis. The head is so united with the atlas as to form a perfect hinge joint, that is, a joint which admits of flexion and extension, or a motion forwards and backwards. The second vertebra, the dentata, forming a pivot on which the atlas turns, and therefore called axis, is united with the | atlas in such a manner as to constitute a perfect rotation joint, or a joint which admits of a rotatory motion. The head being firmly connected with the atlas and carried round with it whenever the latter turns upon its axis, it is plain that by the combination of the two joints, namely, the hinge joint and the rotation joint, the head can be moved in every direction, forwards, backwards, and from side to side. In the construction of these joints, such is the perfection of the mechanism, that these combined motions are attained to the utmost extent and are performed with the greatest ease; the connexion of the different parts with each other forms a union of amazing strength and security; and at the same time certain organs of extreme delicacy and of vital importance are effectually guarded from injury. But the peculiar adaptations by which these objects are effected cannot be understood until the structure of the spinal column has been explained: we shall therefore postpone an account of the peculiar conformation of the atlas and axis until the spinal column is described. [Sec SPINAL COLUMN.] ATLAS, a collection of Maps; so called probably in allusion to the mythological figure of Atlas represented as bearing the world upon his shoulders, symbolical of Mount Atlas. Boucher, in his Glossary, says, the word seems to be derived from the German, in which language atlass means satin; because a collection of maps is usually made of a smooth satin paper. ATMOSPHERE, from the Greek, ἀτμός, and σφαῖρα, sphere of vapour, is the whole body of air or other mixture of gases which envelopes a planet. We shall here devote ourselves exclusively to that which surrounds the earth, merely observing, that we have more or less reason to suppose atmospheres, in density comparable to that of the earth, enveloping the Sun, Venus, Mars, Jupiter, and Saturn; and none for the Moon. See these several names. The subject of the atmosphere, treated in all its extent, would lead us much too far; we shall therefore confine ourselves to the description of its average state. We have already discussed the properties of its constituent material in the article AIR, and we must further refer as follows, both for subjects which we cannot here enter upon, as well as for extensions of various points which must be inci dentally mentioned. For the general subject of the atmosphere, as connected with the weather, see METEOROLOGY, HYGROMETRY, TEMPERATURE, and articles on particular subjects, such as EVAPORATION, DEW, RAIN, WIND, AURORA BOREALIS, HEAT, ELECTRICITY (ATMOSPHERIC), &c. &c. For the atmosphere as a medium of communication (taking this word in its widest sense), see ACOUSTICS, AERODYNAMICS, BALLOON, WINDMILL, SAIL. For its effects upon animal and vegetable life, see RESPIRATION, VEGETATION, ANTISEPTICS, DECOMPOSITION. For the effects of the imponderable substances upon it, and vice versa, see HEAT, ELECTRICITY, REFRACTION. For instruments used to measure its state, see BAROMETER, THERMOMETER, MANOMETER, EUDIOMETER, HYGROMETER: and for its uses in the investigation of the elevations of different parts of the earth, see BAROMETER, Heights (MEASUREMENT OF). The atmosphere, in its average state, must be considered as a body of air revolving with the earth. This gives its several strata an increasing velocity, as we recede from the earth's axis. For instance, at the equator, the air (if any) which is twice as distant from the centre of the earth as the surface, must revolve with twice the actual velocity of the air at the surface. This consideration shows positively that the atmosphere which really accompanies and revolves with the earth cannot certainly extend in the smallest quantity, above 20,000 miles from the surface. For at that height the tendency to recede from the centre, known by the name of centrifugal force, would counterbalance the weight, or tendency of particles towards the earth, and at higher distances would overcome it entirely. But we are not therefore to conclude that there must ba nearly 72° of Fahrenheit. This, if the decrease of temperature be uniform, gives a diminution of 1° of Fahrenheit for every 105 yards, or of 1° centigrade for every 173 metres of ele air, more or less, revolving with the earth up to so great a If there be air throughout the universe, we are obliged to suppose that every planet would collect an atmosphere around itself, proportionate to its attracting power. In this case, we know that Jupiter, at whose surface the force of gravity must be much greater than at that of our earth, would collect a powerful atmosphere around him. The effect of the refraction of light through this atmosphere would become visible in the approach of the satellites to the planet, when they disappear behind his disc, and would cause a sensible retardation in their rate of approach. No such retardation can be observed in the smallest sensible degree; and, consequently, Jupiter has no such atmosphere, nor the means of collecting it: consequently, air, such as we have at the earth, is not diffused in any degree of rarefaction through the whole solar system. Dr. Wollaston argues that this finite character of the atmosphere is more conformable to the atomic theory than to that of the infinite divisibility of matter, since, in the first case, a boundary is possible, and will exist at the point where the weight of a single atom is as great as the repulsive force of the medium; while in the latter case it is difficult to see the possibility of any boundary. The following table was deduced by Humboldt from various observations. It will serve to show how far the temperatures of elevated regions on the earth agrees with those of the same height in the atmosphere, as deduced from the preceding. The first column is the height of the land above the level of the sea (in metres); the second, the mean temperature (centigrade) at and near the equator; the third, the same in about 45° of latitude. The thermometer used is the centigrade; (+) means above, and (-) below, the freezing point. Elevation in 0 974 1949 2923 3900 4872 From the preceding table, it appears that at the equator, on the average of 4872 metres, a rise of 187 metres gives a fall of 1° centigrade. But the fall is more rapid in the higher regions than in the lower. From 0 to 1949 metres of elevation, an elevation of 214" produces a fall of 1°; but from 2923" to 4872", an elevation of 152′′ does the same. The argument in favour of the finite extent of the atmosphere, derived from the preceding, is as follows. If we suppose an elevation of 200 yards to produce a fall of 1° of Fahrenheit's thermometer (which, as we have seen, is likely to fall short of the truth, that is, to give the higher regions of the atmosphere a higher temperature than they really have); it follows, that at a height of forty miles above the level of the sea, the temperature of the air must be 350° It has lately been observed, that Encke's comet appears, in of Fahrenheit below that of the sea, or certainly more than successive revolutions, to show in a slight degree the effect 300° below the freezing point. There is the strongest of some medium resisting its motion; and we believe the reason to suppose that no gas we know of would preserve its same thing has very lately been said of that of Biela. It gaseous state at this low temperature, but would become might therefore appear that the preceding argument is weak-liquid: and though no gas has yet been rendered liquid by ened in force by this circumstance, or vice versa, since the reduction of temperature, yet several have been reduced to large planets might collect sensible atmospheres of the resist- that state by cold and pressure united. ing fluid, whatever it be. But on this we must observe, that If, then, we suppose the atmosphere of finite extent, its supposing the fact of the resisting medium to be established form must be nearly that of an oblate spheroid, the lesser (and several astronomers are of that opinion), it by no axis passing through the poles of the earth; at the same means follows that it is common air, or any thing approach-time the action of the sun and moon must produce certain ing to it in the proportion of its density to its elastic power. On the contrary, the facts observed with regard to the motion of the planets (which show no signs whatever of a resisting medium), and the extreme tenuity of the comets themselves (through which very faint stars may be seen), justify us in supposing that the resisting medium may be of a very high degree of elasticity as compared with air; and it is by no means improbable that the planets actually may have atmospheres of this same medium, not sensible to our instruments, on account of the very small increase of density which is sufficient to counterbalance the action of a planet. To elucidate this subject, see ELASTICITY, FLUID, (ELASTIC). The preceding arguments go to show, that even supposing the temperature of the atmosphere to be uniform throughout, there is no inconsistency in the supposition of a finite atmosphere. But a very strong presumption in favour of such an hypothesis is derived from the rapid decrease of temperature which takes place as we recede from the surface of the earth. The law of this decrease is entirely unknown to us; at least we cannot even guess at the form it assumes in the higher regions of the mass of air. To this circumstance it is owing that all we can say upon those regions must be little more than speculation. Near the earth, even at great elevations above the level of the sea, we cannot say that observed temperatures correctly represent the law of the atmosphere: for example, we cannot say that the average temperature of Quito, which is more than 9000 feet above the sea-level, is the average temperature of the air 9000 feet above, and over, the sea. The only observation worthy of any confidence is that of Gay-Lussac, taken during his celebrated ascent, at a height of 6980 metres, or 7634 yards above the sea-level. The difference of temperature between air at the surface and at the height just mentioned was 401° of the centigrade thermometer, or small atmospheric tides; and the tides of the sea, which are constantly disturbing the base on which the atmosphere rests, must produce periodical alterations of form in the latter also. If any such exist, sensibly, they may be detected by the barometer; for, cæteris paribus, any increase in the height of the superincumbent column of air must be accompanied by a small increase in the height of the counterbalancing column of mercury. Laplace was the first who examined this curious branch of the subject. He showed by analysis that the attraction of the sun and moon could produce no permanent effect upon the currents of the atmosphere; for instance, such as the trade-winds. He also showed that the diurnal oscillations caused by the above-mentioned attractions would only produce a very small effect upon the barometer-in fact, less than one millimetre, or 1-25th of an inch. The reduction of a large number of observations gave, at first, '055 of a millimetre for the quantity in question; those of another set gave 018; from which Laplace concluded, taking into account the smallness of the quantities, and the degree of probability which could be attached to results so different, that the sensible existence of the atmospheric tide was doubtful. In the meanwhile, however, the diurnal variation of the barometer has been completely established by observations made in several different places. But the law and quantity of this oscillation appears to vary so much with latitude, climate, and other circumstances, that no positive conclusion can yet be drawn, either to the exclusion of atmospheric tide, properly so called, or the adoption of any other cause in conjunction with it. Professor Forbes (Report o British Association, p. 230) has discussed all the observa tions, and has given a formula which represents them tolerably well. The average pressure of the atmosphere is found to be the same, or very nearly so, at any one place from year height ascended. The method of doing this will be explained in the article HEIGHTS (MEASUREMENT OF); we notice it here in order to mention a circumstance which shows that our knowledge of the general conditions of the atmosphere has not been overstated. In order to construct the formula, it is necessary to take into account the dimi nution of the weight of the air, not only from its rarefaction, but also from its increasing distance from the earth,—the variation of elastic force, as well from rarefaction as from change of temperature,—the alteration of density in the mercury itself, arising from the alteration of temperature.— and to use the formula in different latitudes, the variation of the force of gravity on the earth's surface. In our ignorance of the variation of the temperature, it is usual to allow to the whole column of air contained between the points of observation, the average temperature of its upper and lower extremities. This is the most doubtful part of the process; and as a verification, recourse is had to the comparison of heights measured by the barometer, and also by the processes of trigonometry. It is thus found that a co-efficient which, when deduced from theory alone, is 18337 46, appears from a number of heights measured trigonometrically to be 18336, differing from the former only by about its 18,000th part. This shows the effect of temperature to be sufficiently well taken into account, for such heights as we can measure, by the preceding supposition. rear, notwithstanding the various temporary alterations | Wind. No. of at noon, 2.98 2.38 6'08 8.67 9.76 9.89 7.04 231 4.60 6'42 S.E. Mean 2.-Mean heights of the barometer for each year, from 1816 to 1826, at 9 in the morning, 3 in the afternoon, and 9 in the evening. In the article AIR some reasons were shown for supposing that its component parts were not united chemically, but only mixed. This opinion, which is now almost universally adopted, has given rise to notions on the constitution of the atmosphere, differing entirely from those of all chemists down to the present day. A law is found to prevail in the mixture of gases and vapours, as universal as the one described in the article AIR, relative to the expansion arising from temperature-namely, that two gases in a state of mixture exercise no influence one upon the other, except communication of temperature, but that each is disposed in exactly the same manner as it would be if the other were not present. Thus it is found, entirely contrary to all previous notions, that no pressure of dry air upon water exerts which goes on exactly as if the space above were a vacuum, the least influence in preventing the formation of steam, and continues until further evaporation is stopped by the pressure of the steam already created. It is found that no Diff. of 1st Diff. of 2nd pressure of one gas can confine another in water; but that supposing a bottle partly full of water, the gas confined in the water will escape to the surface and distribute itself in precisely the same way as if the other gas were not present. By this it is not meant that the action commonly called mechanical cannot take place, or that a stream of hydrogen would not trouble the air; but only that the permanent settlement of one gas is not affected in any way by the presence of another, so long as no chemical action is excited. From this principle, Mr. Dalton (Phil. Trans. 1826), taking into consideration the presumptions which exist against the chemical union of the ingredients of the atmosphere, infers that the atmosphere does not consist altogether of the compound called air, but that the nitrogen atmosphere is higher than the oxygen atmosphere. In fact, if there be no chemical union, the above law of the mixture of gases requires us to allow that each is an atmosphere independent of the other, and that the two are most probably of unequal heights. From some considerations, into which we cannot here enter, Mr. Dalton thinks that the actual pressures exerted by the oxygen and nitrogen are in the proportions of the volumes occupied by them [see AIR], that is as 1 to 4; and concludes that the oxygen atmosphere extends to 38 miles in height, that of nitrogen to 54 miles, that of carbonic acid to 10 miles, and that of aqueous vapour to 50 miles. It must however be observed, that the state of the carbonic acid of the atmosphere is very variable; that there is not the same quantity by night as by day, in moist weather as in dry; and that the higher strata of the atmosphere contain more of it than the lower, which may arise from rapid absorption by the earth. 1816 75 4.359 75 75 columns. 3.683 4.051 ⚫676 1817 6.676 5 914 6.510 ⚫762 1818 6.382 5 473 5.961 ⚫909 vice versa. We do not know whether the experiment of M. Gay-Lussac was made, or even intended to be made, with that degree of accuracy which would justify its being considered a test of Mr. Dalton's theory; but in any case it is an experiment which it is very desirable to repeat. The total quantity of the atmosphere (if the mean height of the barometer at Paris hold good for all other places) is a little less than the millionth part of the whole mass of the earth, supposing the mean density of the latter to be five and a half times that of water. (Poisson, Mécanique, 2d. edit. vol. ii. p. 610.) For the colour of the atmosphere, see SKY. For the quantity of moisture contained in it, see HYGROMETRY. For the history of atmospherical researches, see the following names, HERO, CTESIBIUS, GALILEO, TORRICELLI, PASCAL, FLORENCE (Academy of), BOYLE, MARIOTTE, PRIESTLEY, SCHEELE, BLACK, LAVOISIER, CAVENDISH, &c. The actual constitution of the atmosphere, whether composed of molecules exerting a repulsive force upon each other or not, must remain unsettled until some mathematical hypothesis can be found which shall satisfy all observed phenomena. That probabilities are at present all on the side of the molecular or atomic hypothesis, is pretty generally admitted; and the repulsion of the several parts of air is a fact of every-day experience. Newton entered upon this question, and shewed (Principia, book ii. prop. 23) that if the constitution of the atmosphere be atomic, and if the force exerted by each particle extend only to those nearest to it, and be either nothing or inconsiderable as to all others, that then the observed proportionality of the elastic force to the density is consistent with no hypothesis except that of a repulsive force inversely proportional to the distances of the particles from each other; that is, which becomes double when the distance is halved, and so on. But in the scholium to the same proposition, he takes notice of the imperfection of the hypothesis, and describes his theory as a mathematical handle' to induce philosophers to consider the subject further. The molecular theory, on the supposition that every particle repels all the rest, or, which is as likely to be the case, has alternate spheres of attraction and repulsion, is beyond the reach of the present state of mathematical analysis. For the state of atmospherical knowledge up to 1808, see Robertson, General View of the Natural History of the Atmosphere, Edinburgh, 1808; from thence to 1822, see Daniell's Meteorological Essays, London, 1822; and for an account of what has been lately done, with further references, see Professor Forbes's Report on Meteorology, in the Reports of the British Association, London, 1833. ATMOSPHERIC AIR, a distinction which has been preserved after the necessity for it has ceased. In the time of Priestley all gases were called airs, and common air was called atmospheric to distinguish it from vital air, now oxygen, inflammable air, now hydrogen, &c. [See AIR.] ATOLL, or ATOLLON, is a name given by the natives of the Maldives to the detached coral formations of which their Archipelago is composed. They are commonly of a circular form (the reef seldom exceeding a mile in breadth), from fifteen to thirty miles in diameter, and rise perpendicularly from an unfathomable depth. The openings which occasionally occur in these reefs afford passages for vessels, and safe anchorage is found in many within the circumscribing wall: the space thus included is often interspersed with islands. The principal of these islands, however, are always situated on the outer reef; they abound in cocoa-nut trees, and are long and narrow. In short, they are of the same nature as the coral formations of the South Seas, though generally on a larger scale; the name Atoll is exclusively used among the Maldives. ATOM, or ATOMS (aroμo), the ultimate and indivisible particles of matter, from a Greek compound, signifying indivisible. Anaxagoras, the preceptor of Socrates, who died in the year 428 B.C., imagined the number of elements to be nearly if not absolutely infinite, and that the ultimate atoms composing every substance were of the same kind with that substance. [See ANAXAGORAS.] Leucippus, a philosopher of Abdera, who flourished soon after Anaxagoras, is generally regarded as the original propounder of what has been called the atomic philosophy. It was adopted by Democritus, in his Cosmogony; and afterwards by Epicurus, to whom its celebrity is chiefly owing The following account of this doctrine is copied from Dr. Good's Book of Nature, and is a clear and concise sketch of the theory contained in the writings of Epicurus and his followers : The atomic philosophy of Epicurus, in its mere physica. contemplation, allows of nothing but matter and space, which are equally infinite and unbounded, which have equally existed from all eternity, and from different combinations of which every visible form is created. These elementary principles have no common property with each other: for whatever matter is, that space is the reverse of; and whatever space is, matter is the contrary to. The actually solid parts of all bodies, therefore, are matter; their actual pores space; and the parts which are not altogether solid, but an intermixture of solidity and pore, are space and matter combined. Anterior to the formation of the universe, space and matter existed uncombined, or in their pure and elementary state. Space, in its elementary state, is absolute and perfect void; matter, in its elementary state, consists of inconceivably minute seeds or atoms so small, that the corpuscules of vapour, light, and heat are compounds of them; and so solid, that they cannot possibly be broken or abraded by any concussion or violence whatever. The express figure of these primary atoms is various: there are round, square, pointed, jagged, as well as many other shapes. These shapes, however, are not diversified to infinity; but the atoms themselves of each existent shape are infinite or innumerable. Every atom is possessed of certain intrinsic powers of motion. Under the old school of Democritus, the perpetual motions hence produced were of two kinds : a descending motion, from the natural gravity of the atoms; and a rebounding motion, from collision and mutual clash Besides these two motions, and to explain certain phenomena to which they did not appear competent, and which were not accounted for under the old system, Epicurus supposed that some atoms were occasionally possessed of a third, by which, in some very small degree, they descended in an oblique or curvilinear direction, deviating from the common and right line anomalously; and in this resp resembling the oscillations of the magnetic needle. 'These infinite groups of atoms, flying through all time and space in different directions, and under different laws, have interchangeably tried and exhibited every possible mode of rencounter; sometimes repelled from each other by concussion, and sometimes adhering to each other from their own jagged or pointed construction, or from the casual interstices which two or more connected atoms must produce, and which may be just adapted to those of other figures, as globular, oval, or square. Hence the origin of compound and visible bodies; hence the origin of large masses of matter; hence, eventually, the origin of the world itself. When these primary atoms are closely compacted, and but little vacuity or space lies between, they produce those kinds of substances which we denominate solids, as stones and metals; when they are loose and disjointed, and a large quantity of space or vacuity is interposed, they exhibit bodies of lax texture, as wool, water, and vapour. 'The world, thus generated, is perpetually sustained by the application of fresh tides of elementary atoms, flying, with inconceivable rapidity, through all the infinity of space, invisible from their minuteness, and occupying the posts of those that are as perpetually flying off. Yet nothing is eternal or immutable but these elementary seeds or atoms themselves. The compound forms of matter are continually decomposing and dissolving into their original corpuscules; to this there is no exception: minerals, vegetables, and animals, in this respect, are all alike, when they lose their present make, perishing for ever, and new combinations proceeding from the matter into which they dissolve. But the world itself is a compound though not an organized being; sustained and nourished, like organized beings, from the material pabulum that floats through the void of infinity. The world itself must, therefore, in the same manner perish it had a beginning, and it will have an end. Its present crasis will be decompounded; it will return to its original, its elementary atoms; and new worlds will arise from its destruction. 'Space is infinite, material atoms are infinite, but tl. world is not infinite. This, then, is not the only world, no. the only material system that exists. The cause that has produced this visible system is competent to produce others it has been acting perpetually from all eternity; and there are other worlds, and other systems of worlds, existing around us.' During the most flourishing periods of the Greek philo particles of oxygen to one of azote), it is difficult not to allow the merits of prior conception, as well as of very ingenious illustration, to the elder writer.' (Discourse before the Royal Society, 1826.) sophy, this doctrine of matter consisting of an assemblage | of indivisible particles seems to have kept its ground under various modifications; the idea of one elementary matter deriving its form and properties from the shape, and union of the particles composing it, is a simplification of the doctrine In justice, however, to Mr. Higgins, it must be admitted of Anaxagoras. (See Dr. Daubeny on the Atomic Theory.) that his views were much more extended than those of Dr. Without entering into an account of the opinions enter- Higgins; for it appears that he entertained precisely the tained by other philosophers on this abstruse subject, we same notion of the composition and atomic constitution o shall conclude with the following from Sir Isaac Newton:- water as that now generally admitted, in this country at All things considered, it seems probable, that God, in the least. In his Comparative View of the Phlogistic and beginning, formed matter in solid, massy, hard, impene- Antiphlogistic Theories, published in 1790, p. 37, he says, trable, moveable particles, of such sizes, figures, and withAs two cubic inches of light inflammable air require but such other properties, and in such proportion to space, as one of dephlogisticated air to condense them, we must supmost conduced to the end for which he formed them; and pose that they contain equal number of divisions, and that that these primitive particles, being solids, are incomparably the difference of their specific gravity depends chiefly on harder than any porous bodies compounded of them; even the size of their ultimate particles; or we must suppose that so very hard as never to wear or break to pieces; no ordinary the ultimate particles of light inflammable air require two power being able to divide what God himself made one in or three, or more, of dephlogisticated air to saturate them. the first creation.' If this latter were the case, we might produce water in an intermediate state, as well as the vitriolic or the nitrous acid, which appears to be impossible; for in whatever proportion we mix our airs, or under whatsoever circumstances we com bine them, the result is invariably the same. This likewise may be observed with respect to the decomposition of water. Hence we may justly conclude, that water is composed of molecules formed by the union of a single particle of dephlogisticated air to an ultimate particle of light inflammable air, and that they are incapable of uniting to a third particle of either of their constituent principles.' ATOMIC THEORY, in chemistry, sometimes termed the doctrine of definite proportions. This very important theory, founded on well-ascertained facts, has bestowed on modern chemistry an almost mathematical degree of precision. The hypothetical, which is to be distinguished from the experimental part of the subject, supposes that chemical compounds result from the combination of the ultimate atoms of their constituent parts. It has been determined by experiment, and the fact serves as the basis of the theory, that a compound body, when pure, always contains the same proportions of its constituents thus calcareous It is a remarkable circumstance, that although Mr. spar, and the pure part of marble, chalk, and limestones, Higgins's view of the atomic constitution of the five comconsist of carbonate of lime, composed of the same propor- pounds of oxygen and azote is that which is even now very tions of carbonic acid and lime; the carbonic acid always commonly admitted, he does not state their composition; and contains the same quantity of carbon and oxygen, and the his idea of the comparative atomic constitution of sulphurous lime the same proportions of calcium and oxygen. The same and sulphuric acids is decidedly erroneous. 'Indeed, as law also exists with regard to all similarly-constituted oxides, remarked by Sir H. Davy in the discourses above quoted, sulphurets, and salts, and indeed as to all chemical com-neither of the Higginses attempted to express the quanpounds whatever, whether presented to us by nature or tities in which bodies combine by numbers." formed by art: this is a simple statement of the fundamental facts upon which the superstructure of the atomic theory has been raised. Before we proceed to detail the minutiae of the theory, it will be proper to give a sketch, though a slight one, of the principal discoveries connected with the subject. In 1792, Richter, a Prussian chemist, published a work called Elements of Stochiometrie; or the Mathematics of the Chemical Elements. This author treated the subject almost in the same way as Wenzel had previously done, but extended it very considerably; he endeavoured to determine the capacity of saturation of each acid and base, and to indicate by numbers the weights which mutually saturate each other. He published a table of these, but though the attempt was new and exceedingly ingenious, the results were far from accurate. The discoveries of Proust, a French chemist who was professor of chemistry at Madrid, are well worthy of notice, he being the first person who attempted an accurate analysis of metallic oxides. He found that metals unite only with determinate proportions of oxygen, and that the same law ex The earliest experiments which could have served as a basis for the atomic theory are those of Wenzel, a German chemist, who published, in 1777, a work On the Affinities of Bodies; the experiments detailed in it, though neglected at the time, are now acknowledged to possess a very considerable degree of accuracy. The author showed that when any two neutral salts decomposed each other, the resulting new compounds were exactly neutral. The very attempt,' remarks D Thomson, to analyze the salts was an acknowledgment that bodies united with each other in definite pro-isted with sulphur and the metals, and that these might be pertions; and these definite proportions, had they been followed out, would have ultimately led to the doctrine of atoms. (History of Chemistry, vol. ii. p. 278.) With reference to this subject, it is observed by Sir H. Davy, that there may be found in the works of Dr. Bryan Higgins, Mr. William Higgins, and Professor Richter, hints or conclusions bearing directly on this doctrine. Dr. Bryan Higgins, in his Experiments and Observations relating to Acetous Acid, fixable Air, dense inflammable Air, &c. &c., published in 1786, contends, that elastic fluids unite with each other in limited proportions only; and this depends upon the combination of their particles or atoms with the matter of fire which surrounds them as an atmosphere, and makes them repulsive of each other; and he distinguishes between simple elastic fluids, as composed of particles of the same kind, and compound elastic fluids, as consisting of two or more particles combined, in what he calls molecules, definite in quantity themselves, and surrounded by definite proportions of heat. Dr. Bryan Higgins's notions have, I believe, never been referred to by any of the writers on the atomic theory. Mr. William Higgins's claims have, on the contrary, often been brought forward. Yet, when it is recollected that this gentleman was a pupil and relation of Dr. Bryan Higgins, and that his work, called the Comparative View, was published some years after the treatises I have just quoted, and that his notions are almost identical (with the addition of this circumstance, that he mentions certain elastic fluids, such as the compounds of azote, consisting of one, two, three, four, and five stated in numbers: his opinions were strenuously opposed by Berthollet, but their accuracy is now generally admitted. In the year 1803, Mr. (now Dr.) Dalton, of Manchester, communicated to the Literary and Philosophical Society of Manchester an essay containing an outline of his speculations on the subject of the composition of bodies (Manchester Memoirs, second series, vol. i. p. 286). The following year he explained his notions on the subject to Dr. Thomson, and in 1808 he published the first volume of his New System of Chemical Philosophy, in which he gave an outline of his views of the constitution of matter, and this without any acquaintance with what had been previously done on the subject by Higgins Dr. Dalton was unquestionably the first who laid down, clearly and numerically, the doctrine of multiples, and endeavoured to express by simple numbers the weights of bodies believed to be elementary. He announced it as a general rule, that when only one combination of two bodies can be obtained, it must be presumed to be a binary one, unless some cause appear to the contrary. Consistently with this law, and correctly at the time it was written, Dr. Dalton regarded water as a binary compound of hydrogen and oxygen, and the relative weights, since corrected, are considered as one to eight. As, then, water consists of an atom of hydrogen and an atom of oxygen, either of these elements may be selected as unity, and, in fact, as we shall hereafter notice, both have been occasionally employed as such. Dalton fixed on hydrogen, because it is that body which unites with others in the smallest proportion: thus, |