facets, while the other side is plain. 8. It may have considerable length in a triangular form. No. 1 is called a plane glass or lens, as its sides are parallel; No. 2, a plano-convex lens; No. 3, a double convex; No. 4, a plano-concave; No. 5, a double concave; No. 6, a meniscus; No. 7, a multiplying glass; and No. 8, a prism. The term lens is usually given to such glasses or substances only as either magnify or diminish. Nos. 2, 3, 4, and 5, are therefore lenses; No. 6 is also a lens when its surfaces are portions of different spheres; but when they are of equal radii, or parallel, it has only the effect of a plane glass. A ray entering the plane glass, No. 1, will be refracted; but it will undergo another refraction on its emergence, which will rectify the former; the place of the object will, therefore, be a little altered, but the figure will remain the same. Suppose A B, Fig. 1, to represent a solid piece of glass with two parallel surfaces, an incident ray E F will be refracted into F G, and F G will be refracted on passing from the second surface into G H, parallel to the original direction EF. If parallel rays enter the plano-convex glass, as shown by Fig. 2, the ray E will be refracted upwards to F, and the ray K will be refracted glass, KG, they will be refracted still more From all luminous objects, the rays of light proceed in a state of divergence; but when the distance from which they come is very great, F G the quantity of divergence is too small to require notice. L R The fixed stars and the sun, for example, are so immensely distant, that their rays are always considered as parallel; and it is only parallel rays which are converged to a focus in the manner described. Divergent rays proceeding from a point, as the flame of a candle, will be differently affected. If, therefore, a candle be placed exactly at the focal distance of a single or double convex lens, the rays will emerge parallel to each other. If the candle be placed nearer to the glass than its focal distance, the rays, after passing through the glass, will no longer be parallel, but separate or diverge. If the candle be placed still further off, the rays will then strike the glass more nearly parallel, and will, therefore, upon passing through, converge or unite at a distance behind the glass, more nearly approaching the distance at which parallel rays would be converged. After the rays have united in a focus, they will cross each other, and form an inverted picture of the flame of a candle, which may be received on a piece of paper placed at the meeting of the rays behind. The cause of the inversion of the image is evident, the upper rays being those which come from the under part of the luminous body; and the under rays, on the contrary, coming from the upper part. D a 72 In looking through a plano-convex or double convex lens, the object appears magnified agreeably to the rule, that we see every thing in the direction of the lines in which the rays last approach the eye; consequently, the larger the angle under which an object is seen, the larger that object will appear. From lenses the reverse in form to those we have noticed, we naturally expect opposite effects; accordingly, the attractive and refractive powers of a plano-concave and double-concave lens are not towards the centre, but towards the circumference. Parallel rays falling upon these lenses diverge, or are dispersed Rays already divergent are rendered more so, and convergent rays are made less convergent; hence objects seen through one of these glasses appear smaller than to the naked eye. Let a b, in the subjoined figure, represent an arrow, which would be seen by the eye, if no lens were before it, by the convergent rays a cbi; but if the double-concave glass DH be interposed between the object and the eye, the ray a c will be bent towards g, and the ray bi will be bent towards h, and consequently both will be useless, as they do not enter the eye. The object, then, will be seen by the rays a obr, which, on enter ing the glass, will be refracted into the lines oc and ri; and, according to the rule laid down, the object will be seen in the last direction of these rays; therefore, as the angle ocr is so much smaller than the angle a cb, the arrow necessarily appears diminished; and as, with the diminution of its apparent size, we connect the idea of its being further off, it seems to be at the dis tance n m. h I ь The miniscus acts like a convex lens when it is thickest in the middle, that is, when its convex surface is a portion of a less sphere than its concave one; on the contrary, when it is thinnest in the middle, or has its concave surface a portion of a less sphere than the other, it has the effect of a concave lens. The axis of a lens, is a line supposed to be drawn through the centre of its spherical surfaces. When one side of the lens is plane, the axis is perpendicular to that side. The axis of a lens continued, would pass exactly through the centre of that sphere, of which the lens is the segment. The focus of a plano-convex lens is at a distance from the convex surface equal to the diameter of the sphere of which it is a part; and that of a double and equally convex lens is at half the same distance. The distance of the focus of a solid globe or ball of glass is one quarter of its diameter from the nearest part of the ball. To explain the effect of the multiplying glass, (No. 7,) it will only be necessary to revert to the principle, that objects appear in the direction of the line last described by the rays that render them visible; hence, if the object B, (p. 213.) is seen through the glass E H by the ray B A that passes through the surface F G, the object, by the eye at A, will be seen at B; the ray BG passes through the surface G H, and after refraction comes to the eye in the direction of A D, as it proceeded from D, and therefore the object appears at D; and for the same reason, through the surface F E, it appears at C; consequently, there will be the appearance of as many objects as there are flat surfaces on the glass, for each of them shows the same object in a different place. The disposition of the rays of light to be turned back into the medium from whence they came, is called their reflexibility; the change of direction produced by their being actually turned back, is called reflection. All objects which are not themselves luminous are rendered visible by reflection; and glass, crystal, water, and the most pellucid media, reflect a portion of the rays of light which fall on them, or their forms and substance could not be distinguished. On the other hand, the whole of the incident light is not reflected from any surface, however bright, smooth, and opaque. It is calculated that the best mirrors reflect little more than half the light they receive; the part lost consists of two portions, one of which, and by far the largest, being absorbed by the mirror, and the other, scattered by irregular reflection. Light is always lost, in passing through the most transparent bodies, by the same laws. The different refrangibility of the rays of light is demonstrated by the prism. If a beam of light from the sun be let into a darkened room, and be received upon a white screen or opposite wall, it will form a circular image, and will be of one uniform whiteness. If a prism be interposed, so that the light must pass through it before it reaches the wall, the image is no longer circular or white; it assumes an oblong shape, terminated by semicircular arches, and exhibits seven different colours. This oblong image is called the spectrum. In the whole range of philosophical experiment, a more beautiful appearance cannot be presented to the eye, and instructive nature will appear not less extraordinary than its beauty, when it is considered, that the investigation of the cause of it led Sir Isaac Newton to form the first rational theory of the cause of colours. The seven colours of the spectrum are called the original, or primary colours. If a spectrum be divided into 100 parts, the red part of it is found to occupy 11 of these parts; the orange 8, the yellow 14, the green 17, the blue 17, the indigo 11, and the violet 22. The red part of the spectrum is nearest the prism; and the violet, at the greatest distance. It is clear, from this, that light is not homogenous, because the attractive power of the prism is greater upon some parts of it than upon other parts. Accordingly, it is generally concluded that the solar beam or white light is composed of particles differing in size and density; that this difference of their size and density is the cause of their being differently refrangible; and that the separation of the rays of one or more sizes from the rest, by various means, produces all the diversity of colours which affect our sight. It is found, that the red part of light is capable of struggling through thick and resisting mediums, when all the other colours are stopped. Thus, the sun appears red when seen through a fog. The particles which compose orange light are next, in size and refrangibility, to the red; and so on to the violet, which consists of the smallest particles, and which are, therefore, the most turned out of their course. White is composed of all the primary colours, mixed together in their due proportions. When bodies reflect the rays of light in the proportion in which they exist in the solar beam, they appear white; when they reflect none of the rays, they appear black. Convex lenses in their simple state have been applied to collect the heat of the sun's rays, for purposes similar to those of burning mirrors. A burning lens must be convex; a burning mirror, concave; because both produce their effect by concentrating into a focus the rays of light and heat incident upon a large surface. As the rays which pass through a convex lens, or are reffected from a concave mirror, are united at its focus, their effect is so much the greater, as the surface of the lens or mirror exceeds that of the focus. Thus, if a lens four inches in diameter collect the sun's rays into a focus at the distance of one foot, the focal image will not be more than one-tenth of an inch broad. The surface of this circle is 1600 times less than the surface of the lens, consequently the density of the sun's rays within it is proportionately increased. It has been found, that large lenses and mirrors burn with irresistible intensity when properly constructed, dispersing the hardest metals and other substances into gas, often in a few seconds. See BURNING GLASS, and the various other optical instruments, under their respective names. ORES. The natural bodies whence metals are extracted. Metallic substances, when found pure, are called native; but when combined with other substances, as they generally are, they are denominated ores. As it is of the utmost importance to be acquainted with the materials of which ores are composed, as well as the simplest and easiest processes by which they may be separated from each other, we deem it necessary to give the following brief account of the modes of reduction usually adopted. Ores of Gold.-Gold exists in nature only in the metallic state; but it is scarcely ever found perfectly pure, for it is alloyed in different proportions with silver, copper, tellurium, and some other metals. When it is alloyed with silver or copper, or even with both, the gold retains its ductility; but when combined with tellurium, its distinctive characters entirely disappear. The presence of gold may easily be detected by treating the mineral supposed to contain it with nitro-muriatic acid, and dropping muriate of tin into the solution. If the solution contains any gold, a purple precipitate immediately appears. Native gold ought to be dissolved in nitro-muriatic acid; the silver, if any is present, falls to the bottom in the state of muriate, and may be separated by filtration, and weighed. Pour sulphate of iron into the solution, and the gold is precipitated in the metallic state. The copper, if any is present, may be precipitated by means of a plate of iron; the presence of iron may be ascertained by dropping tincture of nutgalls into a portion of the solution. The auriferous pyrites may be treated with diluted nitrous acid, which dissolves the iron, and separates the sulphur; the gold remains insoluble, and is found in the state of small grains. In the Hungarian gold mines, which are the richest yet known in the old continent, the attention of the miner is not merely limited to the strings of ore, but to the whole contents of the vein, which are usually extracted, and raised to the surface in large masses. These masses are distributed to the workmen, who break them down, first with large hammers, and afterwards with smaller ones, till they are reduced to pieces of the size of a walnut; the native gold, with the matrix attached to it, is again to be broken by hand into still smaller pieces, by which means other impurities and stony matters are separated. The ore is then introduced into a wooden box, floored with cast-iron plates, and, by the action of two or more heavy spars of oak, which are shod with iron, and alternately worked like the common stamping mill, it is reduced to a fine powder; this powder, which is called flour, is then removed into a vessel like a large basin, and mixed with such a quantity of salt and water as will render it damp; the workman then takes a thin, porous leather bag, introduces a quantity of mercury into it, and, by regular and continued pressure, forces the mercury, in very minute drops, through the leather. In this divided state it falls upon the pulverized ore, and is immediately kneaded up with it, till the requisite quantity, which depends on the proportion of gold, has been added. After completing this part of the process, the next object is to incorporate the mercury and gold: this is effected by rubbing the mixture together for some time by means of a wooden pestle; the mixture is then heated in a proper vessel, and subjected, for three or four days, to the temperature of boiling water; and, lastly, the mixture is to be carefully washed by small parcels at a time, so that the earthy particles may be carried off by the water; the mercury, combined with the gold, only remains behind in the form of amalgam. A portion of this mercury is then separated, by pressure, in a leathern bag, and the remainder is driven off by distillation, leaving behind the gold and silver, with which it may be alloyed. When this metal is found in other ores, they are first roasted, to disperse the volatile principles, and to oxidize the other metals. The gold, which is but little subject to oxidation, is extracted by amalgamation or by cupellation, or either methods, adapted to each ore, according to its properties or constituent parts. The metal obtained in these ways is always more or less alloyed, particularly with silver and copper. The first step in its purification is the process of cupellation. (See CUPELLATION.) The gold, after it has been submitted to this process, is often alloyed with silver, which, being nearly as difficult of oxidation, is not removed by the action of lead; and hence the necessity of the operation denominated parting, for which see PARTING. Ores of Platinum.-The whole of the platinum which has been brought to Europe, has been previously subjected to the process of amalgamation in South America; and hence it happens that a.small quantity of mercury remains in it. In treating the ores, therefore, the first object is to separate the mercury by means of heat, either in an open ladle, or in an earthy retort; the platinum remaining after the mercury is thus driven off, appears much yellower, because the particles of gold dispersed through it exhibit their peculiar colour. Proust's method of analysis is, first, to separate the sand with which the grains of platinum are mixed, by exposing them to a blast of air. By heat he evaporates the mercury, which still adheres to them, and then picks out the grains of gold, which are always mixed with platinum, and which are thus rendered visible. The ore is then dissolved in an acid composed of one part of nitre, and three parts of muriatic acid; a black powder remains: this powder, when roasted, gives out phosphorus and sulphur. After the separation of the gold, nitromuriatic acid being poured on the remaining mass will dissolve it, with the exception of a small quantity of black matter, which was formerly mistaken for plumbago, but is now proved to be a compound of osmium and iridium, two of the four new metallic bodies, which were discovered a few years ago by Mr. Tennant. These two metals Dr. Wollaston has since shown to exist also in the crude platinum ore, united together in the form of distinct minute crystals, and dispersed through the other grains, from which they can be distinguished and picked out without difficulty. Muriate of ammonia being now added to the solution, the platinum is precipitated in the form of a yellowish powder, which is a compound of muriatic acid, ammonia, and platinum: the remaining solution, after the platinum has been separated from it, still contains, besides iron, minute quantities of various other substances, amongst which the two other metallic bodies, palladium and rhodium, were discovered by Dr. Wollaston. Having now brought the platinum to the state of salt, the next object is to restore it, thus purified, to its metallic state, and to consolidate it into a malleable mass; this, from the great infusibility of platinum, has long been a matter of considerable difficulty and labour. It had been long discovered that arsenic readily united with platinum, and formed with it an alloy of great fusibility; an alloy, therefore, was made of crude platinum and arsenic; and the latter metal, being easily volatilized, was driven off by heat, whilst the iron, being oxidated during the process, was also separated from the mass; so that the platinum was left in an impure but malleable state. Ores of Silver.-The analysis and reduction of these different ores, it is scarcely necessary to observe, must be conducted according to the nature and proportion of the ingredients which enter into the composition of the ore to be examined or reduced. Pure native silver requires no other assay than fusion, with a little potash to free it from its earthy matter. In the humid way, the silver may be dissolved in nitric acid, and precipitated by common salt; the precipitate may then be fused with soda in a crucible, by which the silver is obtained pure, and the muriate of soda sublimed. The auriferous silver ores may be treated with potash, by fusion in a crucible; the alloy of silver and gold is first obtained, and the two metals may be separated by the process of parting. Those ores which consist of silver combined with antimony, or arsenic, or both, |