an instrument for ascertaining the purity of the atmospheric air. Many have been the contrivances of chemists for this purpose (see Eudiometer, Encycl.), but perhaps the best eudiometer is that of Moreau (or Guyton, as he now chooses to call himself), of which mention has been made in Chemistry, n°420, in this Supplement. The following short description will make the nature and use of this instrument plain to every reader.
AB; (Plate XXVIII.) represents a small glass retort with a long neck; its whole capacity being from seven to nine solid inches. It must be chosen of such a curvature that, when the neck is set upright, the bulb may form at its lower part a cavity to retain the matters introduced. The extremity of the neck of this retort is ground with emery to enter the glass tube CD, which is open at both ends, and about 12 or 15 inches in length. The retort then closes the tube in the manner of a ground stopper, and intercepts all external communication. A cylindrical glass vessel F is provided, of the form of a common jar, in which the glass tube CD may be entirely plunged beneath the level of the water. Lastly, the sulphuret of potash is prepared and broken into pieces sufficiently small to be introduced into the retort. These are to be inclosed, dry and even hot, in a bottle for use. These constitute the whole apparatus and preparation of materials.
When it is required to examine an aeriform fluid, by separating its oxygen, two or three pieces of the sulphuret, of the size of a pea, are put into the retort. It is then filled with water, taking care to incline it so that all the air may pass out from the bulb. The orifice of the retort is then to be closed, and inverted into the pneumatic tube, in order that the gas proposed for examination may be transferred into it in the usual manner. By an easy manoeuvre of alternately inclining the retort in different directions, all the water is made to flow out of the bulb in which the sulphuret remains. When this is done, the retort is placed in the vertical situation, and its extremity introduced into the tube of glass CD, which must always be under water. A small lighted taper is then to be placed under the bulb. To support the retort in its position, the jar is provided with a wooden cover, in which there is a notch to receive it.
The first impression of the heat dilates the gaseous fluid so much that it descends almost to the bottom of the tube, which is disposed expressly for its reception; otherwise the partial escape would prevent an accurate determination of its change of bulk. But as soon as the sulphuret begins to boil, the water quickly rises, not only in the inferior tube, but likewise in the neck of the retort, notwithstanding the application, and even the increase of the heat.
If the fluid be absolutely pure vital air, the absorption is total. In this case, to prevent the rupture of the vessel by too sudden refrigeration, the ascent of the water must be rendered slower, either by removing the taper, or by increasing the perpendicular height; which will not prevent the absorption from continuing while any gas remains which is proper to support combustion.
If the fluid be common air, or oxygen mixed with any other gas, the quantity of water which has entered the retort must be accurately measured after the cooling. It represents the volume of air absorbed. Care must be taken to inclose the remaining gas under the same pressure, by plunging the retort to the level of the line at which the inclosed water rests, before the orifice is flopped.
This operation of measuring, which is very easy when measuring vessels are at hand, may be habitually performed by a slip of paper pasted on the neck of the retort, upon which divisions are drawn from observation, and which must be covered with varnish to defend it from the action of the water.
EUDOXUS of Gnidus was a celebrated philosopher of the school of Pythagoras. His first preceptor was Archytas, by whom he was instructed in the principles of geometry and philosophy. About the age of twenty-three he came to Athens; and though his patrimony was small, by the generous assistance of Theomedon a physician, he was enabled to attend the schools of the philosophers, particularly that of Plato. The liberality of his friends afterwards supported him during a visit to Egypt, where he was introduced by Agathias to king Nectanebus II.; and by him to the Egyptian priests. It has been said that he accompanied Plato into Egypt; but this is inconsistent with chronology; for Nectanebus II. reigned in Egypt from the second year of the hundred and fourth Olympiad, to the second year of the hundred and seventh; and it was before Plato opened his school, that is, before the ninety-eighth Olympiad, about the fortieth year of his age, that he visited Egypt. Eudoxus is highly celebrated by the ancients for his skill in astronomy; but none of his writings on this or any other subject are extant. Aratus, who has described the celestial phenomena in verse, is said to have followed Eudoxus. He flourished about the ninety-seventh Olympiad, and died in the fifty-third year of his age. Enfield's Hist. of Philosophy.
EVECTION is used by some astronomers for the libration of the moon, being an inequality in her motion, by which, at or near the quadratures, she is not in a line drawn through the centre of the earth to the sun, as she is at the syzygies, or conjunction and opposition, but makes an angle with that line of about 2° 51'. The motion of the moon about her axis only is equable; which rotation is performed exactly in the same time as she revolves about the earth; for which reason it is that she turns always the same face towards the earth nearly, and would do so exactly, were it not that her monthly motion about the earth, in an elliptic orbit, is not equable; on which account the moon, seen from the earth, appears to librate a little upon her axis, sometimes from east to west, and sometimes from west to east; or some parts in the eastern limb of the moon go backwards and forwards a small space; and some that were conspicuous, are hid, and then appear again.
The term evetion is used by some astronomers to denote that equation of the moon's motion which is proportional to the sine of double the dilatation of the moon from the sun, diminished by the moon's anomalous EUPHON
This equation is not yet accurately determined; some state it at $1^\circ 30'$, others at $1^\circ 16'$, &c. It is the greatest of all the moon's equations, except the equation of the centre. *Hutton's Dictionary.*
**EVENLY EVEN NUMBER.** See Number, Encycl.
**EVENLY ODD NUMBER.** See Number, Encycl.
**EVOLVENT,** in the higher geometry, a term used by some writers for the involute or curve resulting from the evolution of a curve, in contradiction to that evolute, or curve supposed to be opened or evolved. See Evolute and Involute, Suppl.
**EVOLUTE,** in the higher geometry, a curve first proposed by Huyghens, and since much studied by mathematicians. It is any curve supposed to be evolved or opened, by having a thread wrapped close upon it, fastened at one end, and beginning to evolve or unwind the thread from the other end, keeping the part evolved or wound off tight stretched; then this end of the thread will describe another curve, called the involute. Or the same involute is described the contrary way, by wrapping the thread upon the evolute, keeping it always stretched. For the Involution and Evolution of Curves, see Involution in this Supplement.
**Imperfect Evolute,** a name given by M. Reaumur to a new kind of evolute. The mathematicians had hitherto only considered the perpendiculars let fall from the involute on the convex side of the evolute; but if other lines not perpendicular be drawn upon the same points, provided they be all drawn under the same angle, the effect will still be the same; that is, the oblique lines will all intersect in the curves, and by their intersections form the infinitely small sides of a new curve, to which they would be so many tangents. Such a curve is a kind of evolute, and has its radii; but it is an imperfect one, since the radii are not perpendicular to the first curve or involute.
**EUPHON,** a musical instrument invented lately by Dr Chladni of Wittenberg; well known by his various publications on philosophical subjects, especially the theory of musical sounds. The euphon consists of forty-two immovable parallel cylinders of glass of equal length and thickness; but its construction, tone, and the method of playing it, are totally different from those of the harmonica, with which indeed it has nothing in common but the glass. See Harmonica, Encycl.
Dr Chladni gives the following account of his invention. In his 19th year he began to learn to play the harpsichord; and he afterwards read a great many of the principal works on the theory of music, by which he found that the physico-mathematical part of that science was far more defective than other branches of natural philosophy. Being therefore possessed with an idea that his time could not be better employed than in endeavouring to make discoveries in this department, he accordingly tried various experiments on the vibrations of strings and the different kinds of vibration in cylindric pieces of wood, first discovered, through calculation, by the elder Euler; and found, that though a great deal had been said on the nature of these elastic bodies, yet the manner of vibration and the proportion of tones in other elastic bodies, which do not proceed, as in the former, in straight lines, but depend on the vibration of whole surfaces, were totally unknown, and that the little which had been written on that subject, by some authors, did not correspond with nature. He had already long remarked, that every plate of glass or metal emitted various tones according as it was held and struck in different places; and he was desirous to discover the cause of this difference, which no one had ever examined. He fixed in a vice the axle of a brass plate which belonged to a polishing machine, and found, that by drawing the bow of a violin over it, he produced very different tones, which were stronger and of longer duration than those obtained merely by striking it.
The observation, that not only strings but also other elastic bodies may be made to produce sounds by drawing a violin bow over them, Dr Chladni does not give as a discovery of his own; as the so-called iron violin has been long known; and as he had read of an instrument constructed in Italy*, where glasses or metal bells were made to sound by means of two or more violin bows drawn over them. But the idea of employing this instrument to examine vibrating tones was first entertained by himself. Having accurately remarked the tones produced by the abovementioned metal plate, he found that they gave a progression which corresponded with the squares of $2$, $3$, $4$, &c.
Not long before he had read, in the Transactions of the Royal Society of Gottingen, the observations of Mr Lichtenberg on the phenomena produced by firing pounded resin over a glass plate or cake of resin, and he repeated many of his experiments. This led him to the idea that perhaps the various vibratory movements of such a plate would be discovered by a diversity of phenomena, if he fired over it sand or anything of the like kind. By this experiment there was produced a star-formed figure; and the author, having continued his researches, published the result of them in a work entitled, Discoveries respecting the Theory of Sound, printed at Leipzig in 1787.
Whilst he was employed in these investigations, he resolved to invent a new musical instrument; and he began to consider whether it might not be possible by rubbing glass tubes in a straight line, with the wet fingers, to produce sounds in the same manner as is done in the harmonica by rubbing them circularly. That glass tubes, like those in his euphon, would not merely by such rubbing emit any tones, he had long known by theory and experience; and he therefore applied himself to the solution of the difficult question, in what manner the instrument ought to be constructed to answer the intended purpose? After various fruitless attempts for a year and a half, during which his imagination was so full of the idea, that sometimes in his dreams he thought he saw the instrument and heard its tones, that is, like those of the harmonica, but with more distinctness and less confusion, he at length, in a state between sleeping and waking, obtained a solution of the problem which had given so much employment to his thoughts. On the second of June 1789, being tired with walking, he sat down on a chair, about nine in the evening, to enjoy a short slumber; but scarcely had he closed his eyes when the image of an instrument, such as he wished for, seemed to present itself before him, and terrified him so much that he awoke as if he had been struck by an electric shock. He immediately started up in a kind of enthusiasm; and made a series of experiments, which convinced him that what he had seen was perfectly right, and that he had it now in his his power to carry his design into execution. He made his experiments and constructed his first instrument in so private a manner, that no person knew anything of them. On the 8th of March 1790 his first instrument of this kind was completed; and in a few days he was able to play on it some easy pieces of music. It was now necessary to give to this instrument, as it was entirely new, a new name; and that of euphon, which signifies an instrument that has a pleasant sound, appeared to him the most proper.
It was not, however, brought to perfection at once, for he made a second instrument which was an improvement of the first, and a third which was an improvement of the second. In found, indeed, and particularly in the higher tones, the first was equal to either of the other two; but the construction was deficient in strength, so that every week some hours were necessary to keep it in proper repair; and it was impossible to convey it the distance of a mile without almost totally destroying it. Dr Chadini also, for want of better tubes, employed those used for thermometers, and marked the whole and half tones by a coating of sealing-wax on the under-side; but as the wax, owing to the moisture and vibration, often cracked and flew off, it was attended with danger to the eyes. It was therefore extremely difficult to give to the construction of the instrument sufficient strength; but this the inventor at length accomplished, so that his new euphon cannot be injured or put out of tune either by playing or by carriage. The third instrument was somewhat different from the first and second; as the fore part, which in the two former rose upwards with an oblique angle, stood at right angles, so that it could be transported with ease in a particular carriage made for that purpose. Instead of the thermometer tubes used in the first, the Doctor now employs tubes of different colours. In the second instrument those for the whole tones were of dark green glass; but he used for the half tones, in both, a milk white kind of glass. In a word, the euphon has some resemblance to a small writing-desk. When opened, the abovementioned glass tubes, of the thickness of the barrel of a quill and about 16 inches long, are seen in a horizontal position. They are wetted with water, by means of a sponge, and stroked with the wet fingers in the direction of their length, so that the increase of the tone depends merely on the stronger or weaker pressure, and the slower or quicker movement of the fingers. The number of tubes at present is forty-two. In the back part there is a perpendicular founding-board divided in the middle, through which the tubes pass. It appears therefore that the euphon ought not to be considered as an altered or improved harmonica, but as a totally new and different instrument. In regard to sweetness of sound, it approaches very near to the harmonica; but it has several advantages which no unprejudiced person, who examines both instruments, will deny.
1. It is simpler, both in regard to its construction and the movement necessary to produce the sound, as neither turning nor stamping is required, but merely the movement of the finger. 2. It produces its sound speedier; so that as soon as it is touched you may have the tone as full as the instrument is capable of giving it; whereas, in the harmonica, the tones, particularly the lower ones, must be made to increase gradually.
3. It has more distinctness in quick passages, because Euphonia's tones do not resound so long as in the harmonica, where the sound of one low tone is often heard when you wish only to hear the following tone. 4. The union is purer than is generally the case in the harmonica, where it is difficult to have perfect glissos, which in every part give like tones with mathematical exactness. It is however as difficult to be tuned as the harmonica. 5. It does not affect the nerves of the performer; for a person scarcely feels a weak agitation in the fingers; whereas in the harmonica, particularly in concords of the lower notes, the agitation extends to the arms, and even through the whole body of the performer. 6. The expense of this instrument will be much less in future than that of the harmonica. 7. When one of the tubes breaks, or any other part is deranged, it can be soon repaired, and at very little expense; whereas, when one of the glissos of the harmonica breaks, it requires much time, and is very difficult to procure another capable of giving the same tone as the former, and which will correspond sufficiently with the series of the rest.