ON PHYSICS, OR NATURAL PHILOSOPHY. No. XXXII. (Continued from page 68.) CALORIC. ABSORPTION OF CALORIC, Absorbing Power.-The absorbing power of bodies is that property by which a greater or less quantity of radiant heat is permitted to penetrate their mass. This absorbing power is always in the inverse ratio of their reflecting power; that is, the more that a body reflects radiant caloric the less it absorbs it, and conversely. But the absorbing and reflecting powers are not complementary to each other; that is, the sum of the quantities of the heat reflected and of the heat absorbed do not represent the whole of the radiant heat which falls upon a body. It is always less than this; which shows that the incident heat is really divided into three parts: 1st, that which is absorbed; 2nd, that which is regularly reflected, according to the laws already demonstrated; 3rd, that which is irregularly reflected—that is, in all directions, and which is called diffused heat. In order to determine the absorbing power of bodies, Leslie employed the apparatus already described in the investigation of their reflecting power (see fig. 169, p. 68): but he removed the plate A and placed the bulb of the thermoscope in the real focus of the mirror. This bulb being successively covered with lamp-black, varnish, gold-leaf, silver-leaf, copper-leaf, etc., the thermoscope under the influence of the source of heat м, indicated a temperature which was higher in proportion as the substance which covered the bulb in the focus absorbed a greater quantity of caloric. In this manner, Leslie proved that the absorbing power of a body increased as its reflecting power diminished. Yet, in these experiments, the ratio of the absorbing powers of different bodies cannot be inferred from that of the temperatures indicated by the thermoscope; for here Newton's law is not strictly applicable, this law being only true in the case of bodies whose substance does not change; whilst that which covers the bulb in the focus varies at every observation. The ratios of the absorbing powers are, however, deducible from that of the emissive powers. Emissive Power.-The emissive power of a body is the property by which it emits, at the same temperature and from the same amount of surface, a greater or less quantity of heat. The same apparatus, represented in fig. 168, p. 68, was still employed by Leslie in determining the emissive power of bodies. For this purpose, however, the bulb of the thermoscope was also placed in the focus of the mirror, and the faces of the cube м were formed of different metals, or covered with different substances, as lamp-black, paper, water, etc. The cube being filled with boiling water, and all the other conditions remaining the same, Leslie successively turned each face of the cube towards the reflector, and marked the temperatures indicated by the thermoscope. Now, in the experiment where the face of the cube was covered with lamp-black, the temperature in the focus of the mirror rose higher than in all the other experiments; and the metallic faces produced the lowest temperatures. By applying the law of Newton, and representing the heat emitted by lamp-black at 100, Leslie derived from his experiments the following table of the emissive powers of bodies: Bodies. Lamp-black Water Paper Sealing-wax Crown Glass China Ink Isinglass Dull Lead Mercury Polished Lead Polished Iron E ... ... ... ... Tin, Gold, Silver, Copper, etc. VOL. V. Here it will be observed that the order of the bodies in this table is the reverse of that in the table of reflecting powers. MM. De la Provostaye and Desains, who have recently made researches relating to the emissive powers of bodies, have obtained numbers very considerably different from the preceding. Identity of Absorbing and Emissive Powers.-The absorbing powers of bodies cannot be deduced from their reflecting powers, because, as we have seen, they are not exactly combe determined if we could prove that they are equal, in each plementary to each other. But the absorbing powers would body, to the emissive powers. MM. Dulong and Petit have vessel, which was kept at the freezing point by immersion in inferred this from the following experiment. In a large glass ice, and which was blackened in the interior, they fixed a thermometer heated at first to a certain temperature-say 15o Centigrade; then, having made a vacuum in the vessel by means of a tube in it, which formed a communication between it and an air-pump, they allowed the thermometer to cool by degrees, and they marked the time which it took to fall from 100 to 5°. Repeating the experiment in an inverted order, that is, keeping the sides of the glass vessel at 15° Centigrade, and cooling the thermometer to the freezing point, they observed the time which the thermometer took to rise from 50 t the same as that which it took to fall from 10° to 5o Centi10° Centigrade; and they found that this time was exactly grade; therefore, they concluded from this, that in the same and that of the surrounding medium, the emissive power is body, and for the same difference between its temperature equal to the absorbing power, since the quantity of heat emitted and absorbed in the same time is equal. Modifying Causes.-The emissive and absorbing powers being equal, every cause which modifies the one necessarily As to the reflecting modifies the other in a similar manner. power, since it acts inversely to the other two, every cause which increases them diminishes it, and conversely. We have seen that these different powers vary in different substances; that the metals have the greatest reflecting power, and lamp-black the least. But that in the same body these powers are also modified by the degree of polish, by the denof the incident rays, and lastly by the nature of the source of sity, by the thickness of the radiant substance, by the obliquity heat. It was formerly supposed that the reflecting power gene that the other powers, on the contrary, diminished. But rally increased with the degree of polish in the surface, and M. Melloni has proved, that if a polished metallic plate be roughened by scratching lines across its surface, sometimes its reflecting power is diminished and sometimes it is increased, a phenomenon which he explains by the greater or less density of the reflecting metallic plate. If the plate has first been hardened, its homogeneity has been destroyed by the process of hardening; its particles are closer together at the surface than in the interior of the mass, and the reflecting power is increased. But when lines are scratched across its surface, the interior, which is less dense, is exposed to view, and the reflecting power is diminished. On the contrary, if the plate has not been hardened, and is homogeneous throughout, the reflecting power is increased by the process of drawing lines across its surface with a sharp instrument; and this arises from an increase of density at the surface occasioned by the pressure of the tool employed in drawing the lines. The thickness of the radiant substances may also modify their emissive power, as proved by the experiments of Leslie, Rumford, and Melloni. The latter philosopher found that by varnishing the faces of a metallic cube filled with water at a constant temperature, the emissive power increased with the number of the coats of varnish, until it reached sixteen coats, and that beyond this number this power remained the same, whatever was the number of additional coats. He found by calculation that the thickness of the sixteen coats was about the one 6350 th part of an inch. As to the metals, gold-leaf varying in thickness from the two hundred thousandth part of an inch to the fifty thousandth part of an inch, having been successively applied to the faces of the cube, the diminution of the radiant caloric was the same. Whence it appears that in the metals the thickness of the coat has no influence on its radiating power. 110 M. Melloni has also found that the absorbing power varies with the nature of the source of heat. For example, for the same quantity of incident heat, the carbonate of lead absorbs nearly twice as much of it, when it is emitted from a cube full of boiling water, as when it is emitted from a lamp. Lampblack is the only substance which always absorbs the same quantity of heat whatever may be the nature of its source. The absorbing power varies with the inclination of the incident rays. It is at its maximum at the normal incidence, and it diminishes in proportion as the incident rays depart from the normal position. This is one of the reasons why the sun heats more in summer than in winter; for in summer the solar rays fall less obliquely on the earth's surface. Applications of Radiant Heat.-The properties of the different powers of radiant heat, reflecting, absorbing, and emissive, have numerous applications in domestic economy and the arts. For instance, in selecting raiment for winter or for summer, preference should be given to that which is white; because the emissive power of white garments is less than that of black; consequently they are more opposed, during winter, to the loss of the heat of the human body. Again, in summer, in consequence of their weak absorbing power, they absorb less of the heat of the atmosphere than those that are black; and it is for this reason that they appear to be more cool. For the same reasons, Nature has given to the animals which inhabit the polar regions a covering of white hair, especially during the winter. In vessels employed for heating liquids, such as coffee-pots, it is more advantageous for this purpose that their surface should be black and unpolished, because then their absorbing power is the greatest. The shining appearance which we are accustomed to give them is obtained at the expense of fuel. If, on the contrary, we wish to preserve a liquid warm as long as possible, we must put it in a metal vessel which is polished and clear, because the emissive power is then least, and the cooling more slow. In the Alps, the mountaineers accelerate the melting of the snow by covering it with earth, which increases the absorbing power. In our houses, the exterior coatings of stoves and of heating apparatus should be black, in order to give free emission to the caloric; on the contrary, the interior of our chimneys should be lined with porcelain plates or Dutch ware, white and glazed, in order to increase the reflecting power of the fire, and heat the rooms more effectively. TRANSMISSION OF RADIANT CALORIC. Diathermous Power.-There are some bodies which allow radiant heat to pass through them, just as diaphanous bodies admit of the passage of light through them; other bodies have not this property, or possess it only in a slight degree. M. Melloni has given to the former the name of diathermous bodies, and to the latter the name of athermous bodies. The gases are the most diathermous bodies; the metals are entirely athermous. Notwithstanding the analogy which exists between radiant caloric and light, it should be observed in relation to our subsequent inquiries, that transparent bodies are not always the most diathermous, and that opaque bodies are by no means always athermous. 1st. The nature of the substance of which the screens (the diathermous bodies) are formed, through which the caloric passes. 2nd. The degree of polish given to these screens. 3rd. The thickness of the screens. 4th. The number of the screens through which the calorie passes. 5th. The nature of the screens already passed. on different liquids placed successively in a glass trough, From the preceding tabulated results, we conclude that some substances, more or less impervious to light, as the topaz smoked, is tolerably pervious to heat; while some substances little pervious to heat, as alabaster and alum, are very diaphanous. These experiments would lead also to the conclusion, that there is no point of relationship between the diathermous and the diaphanous powers of bodies. Effect of Polish.-The diathermous power of a screen increases with its degree of polish. Thus, M. Melloni found that the indications of his apparatus varied from 12° to 5° Centigrade, by interposing screens of glass of the same structure and thickness, but different in the nature of the polish-that is, in being more or less ground. Effect of Thickness.—The quantity of heat which passes M. Prevost, at Geneva, and M. Delaroche, in France, in through a diathermous screen decreases as the thickness in1811 and 1812, discovered many of the phenomena which creases, but the absorption is not proportional to the thickdiathermous bodies exhibit; but it was not till 1832 that M.ness. In general, the absorption takes place in the first layers Melloni, by means of an ingenious thermometric apparatus to or coats of the thickness. Beyond a certain thickness the be hereafter described, gave out a complete theory of the dia- quantity of heat transmitted tends to remain a constant quanthermous properties of solids and liquids. In his experiments, tity, even when the thickness continues to increase. M. Melthis philosopher employed five sources of heat, viz. :-1st, a Lo-loni has proved this fact by experimenting on plates of crown catelli lamp, that is, one without a glass, with a reflector, and glass whose thicknesses were as the numbers 1, 2, 3, and 4; with a single current of air; 2nd, an Argand lamp, that is, and he found that out of 1,000 rays of incident heat, these one with a glass, and a double current of air-such are the plates permitted the following rays to pass respectively; viz., Carcel lamps; 3rd, a spiral platinum wire kept at a red-heat 619, 576, 558, and 549; and the differences of these numbers in the flame of a spirit-lamp; 4th, a small hollow copper cube, tend to become zero. blackened on the exterior, and filled with water kept at the boiling point; 5th, a copper plate blackened and heated to about 400° Centigrade by the flame of a spirit-lamp. Thus, by changing successively the diathermous plates and the sources of heat, M. Melloni has proved the facts about to be explained. Effect of the Number of Screens.-The increase of the number of the screens through which caloric passes, produces an effect analogous to the increase of the thickness; that is, the absorption increases less rapidly than the number of the screens; or, in other words, the quantity of heat absorbed decreases from one screen to the following one. Moreover, if several plates of the same kind are placed together, they stop more heat than a single plate of a thickness equal to the sum of their thicknesses; and the effect produced by plates of different substances placed together is independent of the order in which they are arranged. Effect of the Screens employed.-The calorific rays which pass through one or more diathermous substances undergo a modification which renders them more or less proper for transmission through other diathermous substances. Thus, by comparing the results obtained by means of an Argand lamp, whose flame is surrounded with glass, with those obtained by means of a Locatelli lamp, whose flame is not so surrounded, M. Melloni found that out of 100 incident rays, the following were the numbers of rays, or the quantities of heat, respectively transmitted by the two lamps : From these experiments we conclude that the heat, which in the Argand lamp has already passed through the glass, is more easily transmitted through other substances. Rock-salt alone always permits the same quantity of the incident rays to pass through it. Effect of the Nature of the Source.The nature of the source of heat, generally speaking, considerably modifies the diathermous power of bodies, as shown by the results obtained by M. Melloni in employing four different kinds, as in the following table, the number of incident rays of heat being 100, as before: This table shows that the proportion of heat transmitted through solids, with the exception of rock-salt, diminishes with the temperature of the source of heat, and becomes zero when the source is at 100° Centigrade. Liquids exhibit the same phenomenon. Variety in the Calorific Rays.-The properties which heat presents in its passage through bodies, led M. Melloni to form concerning caloric, a hypothesis analogous to that which has long been held respecting light. Thus Newton showed that there were seven different kinds of rays of light, viz. red, orange, yellow, green, blue, indigo, and violet, which are unequally transmissible through diaphanous bodies, and which can either be combined or isolated; in like manner, M. Melloni has shown the existence of several kinds of calorific rays, which are emitted simultaneously, in variable proportions, from different sources of heat, and which are endowed with the property of passing more or less easily through diathermous substances. These substances possess, therefore, a real calorific coloration; that is, they absorb certain rays and allow others to pass, in the same way that a blue glass, for example, is traversed by the colour blue, and not by other colours. The theory of M. Melloni is very well explained by the system of undulations, by admitting that the properties of different kinds of heat are due to the different numbers of the vibrations, or to the calorific waves of unequal length. The properties of diathermous bodies have been employed to separate the light and heat which radiate together from the same source. Rock-salt blackened with smoke completely stops the rays of light, but allows those of caloric to pass through it. On the contrary, plates or solutions of alum stop the rays of heat and give passage to those of light. This latter process is usefully employed in apparatus illuminated by the rays of the sun or by the electric light, when it is necessary to prevent too great a heat. In gardens, the use of bell glasses, which are employed to shelter certain plants, is founded on the diathermous property of glass indicated in the preceding tables; this substance is traversed by the solar rays which have a high temperature, but not by the dark heat which radiates from the sun. Diffusion.-It has already been remarked that the heat which falls on the surface of a body is not wholly reflected according to the laws of reflection above mentioned. A part of this heat is irregularly reflected, that is, in all directions round the point of incidence. This phenomenon is known under the names of diffusion, dispersion, or irregular reflection of caloric; and the name of specular reflection is given to that which follows the regular laws of reflection. The phenomenon of diffusion from the surfaces of bodies was the discovery of M. Melloni. Regular reflection takes place only in polished surfaces; on the contrary, irregular reflection takes place in dull or rough surfaces, as in plates of wood, glass, or metal, ground or unpolished. The diffusive power varies according to the nature of the source and of the reflecting substances. White bodies are very dispersive in the case of the caloric which radiates from an incandescent source. The metals unpolished are still more dispersive than white bodies. CONDUCTIBILITY OF SOLIDS, LIQUIDS, AND GASES. Conductibility of Solids.-The property which bodies possess of transmitting caloric more or less easily through the interior of their mass is called conductibility. It is considered that this kind of propagation of heat takes place by internal radiation from particle to particle. As caloric is not conducted in the same manner through all bodies, those which transmit it easily and readily are called good conductors, as the metals in general; and those which present more or less resistance to the propagation of heat are called bad conductors, such as glass, rosin, wood, and especially the liquids and the gases. In order to compare the conducting power of solids, Ingenhousz, a Dutch physician, who died at the end of the last century, constructed a small apparatus which bears his name, and which is represented in fig. 170. It is composed of a box made of tin plate, to which are fixed, by means of short tubes and corks, rods of different substances, as iron, copper, wood, and glass. These rods penetrate the interior of the box a very little way, and are covered with Instances of the Diathermous Power.-Although no direct ex-white wax which melts at 65° Centigrade. The box being periment has been made on the diathermous power of the filled with boiling water, the wax on some of the rods will gases, it cannot be doubted that air is very diathermous, since soon be observed melting at a greater or less distance, whilst it is in this fluid that all the phenomena of radiant heat take on others there will appear no trace of fusion whatever. The place. It is on account of their great diathermous power that conducting power of each is evidently greater in proportion as the upper strata of the atmosphere are always at a low tempe- the part on which the wax melts is more remote from the box. rature, notwithstanding the solar rays which pass through M. Despretz measured the conducting power of solids with them. Water being little diathermous, the contrary phenome- the apparatus represented in fig. 171. non takes place in the bosom of seas and lakes. The upper strata alone partake of the variations of temperature, according to the seasons, while at a certain depth the temperature is always the same. It consists of a prismatic bar of metal, in which are formed a series of cavities at equal distances, which are filled with mercury, and in each of these cavities is placed a thermometer. This bar being exposed at one of its extremities to a constant source of heat, the mercury in the thermometers is seen successively rising in each, according to its distance from the source; and then indicating a fixed temperature, but diminishing in height as these distances increase. By this process, M. Despretz verified the following 1w, which was first announced by M. Lambert of Berlin, viz.,-If the distances from the source increase in arithmetical progression, the excess of temperature above that of the surrounding air decreases in geometrical progression. This law, however, holds true only for the good conductors among metals, as gold, platinum, silver, and copper; it is only approximately true for iron, zinc, lead, and tin, and not at all applicable to non-metallic bodies, as marble, porcelain, &c. If the conducting power of gold be represented by 1000, that of the following substances will be! only be ascribed to the diathermous power of the liquid, however feeble it may be. Mode of heating Liquids.-When liquids are heated by the application of the source at their under surface, it follows, from their feeble conductibility, that it is only by the ascending and descending currents which take place in their inferior mass that their heating is effected. These currents are explained by the expansion of the lower strata of the liquid, which, becoming less dense, rise in the liquid, and are replaced by the upper strata, which are cooler, and consequently more dense. These currents are rendered visible by throwing into water some saw-dust, which rises and falls along with them. This experiment is arranged in the manner represented in fig. 173. Conductibility of Gases.-We cannot, in a direct inanner, represented as in the table, according to the experiments of determine the conducting power of the gases, on account of M. Despretz: Substances. their great diathermous power, and the extreme mobility of their particles; but when they are restrained in their motions, their conductibility is almost null. It is abserved, indeed, that all substances, between whose filaments the air remains Organic substances are bad conductors of heat; as to wood, M. De La Rive, of Geneva, has shown that its conductibility is greater in the direction of the fibres than across the length, and that the most dense is the best conducting. Bran, straw, wool, and cotton, which are neither dense nor uniform, but composed of discontinuous parts, are very bad conductors. Conductibility of Liquids.-The conductibility of liquids is extremely small, as may be proved by the following experiment:-A piece of ice being kept at the bottom of a glass tube filled with water, and the apparatus arranged as shown in fig. 172, the water is made to boil at the upper part of the tube, by heating it with the flame of a spirit-lamp, and it is then observed that while the column of liquid is at the boiling point at one of its extremities, the ice is scarcely begun to melt at the other extremity. Mercury is the only liquid which is a good conductor of caloric, and this is owing to its metallic nature. It is in consequence of its conductibility that when the hand is immersed in it, at the ordinary temperature, we experience a sensation of cold more striking than in any other liquid at the same temperature. The conductibility of liquids, however, is not null, as some philosophers have asserted. In fact, if we place on the surface of a liquid a small vessel full of boiling water, it is observed that a thermometer placed at a small distance below it, will, at the end of a certain time, indicate a slight increase in temperature, an effect which can stationary, present great resistance to the propagation of caloric; such as straw, eider-down, fur. When a gaseous mass is being heated, it takes place chiefly by its contact with a warm body, and by the ascending currents which arise from expansion, in the same manner as in liquids. Applications of Conductibility.-When it is required to preserve a liquid warm for a length of time, we enclose it in a vessel having double walls, the interval of which is filled with non-conducting matters, as saw-dust, glass, pounded charcoal, and straw. The same means are employed to prevent a body from absorbing caloric; hence, in order to preserve ice, in warm weather, it is enveloped in straw, or with a covering of wool. In our dwellings, the stone-paving appears to be cooler than the wooden flooring, because it is a better conductor of caloric. The sensation of heat or of cold which we feel when we come in contact with certain bodies, is due to their conductibility. If their temperature is lower than ours, they appear to us colder than they are, because they take caloric from us in consequence of their conductibility, as is the case with marble; if, on the contrary, their temperature is higher than ours, they seem to us warmer than they are, because they impart to us caloric at various points of their mass. This phenomenon is exemplified in the case of an iron bar exposed to the rays of the sun. Fig. 173. ever care the process may be conducted, performed by the oldest, the most experienced operators, the results of cupellation are always more or less discordant with the truth, partly from losses experienced in placing the alloy on the cupel and removing it from the same, and partly by the evaporation of minute quantities of silver (for silver is sensibly volatile at high temperatures), and partly from the "spitting" already described, a result which may be diminished within a very narrow range, but which scarcely admits of being altogether prevented. As the subject of cupellation is strictly a practical one, which may be of use to the student, especially at this time, (for it applies also to the estimation of gold, under which head we shall have to review it,) I append a practical table of the discrepancies between the results of cupellation and the more correct process of mint analysis. The table, it must be remarked, refers exclusively to alloys of silver, copper, and lead, and so based on the assumption that all possible care has been taken in order to avoid unnecessary causes of loss. LESSONS IN CHEMISTRY.-No. XXXI. RESUMING the subject of silver assaying by cupellation, it is well to explain the meaning of the term "standard silver," which signifies a silver alloy of the purity legalised by the legislature for the purposes of coinage. When engaged in the performance of experiments on the metal silver, you could not have failed to remark its quality of softness, whereas the silver of coins is moderately hard; the hardness is produced by alloying or incorporating it by fusion with copper. Standard silver, then, is a compound of eighteen parts by weight (say pennyweights) of pure silver alloyed with two parts by weight (pennyweights) of pure copper, and according as the alloy is richer or poorer in silver than the above proportion, so is it said to be better or worse than proof. As standard silver is a mixture of the precious metal and copper in the rates of eighteen to two, so therefore is it spoken of as being 18 pennyweights fine. In our previous operations with the cupel, or its substitute, we have merely taken cognizance of the quality possessed by lead of oxidation, fusion, and final absorption of the oxide by means of bone earth; in other words, we have treated of the cupelling operation as though it could only apply to alloys of the precious metals and lead. It remains, therefore, to state at this time that the powers of the operation are far more extensive. Not only has lead the quality of oxidation, fusion, and final absorption by bone earth, but it promotes these results in most other metals, especially copper; other. wise the cupelling operation would not be of the slightest practical service in the routine of mint operations. Suppose, for example, our object to be the assay of a silver coin, we take it, or rather a part of it, not usually more than 24 grains, envelope it in about three or four times its weight of pure sheet-lead, sold on purpose for the operation; place the enveloped mass on the cupel with all the precautions already indicated, and proceed as described. Not only under these circumstances will the lead become oxidised and finally absorbed, but the copper along with it, leaving the silver pure. From the remarks I have made on cupellation, and from what the experimentalist will himself have seen, he will not fail to discover that the process is imperfect in the chemical sense of the term, inasmuch as it only indicates the amount of base metal contained in any alloy; nor is this all; with what In concluding the subject of the silver assay by cupellation, it may be remarked, that if the operator take 24 grains of the alloy, he will have as many twentieths of a grain as there are half pennyweights in the troy pound; and since no smaller fraction than half a pennyweight fine is reported on by silver assayers, the convenience of commencing operations on 24 grains will be obvious. Nevertheless some assayers prefer 12 grains, in which case the representative of half a pennyweight will obviously be half a grain. Extraction of Silver from its Ores.-Silver occurs in several states of combination, and occasionally "native," or metallic |