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copper. The instant that this metal comes into contact with the solution a remarkable result takes place; the silver begins to reappear in a crystalline metallic state, and the solution assumes a bright blue colour; let the experiment continue until silver ceases to deposit or precipitate, then brush it all away from the copper, allow it to subside, pour off the blue solution, add a little water to the residuum, pour this off after further subsidence, into the first portions, and thus wash the silver until the water is no longer blue; then collect, dry, and weigh the silver; it will present the same weight as that of the pieces originally dissolved*. This is a process of great importance in the art of assaying and metallurgy, and is constantly practised by the workers in silver, to recover the precious metal from its solutions, and, as it appears in a solid state from a solution, they call it " water silver.” The theory of the operation is simply this, the metal copper has a stronger attraction or affinity for nitric acid than the silver has, and, therefore, when put into the solution of the nitrate, it robs the silver of the nitric acid, and compels it to precipitate in its metallic state; the copper taking its place in the solution to form nitrate
may be proved by evaporating the blue solution and washings nearly to dryness, then cooling it; for the production of crystals, which will appear in the full enjoyment of all the characters of deliquescence, solubility in alcohol, &c., which were found to belong to the nitrate produced by the direct union of nitric acid with copper. Take either solution of nitrate of copper, diluted with water in a glass, as before directed, and place in it a plate of silver; no action will ensue, because the nitric acid has the strongest attraction for the copper, but remove the silver, and substitute a plate of clean iron, a very different matter ensues: the instant that it touches the blue solution it becomes covered with a brilliant coating of metallic copper; upon continuing the experiment, the solution assumes a green colour; and ultimately no more copper is deposited; then decant the solution, separate, wash, and dry the copper, as before directed with the precipitated silver, and, if the nitrate employed was that produced by the direct union of the acid and copper, the weight of the precipitated metal will be that of what was originally dissolved. The theory of this process is analogous to that of the former one; the iron, in this instance, has a stronger affinity for the nitric acid than the copper has, therefore robs it of the acid, compels the copper to precipitate, and takes its place in the solution, forming nitrate of iron.
This experiment is not without its practical applications in the arts, for recovering copper from its solutions in various acids; the water which is found in copper-mines very frequently contains a large proportion of soluble salts of copper; this was formerly allowed to run waste. The story goes, that a miner accidentally left his iron pick in the water, and, next day, upon resuming his work with it, he found that it was converted into copper. This led to an examination of the cupreous waters, and chemistry soon divested the matter of the marvellous transmutation, by pointing out that it was a case referrible to affinity, and still further
The most convenient way of collecting | if it is poured on a filter much of the silver the silver is to get it into a small saucer, adheres to the paper, and cannot be readily and let the water slowly evaporate from it; detached.
proving that it might be employed to advantage. Accordingly, quantities of refuse iron are now thrown into such waters, and allowed to remain until they precipitate the copper, which, being collected and melted, furnishes a notable proportion of the metal of commerce.
The green solution of nitrate of iron resulting from this last experiment, if evaporated with the view of obtaining crystals, turns of a deepbrown colour, and refuses to show any tendency to crystallize; it is an uncrystallizable deliquescent salt; nor can we, by placing any other metals in its solution, cause the iron to precipitate in the metallic state: it has got the nitric acid, and holds it in defiance of the attacks of other metals; its affinity for the acid is superior to any of them. There is no direct method of obtaining the iron from the nitrate.
All the foregoing experiments were invented by the alchymists, and described by them under very fanciful names. They were also frequently adduced as instances of the real transmutation of copper into silver, iron into copper, &c., and, in the early dawn of science, such a conclusion was by no means absurd or preposterous. “He who first saw the corrosion of a metal by a limpid liquid,—who beheld the opaque and ponderous body gradually disappear, and become part of a transparent and apparently homogeneous fluid, and who saw the same metal reappear upon the addition of a proper precipitant, must have been infinitely surprised, and struck with admiration of the occult powers of nature *.”
If we subject some of the scarcer metals to the action of nitric acid, the results are very remarkable; potassium may be selected as an instance. Drop a globule of it, about the size of a large pea, into a small cup nearly full of water, containing a drop or two of strong nitric acid; the moment that the metal touches the liquid, it floats upon its surface, enveloped with a beautiful rose-coloured flame, and entirely dissolves. This is an example of intense Chemical Attraction, existing between the two substances, and the usual attendants upon it are the evolution of light and heat. Try the solution with a piece of turmericpaper ; if its colour is changed brown, a drop or two more acid must be cautiously applied,-if, on the contrary, it reddens litmus-paper, a small globule or two of potassium is required; the object being to obtain a neutral solution : this is easily effected, and now if it be carefully evaporated to about half its bulk, and set to crystallize, beautiful crystals will begin to form, which are those of the nitrate of potash; commonly called nitre, or saltpetre.
The theory of the experiment is simple, yet highly instructive; water (as will be hereafter fully proved) consists of oxygen and hydrogen. Potassium has an intense attraction for oxygen, it therefore snatches it energetically from combination with hydrogen, with such rapidity and force, as to evolve heat sufficient to cause the hydrogen to burst into flame; the oxide of potassium (potash,) thus produced, is instantly attracted by the nitric acid in the water, forming solution of nitrate of potash, which readily yields crystals of a six-sided form t.
become oxidized at the expense of part of
its oxygen, and the oxides thus produced, + In all the foregoing experiments upon combine with other portions of the acid tó the solution of metals in nitric acid, they form nitrates.
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.
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 arts. 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. The 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 three quarters of a mile from the sun. we might say, Sir John Herschel's receipt “As to getting correct notions on this for an Orrery: we give it here with some subject by those very childish toys, called alterations, for the sake of supporting our orreries, it is out of the question.” (Sir views on the subject by such an authority. J. Herschel's Astronomy; Lardner's Cy
Choose any large level field. In the clopædia, p. 287.) middle, place a globe two feet in diame- This is the smallest scale on which an ter; this will represent the Sun; place a orrery could be constructed to show the grain of mustard-seed at 82 feet distance satellites and the smaller planets. The from the sun for Mercury; a pea, at the moon, in the above model, would be a distance of 142 feet from the sun, will re- common pin's head, six and a half inches present Venus. Our Earth will be an- from the earth-pea. When we conceive other pea, at 215 feet from the sun; Mars this model, and reflect that the two-feet will be a large pin's head, 327 feet off; globe keeps the plum in its orbit at three four grains of sand, at distances of 5 to 600 quarters of a mile distance, and that the feet, will represent the new planets. Ju- two oranges act on the plum, and the piter will be a moderate-sized orange, a peas, and on one another, and still more quarter of a mile from the sun. Saturn on the pin-head moon, in all possible posia smaller orange, two-fifths of a mile, or tions, as they revolve round the globe, the 1408 feet, from the sun; and, lastly, Ura- mind begins to get a glimpse of the power nus a full-sized cherry, or small plum, of gravity.