Place some dry crystals of nitrate of potash in a glass retort, provided with a proper receiver, and pour on the crystals some oil of vitriol, or sulphuric acid; apply a moderate heat, red fumes will soon appear in great abundance, which passing over and becoming condensed in the cold receiver, present pure liquid nitric acid; the sulphuric acid expelling it from its combination with potash, and therefore sulphate of potash remains in the retort. That this new result is the nitric acid, may be proved by causing it to act upon silver, copper, or iron; their respective nitrates will be as readily produced, as they were by employing the acid of commerce purchased at the chemist's.

After having ascertained this fact, dissolve some lead in diluted nitric acid, and thus make a solution of nitrate of lead, for an experiment which will be required immediately. Now examine the sulphate of potash in the retort; very likely it is a little acid from excess of sulphuric acid remaining in it; if so, neutralize this,-it may be done with potassium; but this is rather too expensive, therefore employ a solution of pure potash. The neutral sulphate of potash is very insoluble in water, and crystallizes in shorter prisms than nitre; but you can obtain a solution of it in about 16 parts of water; add this to the neutral solution of nitrate of lead, and note the result; both solutions were transparent and clear, but upon mixture, a copious mass of white solid matter appears, which, upon a short repose, precipitates, leaving the supernatant liquid clear and colourless. This is an example of double decomposition, the theory of which may be easily understood. Sulphate of potash consists of sulphuric acid and potash, or oxide of potassium, nitrate of lead consists of nitric acid and oxide of lead; but, when the two salts are mixed together, the sulphuric acid having a stronger affinity for the oxide of lead than the nitric acid has, combines with it, forming sulphate of lead, which being very insoluble, precipitates in the solid form the potash thus freed from union with sulphuric acid is instantly attracted by the nitric acid, which existed in the nitrate of lead; therefore a nitrate of potash is formed, which remains in solution, and its crystals may be obtained in the usual manner. So that in this experiment there is an interchange of acids between the oxides of the two metals; two soluble salts producing one that is insoluble, and another remaining in solution.

arts.

Such then are a few of the manifold results of Chemical Affinity, and a passing notice or so regarding their application to some of the The object of this paper is to put the student in possession of a general notion of chemical operations, without entering particularly into all their minutiæ; the application of the theory of definite proportions to some of these results, and the consideration of others in which chemical action takes place more intensely and suddenly, will form matter for future discussion.

DESCRIPTION OF A RATIONAL LUNARIUM.

WHAT vague and false notions of the planetary system common orreries, or planetaria, invariably convey to the learner, who receives his first ideas on the science of Astronomy by means of them, must strike every one who is curious enough to examine a beginner as to the progress he has made. The reason is palpable; those who recommend the use of these machines, as capable of facilitating the acquisition of ideas on what they regard as an abstruse subject, decide from the well-known Horatian maxim; but they do not consider, that unless the associations early excited by impressions from visible objects are perfectly consistent with truth, their vividness tends to render nugatory all attempts to correct those erroneous impressions by subsequent study. These advocates forget that the absurd misrepresentations of relative magnitudes and distances, which result from the attempt to explain a great number of celestial phenomena by one machine, make impressions on the mind of ordinary learners which are too powerful to be subsequently effaced by abstract numerical details, or by pure mathematical reasoning.

If the common Orrery were only had recourse to when the mind of the pupil had been habituated to comprehend very abstract ideas, and to control the impressions derived from his senses by the exercise of his judgment, there is no doubt that it might be advantageously made use of on some occasions; but this is not the case with the popular mode of teaching; the Orrery is shown to the learner before he has the slightest correct conceptions on the subjects,-probably before he has received even the most elementary instructions in plane geometry. Who, therefore, can be surprised if the false ideas imparted by the visible machine before him, cannot be counteracted by the teacher's exhortations not to pay attention to the magnitudes and distances of the representative planets.

This evil might perhaps be submitted to, if there were any counterbalancing advantages; but the ideas which ordinary orreries are intended to convey, are precisely those which there can be no difficulty in acquiring from verbal instructions, or by means of good diagrams. The general conception of bodies revolving in space round a central one, at different distances, and with different velocities, is too simple to present any. difficulty to the slowest comprehension, and a few concentric circles, at the correct proportional distances, drawn on paper, are quite as adequate to assist the understanding as the most elaborate planetarium, and do not convey any false impressions. The common anxiety of machinists to show their skill by ingenious combinations of wheelwork, induces them to aim at making complex machines, by which the planets are carried round in their orbits; but to effect this, they are obliged to violate still more flagrantly proportional distance, and even then without arriving at anything like the correct motions which require illustration. Added to these elementary motions, orreries are intended to explain the phenomena of night and day, of the seasons, the lunar phases, and eclipses, &c.; to effect all this, the falsifications we allude to are carried to a most ludicrous extent, till the machine becomes only a fertile source of every erroneous notion that can be conceived on the subject.

We must here mention that our objections are only against Orreries

or Planetaria, and do not apply in any way to Globes, which are as deserving of eulogium as the former are of ridicule. The astronomical or geographical phenomena which it is the object of a globe to elucidate, are really made more comprehensible by such an auxiliary; and, while affording this help to the learner, the globe actually rectifies the erroneous impressions previously received from his senses, it constantly reminds him that the inequalities on the earth's surface, which are so great in relation to him, and to the minute portion of that surface he can view at one time, are really insensible in relation to the whole mass. The mind conceives the true nature of the "vast unfathomable ocean" in reference to the earth, when it perceives, from a simple calculation, that the thickness of the paper, covering the artificial ball, is a tolerable representative of its real average depth; and how much are the wonders revealed by geology rendered intelligible, when the learner acknowledges that a grain of sand, stuck on his globe, is a correct model of Darwhal Ghiri, or Chimboraço, and a scratch with a pin exaggerates the deepest natural valley, or the slightest puncture the deepest mine which human labour has ever excavated. But the sublime ideas of creative power, which the law of gravitation must excite, when the mind rightly conceives the comparatively minute masses acting on each other at enormous distances, remain undeveloped in that which has imbibed its notions on the subject by means of a two-inch world, stuck on a brass wire at perhaps four inches' distance from a half-inch sun*.

These remarks have been suggested by a Lunarium sent to us by a friend, who, agreeing in our opinions on the worthlessness of common machines, has endeavoured, in that before us, to remove their defects, and to accomplish what they are perfectly incompetent to do. merits of this rational "toy" are, that it can be made by any one who has a little ingenuity, and that, with this simplicity, it effects, with accuracy, all it purposes; it is, in short, the contrivance of a mathematician and philosopher; and we think many of our readers will thank us for such a description of it as will enable them to make and to adjust it.

Every beginner should learn, by heart we might say, Sir John Herschel's receipt for an Orrery: we give it here with some alterations, for the sake of supporting our views on the subject by such an authority. Choose any large level field. In the middle, place a globe two feet in diameter; this will represent the Sun; place a grain of mustard-seed at 82 feet distance from the sun for Mercury; a pea, at the distance of 142 feet from the sun, will represent Venus. Our Earth will be another pea, at 215 feet from the sun; Mars will be a large pin's head, 327 feet off; four grains of sand, at distances of 5 to 600 feet, will represent the new planets. Jupiter will be a moderate-sized orange, a quarter of a mile from the sun. Saturn a smaller orange, two-fifths of a mile, or 1408 feet, from the sun; and, lastly, Uranus a full-sized cherry, or small plum,

three quarters of a mile from the sun. "As to getting correct notions on this subject by those very childish toys, called orreries, it is out of the question." (Sir J. Herschel's Astronomy; Lardner's Cyclopædia, p. 287.)

This is the smallest scale on which an orrery could be constructed to show the satellites and the smaller planets. The moon, in the above model, would be a common pin's head, six and a half inches from the earth-pea. When we conceive this model, and reflect that the two-feet globe keeps the plum in its orbit at three quarters of a mile distance, and that the two oranges act on the plum, and the peas, and on one another, and still more on the pin-head moon, in all possible positions, as they revolve round the globe, the mind begins to get a glimpse of the power of gravity.

The board which serves as a stand, represents a plane parallel to the ecliptic. The cylindrical block a has its upper surface cut obliquely, so that its plane may form an angle of 5° 9 with the ecliptic; that being the inclination of the moon's orbit to it. This oblique surface is covered with paper, and a graduated circle described on it, through which a diameter must be drawn, correctly parallel to the intersection of the two planes, to represent the line of the moon's nodes; the ends of this diameter must be the 0°, and 180° of the division on the circle. The block a turns round in, and concentric with, the ring B, on the surface of which a graduated circle is also drawn on paper: this lower piece в is made with a pivot to fit into a hole in the board, to admit of A and B being turned round together on their centres. The pillar c is screwed fast into A; it has an oblique shoulder to carry the wire axis of the earth at the proper inclination of 66° to the ecliptic; this shoulder turns on c to allow of the earth's axis being set to point to the proper sign of the Zodiac in the circle on B.

H is a light frame to carry the moon, м; the opening is contracted to the thickness of c at one end, by two ivory (or brass or wood) slips fixed on its under-side and flush with that face; when H, therefore, is turned round, resting on the oblique face of A, these slips prevent any lateral motion. At the other end of н a square plate of ivory (wood or brass) is screwed on the under-side also, the upper face of this plate, therefore, coincides with the plane of the surface of a: on this plate a graduated circle is described concentric with the moon's axis. The moon is carried on a double pedestal, the lower piece having a pivot to turn in the central hole of the plate, the under-face of the pedestal is cut bevelled to form an angle of 5° 9' with its axis, so that by turning the pedestal round, the axis of the moon may be made to stand perpendicular to the ecliptic in every position of н, instead of being perpendicular to that frame, as it would be without this contrivance.

The length of the frame, H, being decided on at pleasure, (the one before us is ten inches, but the larger the better,) divide the length of

the opening, from the centre of the circle on the plate, into thirty-two parts, (thirty being the moon's mean distance in earth's diameters.) Then, making the centre of the circle zero, mark the 30th division on the ivory slips, and subdivide a division or two on each side into tenths, to serve as a scale for setting the moon at her true distance from the earth. The ball E, to represent the earth, must be made accurately equal to one division in diameter, or one thirtieth of the mean distance, and it must have an equatorial line drawn on it: the moon, м, is to be made 36 of the earth's diameter, and may be measured from the scale on the slips. In the instrument we are describing, the moon is made in one piece with the upper part of its stand, which turns round on the lower pedestal, before mentioned, to allow of the unblackened half of the surface being turned in the direction of the sun, as indicated by the line BK, without altering the adjustment of the pedestal. But if a larger scale be adopted, the moon may be made to slide on a fine needle stuck perpendicularly into the pedestal; in either case there must be contrived means by which the moon may be set correctly at the same height above the surface of the plate, when the moon is in her node, that the earth is raised above the centre of the surface of a; or in short, that the line joining the centres of the two balls may, in that position, be parallel to the board, and consequently represent one in the true ecliptic.

The dial, D, is intended to supply the place of meridians on the earth, and should be drawn on paper with care, for the latitude of the place, and being then mounted on a conical piece of wood, cut to have its flat surface truly to represent the horizon of that place: a hemispherical hole must be cut out, so that when the earth is sunk in it the radiating lines of the dial may tend to its centre*.

The pillar, K, is to indicate the direction in which the sun is, a piece of card with a hole to represent the apparent diameter of the sun is stuck upright on its top, the centre of this hole must be made exactly as high above the board as the centre of the earth. The diameter of the hole is of course to be made the chord of the angle, subtended by the sun from the earth, the arc being described with the radius equal to the distance of K from E.

There must be a counterpoise put on the end of the frame, н, to keep its under-side close down on the top of a, and to steady it while it is moved.

To adjust the Lunarium. Turn B round on its axis till the line AK of the sun's direction is opposite the degree on the graduated circle, corresponding to the sun's longitude for the time. Then turn A round in B, to bring the moon's ascending node into its proper degree on the same circle. Turn the earth's axis round on c till it lies in the plane of the solstices, perpendicular to the ecliptic. Raise or lower the dial and earth till the centre is perpendicularly over that of the pillar c, or over the centre of the ring B: set the dial in the meridian, as indicated by the earth's axis. Turn н round till the line of the nodes on a coincides with

* This dial is, of course, not essential, tion, even in the latter case: the nicety and may be dispensed with if the earth be required to construct it properly is a good large enough to admit of meridians being exercise, and it materially adds to the drawn on it, but we would advise its adop-merits of the Lunarium.