if we place between them a thick ring of elastic gum, we may represent the natural equilibrium between the forces of cohesion and of repulsion; for the ring would resist any small additional pressure with the same force as would be required for separating the hemispheres, so far as to allow it to expand in an equal degree; and at a certain point the ring would expand no more, the air would be admitted, aud the cohesion destroyed, in the same manner as when a solid of any kind is torn asunder. But all suppositions founded on these analogies must be considered as merely conjectural; and our knowledge of every thing which relates to the intimate constitution of matter, partly from the intricacy of the subject, and partly for want of sufficient experiments, is at present in a state of great uncertainty and imperfection. ETHICS, or MORALITY, the science of manners or duty, which it traces from man's nature and condition, and shews to terminate in his happiness; or, in other words, it is the knowledge of our duty and felicity, or the art of being virtuous and happy. See MORAL PHILOSOPHY. ETHULIA, in botany, a genus of the Syngenesia Polygamia Equalis class and order. Natural order of Compositæ Discoidea. Corymbiferæ, Jussieu. Essential character: receptacle naked; down none. There are six species. ETYMOLOGY, that part of grammar which considers and explains the origin and derivation of words, in order to arrive at their first and primary signification, whence Quintilian calls it originatio. See GRAM MAR. EVAPORATION, in natural philosophy, is the conversion of water into vapour, which in consequence of becoming lighter than the atmosphere, is raised considerably above the surface of the earth, and afterwards by a partial condensation forms clouds. It differs from exhalation, which is properly a dispersion of dry particles from a body. When water is heated to 212°, it boils, and is rapidly converted into steam; and the same change takes place in much lower temperatures; but in that case the evaporation is slower, and the elasticity of the steam is smaller. As a very considerable proportion of the earth's sur face is covered with water, and as this water is constantly evaporating and mixing with the atmosphere in the state of vapour, a precise determination of the rate of evaporation must be of very great importance in meteorology. Accordingly, many experiments have been made to determine the point by different philosophers. No person has succeeded so completely as Mr. Dalton: but many curious particulars had been previously ascertained by the labours of Richman, Lambert, Watson, Saussure, De Luc, Kirwan, and others. From these we learn that, 1. the evaporation is confined entirely to the surface of the water: hence it is in all cases proportional to the surface of the water exposed to the atmosphere. Much more vapour of course rises in maritime countries, er those interspersed with lakes, than in inland countries. 2. Much more vapour rises during hot weather than during cold: hence the quantity evaporated depends in some measure upon temperature. The precise law has been happily discovered by Mr. Dalton, who says, in general, the quantity evaporated from a given surface of water per minute at any temperature, is to the quantity evaporated from the same surface at 212", as the force of vapour at the first temperature is to the force of vapour at 212°. Hence, in order to discover the quantity which will be lost by evaporation from water of a given temperature, we have only to ascertain the force of vapour at that temperature. Hence, we see that the presence of atmospheric air obstructs the evaporation of water; but this evaporation is overcome in proportion to the force of the vapour. Mr. Dalton ascribes this obstruction to the vis inertia of air. 3. The quantity of vapour which rises from water, even when the temperature is the same, varies according to circumstances. It is least of all in calm weather, greater when a breeze blows, and greatest of all with a strong wind. Mr. Dalton has given a table that shews the quantity of vapour raised from a circular surface of six inches in diameter in atmospheric temperatures. The first column expresses the temperature; the second the corresponding force of vapour; the other three columns give the number of grains of water that would be evaporated from a surface of six inches in diameter in the respective temperatures, on the supposition of there being previously no aqueous vapour in the atmosphere. These columns present the extremes, and the mean of evaporation likely to be noticed, or nearly such; for the first is calculated upon the supposition of 35 grains loss per minute, from the vessel of 34 inches in diameter; the second 45, and the third 55 grains per minute. 4. Such is the quantity of vapour which would rise in different circumstances, on the supposition that no vapour existed in the atmosphere. But this is a supposition which can never be admitted, as the atmosphere is in no case totally free from vapour. Now, when we wish to ascertain the rate at which evaporation is going on, we have only to ind the force of the vapour already in the atmosphere, and subtract it from the force of vapour at the given temperature; the remainder gives us the actual force of evaporation; from which, by the table, we readily find the rate of evaporation. Thus, suppose we wish to know the rate of evaporation at the temperature 59°. From the table, we see that the force of vapour at 59° is 0.5, or its force at 212°. Suppose we find by trials, that the force of the vapour already existing in the atmosphere is 0.25, or the half of. To ascertain the rate of evaporation, we must subtract the 0.25 from 0.5; the remainder 0.25 gives us the force of evaporation required; which is precisely one half of what it would be if no vapour had previously existed in the atmosphere. 5. As the force of the vapour actually in the atmosphere, is seldom equal to the force of vapour of the temperature of the atmosphere evaporation, with a few exceptions, may be considered as constantly going on. Various attempts have been made to ascertain the quantity evaporated in the course of a year; but the difficulty of the problem is so great, that we can expect only an approximation towards a solution. The most exact set of experiments on the evaporation from the earth, was made by Mr. Dalton and Mr. Hoyle, during 1796, and the two succeeding years. The method which they adopted was this: having got a cylindrical vessel of tinned iron, ten inches in diameter, and three feet deep, there were inserted into it two pipes turned downwards for the water to run off into bottles: the one pipe was near the bottom of the vessel, the other was an inch from the top. The vessel was filled up for a few inches with gravel and sand, and all the rest with good fresh soil. It was then put into a hole in the ground, and the space around filled up with earth, except on one side, for the convenience of putting bottles to the two pipes; then some water was poured on to sodden the earth, and as much of it as would, was suffered to run through without notice, by which the earth might be considered as saturated with water. For some weeks the soil was kept above VOL. III. the level of the upper pipe, but latterly it was constantly a little below it, which precluded any water running off through it. For the first year the soil at top was bare; but for the two last years it was covered with grass, the same as any green field. Things being thus circumstanced, a regular register was kept of the quantity of rain water that ran off from the surface of the earth through the upper pipe, (whilst that took place), and also of the quantity of that which sunk down through the three feet of earth, and ran out through the lower pipe. A rain gauge of the same diameter was kept close by to find the quantity of rain for any corresponding time. The weight of the water which ran through the pipes, being subtracted from the water in the rain-guage, the remainder was considered as the weight of the water evaporated from the earth in the vessel. From these experiments it appears, that the quantity of vapour raised annually at Manchester is about 25 inches. If to this we add five inches for the dew with Mr. Dalton, it will make the annual evaporation 30 inches. Now, if we consider the situation of England, and the greater quantity of vapour raised from water, it will not surely be considered as too great an allowance, if we estimate the mean annual evaporation over the whole surface of the globe at 35 inches. Now, 35 inches from every square inch, on the superficies of the globe, make 94,450 cubic miles, equal to the water annually evaporated over the whole globe. Was this prodigious mass of water all to subsist in the atmosphere at once, it would increase its mass by about a twelfth, and raise the barometer nearly three inches: but this never happens; no day passes without rain in some part of the earth; so that part of the evaporated water is constantly precipitated again. Indeed it would be impossible for the whole of the evaporated water to subsist in the atmosphere at once, at least in the state of vapour. See Manchester Memoirs. EUCALYPTUS, in botany, a genus of the Icosandria Monogynia class and order. Essential character: calyx superior, permanent, truncate, before flowering time covered with a hemispherical, deciduous lid; corolla none; capsules four-celled, opening at the top, inclosing many seeds. There are two species, viz. E. obliqua; oblique leaved eucalyptus, and E. resinifera; red gum tree. These are both very large and lofty trees, much exceeding the English oak both in height and bulk. E. resinifera, con F tains a large quantity of resinous gum; the wood is of a brittle quality; the flowers grow in little clusters, or rather umbels, about ten in each, and every flower has its proper partial foot stalk, a quarter of an inch in length, besides the general one; the flowers are yellowish, and of a singular structure; the calyx is hemispherical, perfectly entire on the margin, it afterwards becomes the capsule; the anthers are small and red, in the centre is a single style terminated by a blunt stigma; the stamens. are resinous and aromatic; the germ appears when cut across to be divided into three cells; each containing the rudiments of one or more seeds. EUCLEA, in botany, a genus of the Dioecia Dodecandria, or Polygamia class and order. Essential character: male calyx, four or five-toothed; corolla four or five-parted; stamens twelve to fifteen: female calyx and corolla as in the male; germ superior; styles two; berry two celled. There is but one species, viz. E. racemosa; round-leaved euclea, a native of the Cape of Good Hope. EUCLID, of Megara, a celebrated philosopher and logician; he was a disciple of Socrates, and flourished about 400 years before Christ. The Athenians having prohibited the Megarians from entering their city, on pain of death, this philosopher disguised himself in women's clothes to attend the lectures of Socrates. After the death of Socrates, Plato and other philosophers went to Euclid at Megara, to shelter themselves from the tyrants who governed Athens. This philosopher admitted but one chief good; which he at different times called God, or the Spirit, or Providence. EUCLID, the celebrated mathematician, according to the account of Pappus and Proclus, was born at Alexandria, in Egypt, where he flourished and taught mathematics, with great applause, under the reign of Ptolemy Lagos, about 280 years before Christ. And here, from his time, till the conquest of Alexandria by the Saracens, all the eminent mathematicians were either born or studied; and it is to Euclid, and his scholars, we are beholden for, Eratosthenes, Archimedes, Apollonius, Ptolemy, Theon, &c. &c. He reduced into regularity and order all the fundamental principles of pure mathematics, which had been delivered down by Thales, Pythagoras, Eudoxus, and other mathematicians before him, and added many others of his own discovering: on which account it is said he was the first who reduced arithmetic and geometry into the form of a science. He likewise applied himself to the study of mixed mathematics, particularly to astronomy and optics. His works, as we learn from Pappus and Proclus, are the Elements, Data, Introduction to Harmony, Phenomena, Optics, Catoprics, a Treatise of the Division of Superficies, Porisms, Loci ad Superficiem, Fallacies, and four hooks of Conics. The most celebrated of these is the first work, the "Elements of Geometry;" of which there have been numberless editions, in all languages; and a fine edition of all his works now extant, was printed in 1703, by David Gregory, Savilian Professor of Astronomy at Oxford. The "Elements," as commonly publish. ed, consist of fifteen books, of which the two last, it is suspected, are not Euclid's, but a comment of Hypsicles of Alexandria, who lived 200 years after Euclid. They are divided into three parts, viz. The Contemplation of Superficies, Numbers, and Solids; the first four books treat of planes only; the fifth of the proportions of magnitudes in general; the sixth of the proportion of plane figures; the seventh, eighth, and ninth give us the fundamental properties of numbers; the tenth contains the theory of commensurable and incommensurable lines and spaces; the eleventh, twelfth, thirteenth, fourteenth, and fifteenth treat of the doctrine of solids. There is no doubt but, before Euclid, elements of geometry were compiled by Hippocrates of Chius, Eudoxus, Leon, and many others, mentioned by Proclus, in the beginning of his second book; for he af firms, that Euclid new ordered many things in the Elements of Eudoxus, completed many things in those of Theatetus, and besides strengthened such propositions as before were too slightly, or but superficially established, with the most firm and convincing demonstrations. History is silent as to the time of Euclid's death, or his age. He is represented as a person of a courteous and agreeable behaviour, and in great esteem and familiarity with King Ptolemy; who once asking him whether there was any shorter way of coming at geometry than by his Elements, Euclid, as Proclus testifies, made answer, that there was no other royal way or path to geometry. EUCOMIS, in botany, a genus of the Hexandria Monogynia class and order. Natural order of Coronaria. Asphodeli, Jus sien. Essential character: corolla inferior, six-parted, permanent, spreading; filaments united at the base into a nectary growing to the corolla. There are four species, all natives of the Cape. EUDIOMETRY. The measurement of the quantity of oxygen contained in atmopheric air, or indeed in any gas in which it is not intimately combined, is named eudiometry, and the instrument by which it is performed, the eudiometer. To attain such a measurement, it is merely necessary to present to atmospheric air, some substance which combines with its oxygen, and which either does not afford any gaseous product, or affords one that is easily abstracted and measured. Different substances have been applied to this purpose. The fluid originally employed by Scheele, in the analysis of the air, the solution of sulphuret of potash, or what is rather more convenient, the sulphuret of lime, is perhaps superior in accuracy to any, at least if the air be not too long exposed to it, and be not in too small quantity proportioned to the quantity of fluid. Phosphorus is applied by a very simple apparatus, but by its so_lubility in nitrogen gas, it adds to the bulk of the residual air, for which a correction must be made. Nitrous gas was employed by Priestley, it exhibits the result immediately, but is liable to several sources of fallacy. Hydrogen gas was employed by Volta: a given measure of it being put along with a quantity of the air, designed to be submitted to trial, into a graduated tube, and inflamed by the electric spark, the diminution of volume indicating the quantity of oxygen; 100 measures of oxygen require rather less than 200 measures of hydrogen for saturation; about 40 measures of hydrogen are therefore sufficient to saturate the oxygen contained in 100 measures of atmospheric air, but it is proper to use an excess of hydrogen, as otherwise part of the oxygen is liable to escape combination. From 60 of hydrogen, with 100 of atmospheric air, Mr. Dalton states, that the residuum after explosion is 100, 21 of oxygen combining with 39 of hydrogen. The method is simple and expeditious, and as Humboldt and Gay Lussac have remarked, has the great advantage, from the bulk of the mixture, and the great diminution of volume, from the consumption of a given quantity of oxygen, of being more delicate than any other. It also requires no corrections for variations of temperature or atmospheric pressure; and any impurity in the hydrogen gas, which it has been supposed might be a source of error, may be avoided by care. It affords also the best method of determining the purity of oxygen gas, or the proportion of oxygen in any mixed gas containing it. Humboldt and Gay Lussac, in an elaborate memoir, have pointed out all the circumstances to be attended to in employing it as an eudiometer. (Journal de Physique, t. lx. p. 129.) From the practice of eudiometry, it was at one time expected, as the name implies, that we should be able to ascertain the purity of the air, with regard to its salutary or noxious power on life. It was soon found, however, particularly by Priestley, (and the fact has also since been established by De Marti), that the air of places the most offensive and unhealthy, afforded as much oxygen as that of others of an opposite description; the air, for example, of crowded cities, of low, damp situations, or of crowded manufactories, has not been found less pure than that of the country; the noxious quality of the air depending not so much on any deficiency of oxygen, as on the presence of effluvia not discoverable by this test. It was at one time imagined, that the composition of atmospheric air is not uniform, but that it varies both at different parts of the earth's surface, and still more at different heights. Ingenhouz made a number of experiments to prove the former fact, from which he concluded, that the air is purer, or contains more oxygen at sea than on land, and that in the neighbourhood of marshy situations it contains less oxygen than the standard. (Philosophical Transactions, vol. lxx. p. 354). Saussure made some experiments on the air at some of the elevated parts of the Alps, the summit of the great St. Bernard, the Buet, &c.; in this air the proportion of oxygen was less than in the air on the plains. (Voyages, t. ii. p. 357; t. iv. p. 451.) Von Humboldt relates also, that air brought from a great height in the atmosphere, by a person who had ascended in a balloon, contained in 100 parts 25.9 of oxygen, while air at the surface contained 27.6; and that at the summit of the Peak of Teneriffe, the proportion of oxygen amounted only to 19, while at the foot of the mountain it was 27. The proportions which he states prove sufficiently the error of the eudiometrical method he employed, and the eudiometer he did use, that with nitrous gas, corrected by trying its purity with sul phate of iron, is indeed the one which is most liable to fallacy. The analysis of the air in the upper regions of the atmosphere, has since been executed with accuracy by Gay Lussac, assisted by Thenard. A glass balloon was filled with air, at the height of 21,735 feet from the surface, the greatest which has yet been reached, and when opened under water by Gay Lussac after his descent, one half of its capacity was filled by the water, a sufficient proof that it had been accurately closed. The air was subjected to trial, both by Volta's eudiometer, and by the solution of sulphuret of potash; it afforded by the former method 21.49 of oxygen, in 100; by the latter 21.63. Atmospheric air at the surface, analysed at the same time in the eudiometer of Volta, gave precisely the same result, 21.49. (Nicholson's Journal, vol. x. p. 286). Saussure, junior, also found, that the air on the summit of the Col-du-Geant contained within one-hundredth part as much oxygen as that on the plain, and even this difference may be ascribed to the difficulty of making the experiment with perfect.accuracy. The uniformity of the composition of the atmosphere at different parts of the earth's surface, appears also to be established. Mr. Cavendish originally observed, that air subjected to examination at different times, and air likewise from different places, was of perfectly similar composition; (Philosophical Transactions, vol. lxxiii. p. 129) and the same observation had been made by Fontana, from his own experiments. (Philosophical Transactions, vol. Ixix.) Mr. Davy states, that no sensible difference was found in the air sent from the coast of Guinea, and the air in England. (Journal of the Royal Institution, vol. i. p. 48). Berthollet found, that the air in Egypt and in France was similar, affording 22 of oxygen in the 100, any difference observed not amounting to a two-hundredth part of the air submitted to trial. (Memoirs relative to Egypt, p. 326). De Marti, by experiments in Spain, obtained the same uniformity of composition between 21 and 20 of oxygen in the hundred parts) in the air at places at a distance from each other; and he adds also, as-established by his experiments, that in every state of the atmosphere, whether with regard to temperature, to pressure, as indicated by the barometer, to winds, to humidity, to the season of the year, or the hour of the day or night, the results were precisely the same. (Journal de Physique, t. iii. p. 173). And more lately the researches of Humboldt and Gay Lussac, made with the view of determining this question, have establishthe same conclusion. (Journal de Physique, t. lx. p. 152). The instruments for subjecting atmospheric air to such changes as may indicate its proportion of oxygen, have been called eudiometers. When a mixture of nitrous gas is to be made with atmospheric air, the most convenient apparatus consists in a glass tube closed at top, and graduated by a diamond into cubic inches and parts. The lower aperture may be widened, in order that the gases may more easily be passed up, and likewise to afford the facility of its standing alone upon the pneumatic shelf. It is likewise usual and advantageous to fit a stopper in the month by grinding; a cubic inch measure will be required for determining the quantities poured up. A bottle will do for this purpose, and the instrument may be made very well by a chemist who is obliged to work for himself; by taking any small bottle whatever, and pouring its contents of water, by successive times, into the tube placed mouth upwards. By this means he will obtain a graduation, which, whether of the cubic inch or not, will answer the purposes of eudiometry. When air is to be exposed to a liquid sulphuret, which absorbs the oxygen, the eudiometric tube may be immersed in the liquid. Professor Hope, of Edinburgh, has contrived a very simple, elegant, and accurate apparatus for this purpose, announced in "Nicholson's Journal," iv. 210. It consists of a small bottle, of the contents of about three ounces, intended to contain the eudiometric liquid; into the neck a tube is accurately fitted by grinding, which holds precisely a cubic inch, and is divided into a hundred equal parts, and on one side the bottle, near its bottom, there is a neck into which a stopper is ground in the usual man ner. In the use of this apparatus, the bottle is first filled with the liquid employed, which is best prepared by boiling a mixture of quick lime and sulphur with water, filtering the solution, and agitating it for some time in a bottle half filled with common air. The tube, filled with the gas under examination, or with common air, if that be the subject of the experiment, is next put into its place, and, on inverting the instrument, the gas ascends into the bottle, where it is brought extensively into contact with the |