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with it was dissolved. It dissolved the volatile oils, and also phosphorus. . Its specific gravity was inferior to that of alcohol, being as 94 to 100. After its production, when the heat was much raised, a quantity of oily matter was distilled over, and carburetted hydrogen was disengaged, the residual liquor was of a dark brown colour, and contained a o: quantity of phosphoric acid. (Annales de Chimie, tom. xi. p. 123.) Fluoric ether has been said to be formed by putting fluate of lime, previously ignited and in powder, into a retort, with equal weights of alcohol and sulphuric acid, and distilling to dryness. The product of this distillation was again distilled to one half, and a portion of fluoric acid abstracted from it by a solution of potash, which at the same time precipitated a portion of silex, so as to render the whole gelatinous. This, on being again distilled, afforded an ether of the specific gravity of 0.720, which burnt with a blue flame, and had a bitter taste. It is added, that it greatly resembled sulphuric ether; and it is not improbable that it may have been merely this ether disguised. (Nicholson's Journal, vol. viii. p. 143.) ./lcetic ether has been known for a considerable time to chemists, Lauragais having given, in 1759, the process for preparing it, by distilling alcohol, with the concentrated acetic acid that is procured by the decomposition of acetate of copper by heat. Scheele, as well as other chemists, have been unable to form it; but Pelletier has observed that it is procured with certainty by distilling alcohol repeatedly from the acetic acid. The alcohol at first acquires an ethereal odour, but is miscible with water; by returning it on the residual liquor, distilling it, and repeating this for a third time, this becomes stronger: the acid contained in the liquor thus procured was saturated by the addition of carbonate of potash; and by distillation there was procured from it a pure acetic ether, in quantity about half of the alcohol employed. (“Mémoirs de Chimie,” tom. i. p. 237.) It was soluble in water in a limited quantity, seven measures dissolving three. It has an agreeable odour, ethereal, but in which the smell of acetic acid is also perceptible. It is very volatile and inflammable: it burns with a clear light, and leaves a little charcoal. According to Pelletier, acetic ether may likewise beformed by distillation, from a mixture of sulphuric acid, acetate of copper, and alcohol ; and according to Lap

lanche, it may be obtained from a mixture. of sulphuric acid, alcohol, and acetate of lead. Ethen of Sir Isaac Newton. When we have separated the actions of bodies upon each other, so far that the effects appear to us to be simple, we resolve the causes of motion into two; namely, a disposition of bodies to come together, called attraction, and a disposition to recede from each other, called repulsion. Impulse, or the communication of motion by apparent contact, will not constitute a peculiar case, because we know that bodies cannot be, or are not, in any of our observations, brought close to each other. But as in all our philosophising we endeavour to simplify the general principles, it becomes a question, whether the effects of attraction and repulsion may not depend upon the same cause; and as we have many gross instances of bodies being urged together by the action of fluids, it naturally occurs to enquire, whether the apparent attractions in nature may not be caused by some fluid medium. Sir Isaac Newton was strongly of this opinion, as appears by his letter to Boyle, published in Birch's life of that philosopher, as well as by the famous paragraph at the end of his “Principia,” and one of the queries at the end of his “Optics,” in the preface to the second edition of which he remarks, that he does not take gravity for an essential property of bodies. In the query here mentioned, he proceeds upon the supposition of an elastic medium pervading all space ; , a supposition which he advances with considerable confidence, and which he supports b very strong arguments, deduced as well from the phenomena of light and heat, as from the analogy of the electric and magnetic influences. This medium he supÉ. to be much rarer within the dense odies of the sun, the planets, and the comets, than in the empty celestial spaces between them, and to grow more and more dense at greater distances from them, so that all these bodies are naturally forced towards each other by the excess of pressure. The effects of gravitation might be produced by a medium thus constituted, if its particles were repelled, by all material substances, with a force decreasing like other repulsive forces, simply as the distances increase; its density would then be every way such as to produce the appearance of an attraction varying, like that of gravitation such an ethereal me. dium would therefore have the advantage of simplicity in the original law of its action, since the repulsive force, which is known to belong to all matter, would be sufficient, when thus modified, to account for the principal phenomena of attraction. It may be questioned whether a me: dium, capable of producing the effects of gravitation in this manner, would also be equally susceptible of those modifications which we have supposed to be necessary for the transmission of light : in either case it must be supposed to pass through the apparent substance of all material bodies with the most perfect freedom, and there would, therefore, be no occasion to apprehend any difficulty from a retardation of the celestial motions; the ultimate impenetrable particles of matter being perhaps scattered as thinly through its external form as the stars are scattered in a nebula, which has still the distant appearance of an uniform light, and of a continuous surface : and there seems no reason to doubt the possibility of the propagation of an undulation through the Newtonian medium, with the actual velocity of light. It must be remembered, that the difference of its pressure is not to be estimated from the actual bulk of the earth, or any other planet alone, but from the effect of the sphere of repulsion of which that planet is the centre; and we may then deduce the force of gravitation from a medium of no very enormous elasticity. A similar combination of a simple pressure with a variable repulsion is also observable in the force of cohesion ; and Dr. Young, in his Lectures, remarks, that supposing two particles of matter floating in such an elastic medium, capable of producing gravitation, to approach each other, their mutual attraction would at once be changed from gravitation to cohesion, upon the exclusion of the portion of the medium intervening between them: this supposition is, however, as he adds, directly opposite to that which assigns to the elastic medium the power of passing freely through all the interstices of the ultimate atoms cohering in this manner; but that, as we see some effects so nearly resembling them, which are unquestionably produced by the pressure of the atmosphere, we can scarcely avoid suspecting that there must be some analogy in the causes. Two plates of metal, which cohere enough to support each other in the open air,will often separate in a vacuum. When

a boy draws along a stone by a piece of wet leather, the pressure of the atmosphere seems to be materially concerned. The well-known experiment of the two exhausted hemisphères of Magdeburgh affords a still more striking instance of apparent cohesion, derived from atmospherical pressure: and 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 al. low it to expand in an equal degree and at a certain point the ring would expand no more, the air would be admitted, and 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 Mohality, 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. ETHULLA, in botany, a genus of the Syngenesia Polygamia Aequalis class and order. Natural order of Compositae Discoideae. Corymbifera, 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 GRAMMAR. EVAPORATION, in natural philoso. phy, is the conversion of water into vapour, which, in consequence o lighter than the atmosphere, is raise 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 surface 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, mamy 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, or 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 o 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 inertiae 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 dia. meter 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 noLiced, 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 find 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 go 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 the level of the upper pipe, but latterl it was constantly a little below it, ...}. 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 guage 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 evaporatcd 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 fourcelled, opening at the top, inclosing many seeds. There are two species, viz. E. obliqua; oblique leafed eucalyptis; 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, contains 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 be: comes 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 twocelled. 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 hav. ing 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. EucLin, 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 Eradtosthenes, Achimedes, 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, Catoptrics, a Treatise of the Division of Superficies, Porisms, Loci ad Super

ficiem, Fallacies, and four books of Co-.

Ilics. 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 Astronmy at Öxford. The “Elements,” as commonly published, 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, eight, and ninth, give us the fundamental properties of numbers; the tenth contains the theory of commensurable and incommensurable lines and spaees; 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 affirms, 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 th Hexandria Monogynia class and order. Natural order of Coronariae. Asphodeli, Jussieu. 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. EUDIOM ETRY. The measurement of the quantity of oxygen contained in atmospheric 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 so. lution 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 a quantity proportioned to the quantity of fluid. Phosphorus is applied by a very simple apparatus, but, by its solubility in nitrogen gas, it adds to the bulk of the res. dual air, for which a correction must be made. Nitrous gas was employed by Priestley; it exhibits the resultimmediate. ly, but is liable to several sources of fai. lacy. 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 ratherless than 200 measures of hydrogen for saturation; about 40 measures of hydrogen are therefore sufficient to saturate the . 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

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