154 ALLOTROPY. [86. Some substances are stated to be even trimorphous, that is, they crystallize in three different systems. Both zincic seleniate (ZnSeO,, 7 H2O) and zincic sulphate (ZnSO,, 7 H,O), and nickel seleniate (NiSeO,, 7 H,O) and nickel sulphate (NiSO4, 7 H,O), according to Mitscherlich, exhibit this peculiarity. Nickel sulphate crystallizes below 15° C. in right rhombic prisms; between 15° and 20° C. in acute square-based octohedra; and when the temperature is above 30° C. in oblique rhombic prisms. In the first case the crystals belong to the prismatic, in the second to the pyramidal, and in the third to the oblique system. If the right rhombic crystals be placed in the summer's sun for a few days they become opaque, but still retain the form of the prism, which is found, when broken, to consist of a mass of octohedra.* It is not unlikely that the change of tenacity produced in some of the metals by elevation of temperature, and exhibited in a marked degree by zinc, is produced by some modification of their crystalline form under the action of heat. The influence of temperature in thus subverting the direction of the molecular attractions in obedience to which crystals are formed, has as yet scarcely been made the subject of systematic research; its further prosecution, however, cannot fail to throw much additional interesting light upon our knowledge of the operation of molecular attraction. In certain cases two isomorphous bodies are similarly dimorphous, or isodimorphous, such, for instance, as arsenious anhydride and antimonious oxide, both of which crystallize in regular octohedra and in four-sided prisms; palladium and iridium, as well as potassic and sodic nitrates, which all crystallize in rhomboidal prisms or in hexagonal prisms; cuprous and argentic sulphides, which occur either in cubes or in rhomboidal prisms. (87) Allotropy.-Independently of dimorphism, the particles of many solids are capable of other modes of arrangement, which, without altering the chemical composition of the body, yet produce a very important modification of many of its properties, both chemical and physical. There appear to be four different conditions in which solid bodies may exist. They may be-1st, crystalline, as diamond, garnet, felspar; 2nd, vitreous or glassy, as glass itself, transparent arsenious anhydride, and barley sugar; 3rd, amorphous, or destitute of crystalline form altogether, as tinder, chalk, or clay; and 4th, organized, or arranged in masses, consisting of cells, fibres, or membranes, like the tissues of animals or vegetables, as hair, * According to De Marignac, however, nickel sulphate in the second and third forms contains 1 H2O less than it does when crystallized in right rhombic prisms; and if this be true for nickel sulphate, it is most probably the case with the other salts above mentioned as trimorphous. 87.] ALLOTROPY. 155 muscle, skin, wood, bark, leaves, &c. To these organized structures no further allusion will for the present be made, since they are producible only by the living organism. Many substances are capable of assuming indifferently any one of the first three of these conditions. Sulphur, for example, often occurs naturally in beautiful octohedral crystals, and may always be obtained in this form by allowing its solutions to evaporate spontaneously in the air. These crystals are hard and brittle, and they may easily be dissolved in carbonic disulphide. But if a quantity of these crystals be melted, and heated considerably beyond the boiling point of water, and the liquid be then suddenly cooled by pouring it into cold water, a tough, flexible, transparent substance, of an amber colour, is procured, which may be kneaded in the hand or drawn out into long threads, and is less easily inflamed than ordinary sulphur. This constitutes vitreous sulphur; but if it be left for a few days, it becomes brittle, opaque, and partly crystalline. However, it is not all crystallized, for if digested with carbonic disulphide, part of it only will be dissolved; the crystallized portion is taken up, and a buff-coloured powder is left, which is insoluble. It has no crystalline appearance, and is amorphous sulphur. This, if melted by heat, becomes as soluble as before. In addition to these alterations in consistence, colour, inflammability, and solubility, differences in the density are observed: Octohedral sulphur has a density relative to water of 2:05 Corresponding differences in the specific heat have been observed in these different conditions. These three different forms of sulphur are called allotropic modifications of sulphur, and the existence of the same substance in different forms, each endowed with different properties, is called allotropy (from aλλoç, another, and rρóπоç, manner). Phosphorus affords another excellent instance of this singular series of modifications. Phosphorus, when first prepared and as sold in the shops, is in the form of transparent, flexible, waxylooking sticks which are of the vitreous variety. In this form it is freely soluble in carbonic disulphide, melts in warm water at a temperature very little above that of the human body, and is so inflammable, that if left exposed to the air, even for a few minutes, in warm weather, it often takes fire and burns with great violence. Phosphorus has also been obtained in crystals, 156 ALLOTROPY. [87. But if which are equally inflammable with the common form. phosphorus be put into a flask filled with nitrogen or carbonic anhydride, to prevent it from taking fire, and be heated, with various precautions to avoid accident, up to the melting point of tin (228° C.), or rather higher, in a few hours it will be changed into a red powder which, when properly purified, may be exposed to the air without any danger of taking fire. In this condition it does not melt until heated to 500° (260° C.), or even beyond that temperature; and it is quite insoluble in carbonic disulphide. Yet it is pure phosphorus, although in the amorphous condition. By heating it to about 572° (300° C.), in a retort or vessel from which the air is excluded, it melts, and then cannot be distinguished from the original phosphorus that furnishes it. In addition to these alterations in solubility, colour, inflammability and external appearance, differences in the density and in specific heat have been observed. Many other elementary bodies exhibit analogous allotropic modifications, and their number will no doubt be increased as researches in this direction become multiplied. It is probable, indeed, that such modifications exist in all the elements, although the properties of the different forms are certainly not always so dissimilar as in the cases already quoted. Even in permanent gases we have indications of allotropy, the remarkable substance ozone having been ascertained to be oxygen in a particularly active condition, its molecule consisting of three atoms of oxygen, ordinary oxygen containing only two. The formation of this allotropic modification by an alteration in the constitution of the molecule renders it probable that the explanation of allotropy must be sought in the atomic structure of the elementary bodies. The consideration of special instances of allotropy will be deferred until the properties of the bodies themselves are detailed. Allotropy does not appear to be confined to elementary bodies; but in compounds it is not always easy to determine whether the corresponding modifications may not be due to alterations in chemical composition, arising from a change in the mode of combination of the different component elementary bodies with each other. It is certain, whatever be the causes which thus influence molecular arrangement, that the particular arrangement which such causes may produce in any given case, has a very material influence in modifying the physical properties of the body. When a body is homogeneous, or when it is symmetrically arranged, as in the crystals belonging to the regular system, the transmission 88.] CHEMICAL RELATIONS OF LIGHT. 157 of light, the expansion by heat, and the conductivity of the body for heat, is uniform in every direction; but when the molecular attractions, as shown by the form of the crystal, are more powerful in one direction than in others, immediately a corresponding irregularity in the action of the body on light, and in its expansive and conducting powers for heat, may be traced. Wertheim has proved that the velocity of sound, when transmitted through wood, is nearly five times as great when transmitted in the direction of the fibre, as when transmitted across it; and Wiedemann (Poyg. Ann. 1849, lxxvi. 404) has shown that electric induction occurs with varying degrees of facility in different directions in the same doubly-refracting crystal. CHAPTER IV. LIGHT. Nature of Light-Undulations-Reflection-Refraction-Production of Colour-Chemical Actions, Photography-Interference -Double Refraction-Polarization. (88) Chemical Relations of Light.-The light which, operating through the eye, gives exercise to the sense of vision, until within the last few years, would have been thought to have little connexion with chemistry. Now, however, the case is otherwise, and an acquaintance with the fundamental laws and properties of light is indispensable to the chemist. The physical characters of an object, revealed by its action on light, are often of the greatest chemical value. Differences in refractive power, for example, furnish in many cases the most rapid and satisfactory proof of the genuineness or adulteration of an essential oil. Varieties in the amount and direction of circular polarization afford the best means in certain cases of arriving at a knowledge of the varieties and proportions of sugar in complex saccharine liquids. By the action of polarized light, the diamond and other precious gems may be distinguished from spurious imitations. But besides the indirect assistance thus afforded to chemistry, the researches of the last sixty years have been gradually developing the vast importance of light as an agent in producing the chemical changes which are continually in operation upon the surface of the earth; and they have at length shown that this wonderful emanation from the sun is, conjointly with heat, the 158 SOURCES OF LIGHT. [88. mainspring which maintains the chemical actions, and with them the existence, of all the varied forms of organic life which teem around us. The fixation of carbon in the vegetable creation, the accumulation of materials for our habitations and for fuel, and the maintenance of a uniform composition in the atmosphere, may be mentioned in illustration of the importance of its chemical actions: whilst the fascinating art of photography gives proof of the rapidity and the variety of the changes which it produces; and in the new department of spectrum analysis we are furnished with a method of investigation which reveals to us the composition, not only of the flames of furnaces and of volcanoes, but even of the luminous atmosphere of the sun, the fixed stars, and the nebulæ, extending the range of our inquiries through distances limited only by that through which the object is visible. The investigation of the laws of light belongs to the science of optics in the following pages, therefore, reference will only be made to some of its principal properties, a knowledge of which will be a necessary preparation to the study of its chemical effects. (89) Sources of Light.-1. The great natural sources of light are the sun and the heavenly bodies, but there are several modes of procuring light by artificial means. The other chief sources of light are the ignition of solids; phosphorescence by heat; luminous animals; phosphorescence of decaying animal matter; electricity; and certain cases of crystallization. 2. Ignition of Solids.-Whenever any solid object is raised to a high temperature (about 980° or 1000° F., equivalent to 526° or 538° C.), it becomes luminous. A current of gaseous matter may have a temperature of upwards of 2000° (1093° C.) without becoming luminous. If, however, a solid be introduced into such a current of heated gas, it begins to throw off light in all directions, even though it may not burn, and may experience no chemical change; under such circumstances, it is said to become incandescent. The colour of the light varies with the temperature. When first perceptible it is of a dull red colour, and as the temperature rises, it passes through orange and yellow into a full white, which, when the temperature becomes extremely high, assumes something of a violet tinge. The experiments of Draper (Phil. Mag. 1847 [3], xxx. 345) show that platinum begins to emit light in the dark at a temperature which he estimates at about 977° (525° C.). He also found, by introducing different substances into a clean gun-barrel, and raising the barrel to a dull red heat, and then looking down into the barrel, that they all became red hot at the same time |