knowledges to be inaccurate, because deduced fom the short period of 150 years, and from the observations of Timochares, in which he had no great confidence. This motion of the starry heavens was long a matter of discussion, as a thing for which no physical reason could be assigned. But the establishment of the Copernican system reduced it to a very simple affair; the motion which was thought to affect all the heavenly bodies, is now acknowledged to be a deception, or a false judgment from the appearances. The earth turns round its own axis while it revolves round the sun, in the same manner as we may cause a child's top to spin on the brim of a mill-stone, while the stone is turning slowly round its axis. If the top spin steadily, without any wavering, its axis will always point to the zenith of the heavens; but we frequently see, that while it spins briskly round its axis, the axis itself has a slow conical motion round the vertical line, so that, if produced, it would slowly describe a circle in the heavens round the zenith point. The flat surface of the top may represent the terrestrial equator, gradually turning itself round on all sides. If this top were formed like a ball, with an equatorial circle on it, it would represent the whole motion very prettily, the only difference being, that the spinning motion and this wavering motion are in the same direction; whereas the diurnal rotation and the motion of the equinoctial points are in contrary directions. Even this dissimilarity may be removed, by making the top turn on a cap, like the card of a matiner's compass. It is now a matter fully established, that while the earth revolves round the sun from west to east, in the plane of the ecliptic, in the course of a year it turns round its own axis from west to east in 23 56' 4", which axis is inclined to this plane in an angle of nearly 23° 28: and that this axis furns round a line perpendicular to the ecliptic in 25,745 years from east to west, keeping nearly the same inclination to the ecliptic.-By this means, its pole in the sphere of the starry heavens describes a circle round the pole of the ecliptic at the distance of 23° 28' nearly. The consequence of this must be, that the terrestrial equator, when produced to the sphere of the starry heavens, will cut the ecliptic in two opposite points, through which the sun must pass when he makes the day and night equal; and that these points must shift to the westward, at the rate of 501 seconds annually, which is the precession of the equinoxes. Accordingly this has been the received doctrine among astronomers for nearly three centuries, and it was thought perfectly conformable to appear ances. But Dr. Bradley, the most sagacious of modern astronomers, hoped to discover the parallax of the earth's orbit by observations of the actual position of the pole of the celestial revolution. Dr. Hooke had attempted this before, but with very imperfect instruments. The art of observing being now prodigiously improved, Dr. Bradley resumed this investigation. It will easily appear, that if the earth's axis keep parallel to itself, its extremity must describe in the sphere of the starry heavens a figure equal and parallel to its orbit round the sun; and if the stars be so near that this figure is a visible object, the pole of diurnal revolution will be in different distinguishable. points of this figure. Consequently, if the axis describe the cone already mentioned, the pole will not describe a circle round the pole of the VOL. IX. ecliptic, but will have a looped motion along this circumference, similar to the absolute motion of one of Jupiter's satellites, describing an epicycle whose centre describes the circle round the pole of the ecliptic. He accordingly observed such an epicyclical motion, and thought that he had now overcome the only difficulty in the Copernican system; but, on maturely considering his observations, he found this epicycle to be quite inconsistent with the consequences of the annual parallax, and it puzzled him exceedingly. One day, while taking the amusement of sailing about on the Thames, he observed that every time the boat tacked, the direction of the wind, estimated by the direction of the vane, seemed to change. This immediately suggested to him the cause of his observed epicycle, and he found it an optical illusion, occasioned by a combination of the motion of light with the motion of his telescope while observing the polar stars. Thus he unwittingly established an incontrovertible argument for the truth of the Copernican system, and immortalized his name by his discovery of the aberration of the stars. He now engaged in a series of observations for ascertaining all the circumstances of this discovery. In the course of these, which were continued for 28 years, he discovered another epicyclical motion of the pole of the heavens, which was equally curious and unexpected. He found that the pole described an epicycle, whose diameter was about 18', having for its centre that point of the circle round the pole of the ecliptic in which the pole would have been found independent of this new motion. He also observed, that the period of this epicyclical motion was 18 years and seven months. It struck him, that this was precisely the period of the revolution of the nodes of the moon's orbit. He gave a brief account of these results to lord Macclesfield, then president of the Royal Society, in 1747. Mr. Machin, to whom he also communicated the observations, gave him in return a very neat mathematical hypothesis, by which the motion might be calculated. Let F (pl. 142, fig. 1), be the pole of the ecliptic, and SPQ a circle distant from it 23° 28', representing the circle described by the pole of the equator during one revolution of the equinoctial points. Let P be the place of this last mentioned pole at some given time. Round P describe a circle ABCD, whose diameter AC is 18. The real situation of the pole will be in the circumference of this circle; and its place, in this circumference, depends on the place of the moon's ascending node. Draw EPF and GPL perpendicular to it; let GL be the colure of the equinoxes, and EF the colure of the solstices. Dr. Bradley's observations showed that the pole was in A when the node was in L, the vernal equinox. If the nude recede to H, the winter solstice, the pole is in B. When the node is in the autumnal equinox at G the pole is at C; and when the node is in F, the summer solstice, the pole is in D. In all intermediate situations of the moon's ascending node, the pole is in a point of the circumference ABCD, three signs or 90° more advanced. Dr. Bradley, by comparing together a great number of observations, found that the mathematical theory, and the calculation depending on it, would correspond much better with the observations, if an eclipse were substituted for the circle ABCD, making the longer axis AC 18", and the shorter, BD, 16". Mr. D'Alembert determ Q Q by the physical theory of gravitation, the axis to be 18" and 13"4 These observations, and this mathematical the ory must be considered as so many facts in astronomy, and we must deduce from them the methods of computing the places of all celestial phenomena, agreeable to the universal practice of determining every point of the heavens by its longitude, latitude, right ascension, and declina tion. It is evident, in the first place, that this equation of the pole's motion makes a change in the obliquity of the ecliptic. The inclination of the equator to the ecliptic is measured by the arch of a great circle intercepted between their poles. Now, if the pole be in O instead of P, it is plain that the obliquity is measured by EO instead of EP. If EP be considered as the mean obliquity of the ecliptic, it is augmented by 9" when the moon's ascending node is in the vernal equinox, and consequently the pole in A. It is, on the contrary, diminished "when the node is in the autumnal equinox, and the pole in C; and it is equal to the mean when the node is in the colure of the solstices. This change of the inclination of the earth's axis to the plane of the ecliptic was called the nutation of the axis by sir Isaac Newton; who showed that a change of nearly a second must obtain in a year by the action of the sun on the prominent parts of the terrestrial spheroid. But he did not attend to the change which would be made in this motion by the variation which obtains in the disturbing force of the moon, in consequence of the different obliquity of her action on the equator, arising from the motion of her own oblique orbit. It is this change which now goes by the name nutation, and we owe its discovery entirely to Dr. Bradley. The general change of the position of the earth's axis has been termed deviation by modern astronomers. The quantity of this change of obliquity is easily ascertained. It is evident from what has been already said, that when the pole is iu O, the arch ADCO is equal to the node's longitude from the `vernal equinox, and that PM is its confine; and (on account of the smallness of AP in comparison of EP) PM may be taken for the change of the obliquity of the ecliptic. This is therefore =9"+cos. long, node, and is additive to the mean obliquity, while O is in the semicircle BAD, that is, while the longitude of the node is from 9 signs to 3 signs; but subtractive while the longitude of the node changes from 3 to 9 signs. But the nutation changes also the longitudes and right ascensions of the stars and planets, by changing the equinoctial points, and thus occasioning an equation in the precession of the equinoctial points. It was this circumstance which made it necessary for us to consider it in this place, while expressly treating of this precession. Let us at tend to this derangement of the equinoctial points, The great circle or meridian which passes through the poles of the ecliptic and equator is always the solstitial colure, and the equinoctial colure is at right angles to it: therefore when the pole is in P or in O, EP or EO is the solstitial colure. Let S be any fixed star or planet, and let SE be a meridian or circle of longitude; draw the circles of declination PS, OS, and the circles M'EM', mEm', perpendicular to PE, OE. If the pole were in its mean place P, the equinoctial points would be in the ecliptic meridian M'EM', or that meridian would pass through the intersections of the equator and ecliptic, and the angle M'ES would measure the longitude of the star S. But when the pole is in 0, the ecliptic meridian mEm' will pass through the equinoctial points. The equinoctial points must therefore be to the westward of their mean place, and the equs. tion of the precession must be additive to that precession: and the longitude of the star S will now be measured by the angle m ES, which, in the case here represented, is greater than its mean longitude. The difference, or the equation of longitude arising from the nutation of the earth's ом ОЕ axis, is the angle OEP, or OM is the sine of the angle CPO, which, by what has been already observed, is equal to the longitude of the mode Therefore OM is equal to 9" long. node, and 9" X sin. long. node is equal to ом ОЕ This sin. obliq. eclip. equation is additive to the mean longitude of the star when O is in the semicircle CBA, or while the ascending node is passing backwards from the ver nal to the autumnal equinox; but it is subtractive from it while O is in the semicircle ADC, or while the node is passing backwards from the autumes to the vernal equinox; or, to express it more briefly, the equation is subtractive, from the mean longitude of the star, while the ascending node is in the first six signs, and additive to it while the node is in the last six signs. This equation of longitude is the same for all the stars, for their longitude is reckoned on the ecliptic (which is here supposed invariable); and therefore is affected only by the variation of the point from which the longitude is computed. The right ascension, being computed on the equator, suffers a double change. It is computed from, or begins at, a different point of the equator, and it terminates at a different point; because the equator having changed its position, the circles of declination also change theirs. When the pale is at P, the right ascension of S from the solstit colure is measured by the angle SPE, contained between that colure and the star's circle of deci nation. But when the pole is at O, the right difference of SPE and SOF is the equation of right cension is measured by the angle SOE, and the ascension. The angle SOE consists of two parts, GOE and GOS; GOE remains the same whereve the star S is placed, but GOS varies with the place of the star. We must first find the variation by which GPE becomes GOE, which variation is com mon to all the stars. The triangles GPE, GOL, have a constant side GE, and a constant angle G, the variation PO of the side GP is extremely small, and therefore the variation of the angles may be computed by Mr. Cotes's Fluxionary The orems. See Simpson's Fluxions, § 253, &c. A the tangent of the side EP opposite to the constan angle G, is to the sine of the angle EPG, opposite to the constant side EG, so is PO the variation of the side GP adjacent to the constant angle, to the variation of the angle GPO, opposite to the coa stant side EG. This gives x = 9x sindo ng ipi tang. obl. eclip. This is subtractive from the mean right ascension for the first six signs of the node's longitude, and additive for the last six signs. This equation is con mon to all the stars. The variation or the other part SOG of the an gle, which depends on the different position of the hour circles PS and OS, which causes them to cat the equation in different points, where the arches of right ascension terminate may be discovered as flows. The triangles SPG, SOG, have a constant de SG, and a constant angle G. Therefore, by the same Cotesian theorem, tan. SP: sin. SPG PO: y, and y, or the second part of the nutation in right ascension,= 9x sin. diti. R. A. of star and node. cotan. declin. star. The nutation also effects the declination of the stars. For SP, the mean codeclination, is changed into SO.-Suppose a circle described round S with the distance SO cutting SP in f; then it is evident that the equation of declin. is Pf= POX consine OPf=9" x sign. r. ascen, of star-long, of node. Such are the calculations in constant use in our stronomical researches, founded on Machin's Theory. When still greater accuracy is required, the elliptical theory must be substituted, by taking as is expressed by the dotted lines) O in that point of the ellipse described on the transverse ixis AC, where it is cut by OM, drawn according to Machin's theory. All the change made here is he diminution of OM in the ratio of 18 to 13,4, and a corresponding diminution of the angle CPO. The detail of it may be seen in De la Lande's Astronomy, art. 2874; but is rather foreign to our present purpose of explaining the precession of the equinoxes. The calculations being in every tase tedious, and liable to mistakes, on account of he changes of the signs of the different equations, he zealous promoters of astronomy have calcuated and published tables of all these equations, both on the circular and elliptical hypothesis. And still more to abridge calculations, which occur n reducing every astronomical observation, when he place of a phenomenon is reduced from a Comparison with known fixed stars, there have ween published tables of nutation and precession, for some hundreds of the principal stars, for every position of the moon's node and of the sun. It now remains to consider the precession of the quinoctial points, with its equations, arising from The mutation of the earth's axis as a physical phesomenon, and to endeavour to account for it upon those mechanical principles, which have so happily explained all the other phenomena of the celestial motions. This did not escape the penetrating eye of sir Isaac Newton; and he quickly found it to be a consequence, and the most beautiful proof, of the universal gravitation of all matter to all matter; and there is no part of his immortal work where his sagacity and fertility of resource shine more conspicuously than in this investigation. It must be acknowledged, however, that Newton's investigation is only a shrewd guess founded on assump. tions of which it would be extremely difficult to demonstrate either the truth or falsity, and which required the genius of a Newton to pick out in such a complication of abstruse circumstances. The subject has occupied the attention of the first mathematicians of Europe since his time; and is still considered as the most curious and difficult of all mechanical problems. The most elaborate aud accurate dissertations on the precession of the equinoxes are those of Sylvabella and Walmesly, in the Philosophical Transactions, published about the year 1754; that of Thomas Simpson, published in his Miscellaneous Tracts; that of Father Frisius, in the Memoirs of the Berlin Academy, and afterwards, with great improvements in his Cosmographia; that of Euler in the Memoirs of Berlin; that of D'Alembert in a separate disser tation; and that of De la Grange on the Libra tion of the Moon, which obtained the prize in the Academy of Paris, in 1769. We think the dissertation of Father Frisius the most perspicuous of them all, being conducted in the method of geometrical analysis; whereas most of the others proceed in the fluxionary and symbolic method, which is frequently deficient in distinct notions of the quantities under consideration, and therefore does not give us the same perspicuous conviction of the truth of the results. In a work like ours, it is impossible to do justice to the problem, without entering into a detail which would be thought extremely disproportioned to the subject by the generality of our readers. Yet those who have the necessary preparation of mathematical knowledge, and wish to understand the subject fully, will find enough here to give them a very distinct notion of it; and in the article ROTATION they will find the fundamental theorems, which will enable them to carry on the investigation. We shall first give a short sketch of Newton's investigation, which is of the most palpable and popular kind, and is highly valuable, not only for its ingenuity, but also because it will give our unlearned readers distinct and satisfactory conceptions of the chief circumstances of the whole phenomena. Let S (fig. 2.) be the sun, E the earth, and M the moon, moving in the orbit NMCDn, which cuts the plane of the ecliptic in the line of the nodes Nn, and has one half raised above it, as represented in the figure, the other half being hid below the ecliptic. Suppose this orbit folded down; it will coincide with the ecliptic in the circle Nm c d n. Let EX represent the axis of this orbit perpendicular to its plane, and therefore inclined to the ecliptic. Since the moon gravitates to the sun in the direction MS, which is all above the ecliptic, it is plain that this gravitation has a tendency to draw the moon towards the ecliptic. Suppose this force to be such that it would draw the moon down from M to i in the time that she would have moved from M to t, in the tangent to her orbit. By the combination of these motions, the moou will desert her orbit, and describe the line Mr, which makes the diagonal of the parallelogram; and if no further action of the sun be supposed, she will describe another orbit Min, lying between the orbit MCDn and the ecliptic, and she will come to the ecliptic, and pass through it in a point n', nearer to M than n is, which was the former place of her descending node. By this change of orbit, the line EX will no longer be perpendicular to it; but there will be another line E, which will now be perpendicular to the new orbit. Also the moon, moving from M to r, does not move as if she had come from the ascending node N, but from a point N lying beyond it; and the line of the nodes of the orbit in this new position is N'n'. Also the angle MN'm is less than the angle MNm. Thus the nodes shift their places in a direction opposite to that of her motion, or move to the westward; the axis of the orbit changes its position, and the orbit itself changes its inclination to the ecliptic. These momentary changes are different in different parts of the orbit, according to the position of the line of the nodes. Sometimes the inclination of the orbit is increased, and sometimes the nodes move to the eastward. But, in general, the inclination increases from the time that the nodes are in the line of syzigee, till they get into quadrature, after which it diminishes till the nodes are again in syzigee. The nodes advance only then shows that the effect of the disturbing forte while they are in the octants after the quadratures, and while the moon passes from quadrature to the node, and they recede in all other situations. Therefore the recess exceeds the advance in every revolution of the moon round the earth, and, on the whole, they recede. What has been said of one moon would be true of each of a continued ring of moons surrounding the earth, and they would thus compose a flexible ring, which would never be flat, but waved according to the difference (both in kind and degree) of the disturbing forces acting on its different parts. But suppose these moons to cohere, and to form a rigid and flat ring, nothing would remain in this ring but the excess of the contrary tendencies of its different parts. Its axis would be perpendicular to its plane, and its position in any moment will be the mean position of all the axis of the orbits of each part of the flexible ring; therefore the nodes of this rigid ring will continually recede, except when the plane of the ring passes through the sun, that is, when the nodes are in syzigee; and (says Newton) the motion of these nodes will be the same with the mean motion of the nodes of the orbit of one moon. The inclination of this ring to the ecliptic will be equal to the mean inclination of the moon's orbit during any one revolution which has the same situation of the nodes. It will therefore be least of all when the nodes are in quadrature, and will increase till they are in syzigee, and then diminish till they are again in quadra ture. Suppose this ring to contract in dimensions, the disturbing forces will diminish in the same proportion, and in this proportion will all their effects diminish. Suppose its motion of revolution to accelerate, or the time of a revolution to diminish; the linear effects of the disturbing forces being as the squares of the times of their action, and their angular effects as the times, those errors must diminish also on this account; and we can compute what those errors will be for any diameter of the ring, and for any period of its revolution. We can tell, therefore, what would be the motion of the nodes, the change of inclination, and deviation of the axis, of a ring which would touch the surface of the earth, and revolve in 24 hours; nay, we can tell what these motions would be, should this ring adhere to the earth. They must be much less than if the ring were detached; for the disturbing forces of the ring must drag along with it the whole globe of the earth. The quantity of motion which the disturbing forces would have produced in the ring alone will now (says Newton) be produced in the whole mass; and therefore the velocity must be as much less as the quantity of matter is greater: But still all this can be computed. Now there is such a ring on the earth: for the earth is not a sphere, but an elliptical spheroid. Sir Isaac Newton therefore engaged in a computation of the effects of the disturbing force, and has exhibited a most beautiful example of mathematical investigation. He first asserts, that the earth must be an elliptical spheroid, whose polar axis is to its equatorial diameter, as 229 to 230. Then he demonstrates, that if the sine of the inclination of the equator be called, and ift be the number of days (sidereal) in a year, the annual motion of on this ring is to its effect on the matter of the same ring, distributed in the form of an elliptical stratum (but still detached) as 5 to 2; therefre 3√1-2 the motion of the nodes will be 360° x 101 or 16' 16" 24" annually. He then proceeds to show, that the quantity of motion in the sphere is to that in an equatorial ring revolving in the same time, as the matter in the sphere to the matter in the ring, and as three times the square of a qua drantal arch to two squares of a diameter, jointly Then he shows, that the quantity of matter in the terrestrial sphere is to that in the protuberant mat ter of the spheroid, as 52900 to 461 (supposing all homogeneous). From these premises it fol lows, that the motion of 16' 16" 24", must be d minished in the ratio of 10717 to 100, which reduces it to 9" 07" annually. And this (he s) is the precession of the equinoxes, occasioned by the action of the sun; and the rest of the 50, which is the observed precession, is owing to the action of the moon, nearly five times greater thas that of the sun. This appeared a great difficulty: for the phenomena of the tides show that it cas not much exceed twice the sun's force. Nothing can exceed the ingenuity of this pr cess. Justly does his celebrated and candid comentator, Daniel Bernoulli, say (in his Disserta tion on the Tides, which shared the prize of the French Academy with M'Laurin and Euler), t Newton saw through a veil what others could hardly discover with a microscope in the light of the meridian sun. His termination of the form and dimensions of the earth, which is the founda tion of the whole process, is not offered as sy thing better than a probable guess, in re d lima; and it has been since demonstrated with geometrical rigour by M'Laurin. His next principle, that the motion of the nodes of the rigid ring is equal to the mean motion d the nodes of the moon, has been most critica discussed by the first mathematicians, as a thing which could neither be proved nor refuted. Fr sius has at least shown it to be a mistake, and that the motion of the nodes of the ring is docar the mean motion of the nodes of a single me and that Newton's own principles should hav produced a precession of 184 seconds analy which removes the difficulty formerly mentioned His third assumption, that the quantity of tion of the ring must be shared with the inch sphere, was acquiesced in by all bis commentat till D'Alembert and Euler, in 1749, shewed that was not the quantity of motion round an ax rotation which remained the same, but the qua tity of momentum or rotatory effort. The q tity of motion is the product of every particle its velocity; that is, by its distance from the at while its momentum, or power of producing r tion, is as the square of that distance, and is to be had by taking the sum of each particle muty by the square of its distance from the axis. Sy the earth differs so little from a perfect spies, this makes no sensible difference in the result. will increase Newton's precession about the fourths of a second. The real source of Newton's mistake in the solution of this intricate and interesting prakis was first detected by Mr. Landen, in the first v lume of his Memoirs. That excellent mathema tician discovered that when a rigid annulus revolve and 46 0619+ 20" 084 x sin. a x tang. d = that of right ascension. See the Connoissance des Temps for 1792, pa. 206, &c. The PRECIE. In botany. Early ripe. name of an early sort of grape in Virgil. The fifty-first order in Linnéus's Fragments; and the twenty-first in his Natural Orders: comprehending such plants as flower early in the spring. PRECINCT. s. (precinctus, Latin.) Outward limit; boundary (Hooker). PRECIOSITY. s. (from pretiosus, Latin.) 1. Value; preciousness: not used. 2. Any thing of high price: not used (More). PRECIOUS. a. (precieux, Fr. pretiosus, Lat.) 1. Valuable; being of great worth (Addison). 2. Costly; of great price (Milton). PRECIOUSLY. ad. (from precious.) Valuably; to a great price. PRECIOUSNESS. s. (from precious.) Valuableness; worth; price (Wilkins). PRECIPICE. s. (præcipitium, Latin.) A headlong steep; a fall perpendicular (Sanlys). "PRECIPITANCE. PRECIPITANCY. S. (from precipitant.) Rash haste; headlong hurry (Milton). PŘÈCIPITANT. a. (precipitans, Latin.) 1. Falling or rushing headlong (Phillips). 2. Hasty; urged with violent haste (Pope). 3. Rashly hurried (King Charles). PRECIPITANTLY. ad. (from precipitant.) In headlong haste; in a tumultuous hurry. To PRECIPITATE. v. a. (præcipito, Lat.) 1. To throw headlong (Wilkins). 2. To urge on violently (Dryden). 3. To hasten unexpectedly (Harvey). 4. To hurry blindly or rashly (Bacon). 5. To throw to the botcom. A terin of chemistry opposed to sublime Grew). To PRECIPITATE. v. n. 1. To fall headong (Shakspeare). 2. To fall to the bottom as a sediment (Bacon). 3. To hasten without just preparation (Bacon). PRECIPITATE. a. (from the verb.) 1. Steeply falling (Raleigh). 2. Headlong; hasty; rashly hasty (Clarend.). 3. Hasty; violent (Arbuthnot). PRECIPITATE, in chemistry, (heederschlag, Germ.) In a more limited and appropriate sense a substance thrown down from its solution in a menstruum by the application of a third or additional material, which detaches it, either by a greater affinity to itself or to the menstruum with which it has thus far been combined. (See the article PRECIPITATION.) It is necessary, however, for the name of a precipitate in this restricted sense, that the substance thus subsiding should fall in a flocculent or pulverulent form, since if the different particles unite in an angular polyhedral figure it will be a crystallization. The term, however, is often used in a more extended sense to impart any visible substance, simple or compound, separated from its menstruum whether it sink or swim, or whether it be a crystal or a powder; and in this view precipitation forms one of the great operations in chemistry, and instead of being opposed to the phænomenon of crystallization, is directly opposed to that of solution. It is chiefly from the process of precipitation, as Berthollet has shown in his valuable essays on chemical statics, that philosophers have been led to deduce the supposed order of affinities between different substances. The precipitates of most importance to attend to are those which occur in the case of acids, alkalies, or earths, and those in the case of metals and metallic compounds. We shall subjoin from Berthollet a few observations upon both these, connected with remarks offered by the Messrs. Aikin. Among the former there are two principal kinds of precipitates which have been distinguished in chemistry; one, where to a compound produced by an acid and a base, each highly soluble in water, another acid is added which has a great affinity with the base, and forms with it a compound less soluble than the first. In this case the precipitate (which is sometimes but not always crystallized) is composed of the acid last added, in proportions that do not essentially vary. As for example, when sulphuric acid is added to a saturated solution of nitrat of potash a crystallized precipitate of sulphat of potash is produced, the proportions of which scarcely yary in any circumstances, though the actual quantity of precipitate is determined by the quantity of water and of sulphuric acid present. The second kind of precipitate is that which has been usually supposed to be simple, and which takes place when to a compound of an acid and a base, insoluble or nearly so in water, an alkali or soluble earth is added, which by elective affinity unites with the acid and displaces the former base: and this latter now supposed to be uncombined, and insoluble in the quantity of liquid present, falls to the bot |