! their modifications and relative positions, as the same group presents the primitive form as well as its truncations and decrements. Other salts yield other figures, and by more complicated chemical action, as of acids upon carbonate of lime, the metals, &c., analogous results are obtained. Here then, instead of dividing a crystal by mechanical force its structure is gradually developed by the process of solution. In these cases two circumstances are particularly remarkable: the crystals are different; and their forms vary with the different faces of the original mass. In one direction we observe octoëdra and sections of octoëdra; in another, parallelograms of every dimension, modified with certain determinate intersections. Several conditions for a the. ture of all cryssame body uni If, in neither of these positions, we turn the mass upon on its axis, the same figures will be perceived at every quadrant of a circle; and, if we suppose the planes continued, they will mutually intersect each other, and various geometrical solids will be constructed. In this way, alum alone furnishes octoëdrons, tetraëdrons, cubes, four and eight-sided prisms either with plain or pyramidal terminations, and rhombic parallelopipedons. It is evident, then that no theory of crystallization can be admitted, which is not founded upon such a disposition of constituent particles, as may furnish all these modifications, by mere ab-ory. straction of certain individuals from the congeries, without altering the original relative position of those which remain; and these conditions may be fulfilled by such an arrangement of spherical particles, as would arise from the combination of an indefinite number of balls endued with mutual attraction and no other geometrical solid is adequate to the purpose; and where bodies afford crystals differing from the octoedral series, an analogous explanation is furnished, by supposing their constituent particles to consist of oblate spheroids, whose axes bear different proportions to each other in different substances. Hence we may also conclude, that the internal structure of all crystals of the Internal struesame body is alike, however the external shapes differ. In corrobo-tals, of the ration of the above hypothesis, we may remark, that the hexaëdron is, form. of all geometrical figures, that which includes the greatest capacity under the least surface. If, therefore, the ultimate particles of crystalline bodies be spheres or spheroids, the greatest possible number in the least space will be included in this form. It is probable that the Probability exterior shape of every crystal is determined by the nucleus first form-rior shape vaed by a certain definite number of particles, which, by the power of depends upon mutual attraction, overcome the resistance of the medium in which the number of particles unitthey were suspended or from which they were separated. This num-ed in the nuber may vary with the solvent, or other contingent circumstances, cleus. Four spherical particles, thus united, would balance each other in a Their number tetraedral group, six in an octoëdral group, and each would present may vary with particular points of attraction to which all subsequent deposits would &c. be directed. Now, let us imagine two nuclei formed in the same solution, whose axes run in contrary directions; their increase will consequently be in contrary directions, and each will attract a particular system of particles from the surrounding medium. If these two sys- The plane of tems should cross each other in their course, a greater number will be intersection brought within the sphere of mutual re-action at the point of junction, crysta and they ought to arrange themselves in the least possible compass.agon. The facts here answer to the theory. If we select any crystals, having others crossing them nearly at right angles, and separate them, the points of junction invariably present an hexaedral arrangement. that exte cries with and the solvents, between two a hex us Attraction, or Affinity. vered Attraction as disposing the parti>form masses or aggregates; and, in selves according to peculiar laws, and ures. We are now to regard this power articles; as presiding over the composing their chemical varieties. This is FINITY. exhausted of air, be introduced some sula neat applied so as to melt the former, it the latter. We observe, as the results of wiphur and copper, 1. That the substance mediate properties of its elements, but that 2. That much heat and light are evolved 3. That sulphur and copper will unite in similar changes of properties may be exa change of form or state results. Thus wr bodies produces a solid, as when muriatic produce the salt called muriate of ammonia. wder offers a familiar instance of the conerm matter. Gases form a liquid, as when chlorine. Solids also produce liquids, as is Matthrystals of Glauber's salt with nitrate of ammonia: centrated solutions of muriate of lime and carboorm a solid. Liquids produce gases, as when iis mixed with two of alcohol an effervescence matter is copiously evolved. of combination, the resulting compound differs mponent parts, and their leading characters are still • is especially remarked in solutions of different and other fluids. Salt and sugar dissolved in wate and sweet tastes, the only physical quality that is of cohesion. cases, the properties of the compound differ essential ly from those of its component parts, and a series of new bodies, possessed of distinct and peculiar characters, are produced. Thus, when two volumes of nitric oxide gas are mixed with one of oxygen, an orange-coloured gas results, very sour, and soluble in water; whereas, the gases before mixture were colourless, tasteless, and insoluble in water. Such operations are not confined to art: Nature presents them on an extended scale; and, in connexion with the functions of life, renders them subservient to the most exalted purposes. 36. The new chemical powers, that bodies thus acquire in consequence of combination, are often extremely remarkable, and can only be learned by experiment. It frequently happens that inert bodies produce inert compounds, and that active substances remain active when combined; but the reverse often occurs: thus oxygen, sulphur, and water, in themselves tasteless and comparatively inert, produce sulphuric acid when chemically combined; and potassa, which is a powerful caustic, when combined with sulphuric acid, forms a salt possessed of little activity. produced by ac 37. The colours, the specific gravity, and the temperature of bodies are also commonly altered by chemical action. Thus the blue infusion obtained by macerating violets in warm water is rendered red by acids, Other changes green by alcalis, and its colour is wholly destroyed by chlorine. When chemical equal parts of sulphuric acid and water are mixed, the resulting liquid tion. has a specific gravity much above the mean; the temperature is also Chemical ac much increased; and ignition frequently attends chemical action. (32.) ion take ween 38. As chemical action takes place among the ultimate or constituent elementary elements of bodies, it must obviously be opposed by the cohesion of attepped to their particles, and chemical attraction is often prevented by mechanic-simple coheal aggregation. A piece of the metal antimony, put into the gas called chlorine, is only slowly and superficially acted upon; but if the mecha- Attraction of nical aggregation be previously diminished, by reducing the metal to acts in opposi powder, it in that state rapidly unites with the gas, and burns the instant tion. that it is introduced. 39. Heat increases the chemical energies of bodies. Its effects are sometimes only referable to the diminution of adhesion by expansion or liquefaction; but in other cases they are peculiar and complicated and probably concerned in modifying the electrical energies of the acting substances. matteris there sion. aggregation Caloric pros motes Chem. action. 40. Different bodies are possessed of different attractive powers, and if several be brought together, those which have the strongest mutual simple or elecaffinities enter first into union. Thus, if nitric acid be poured upon a tive attraction. mixture of lime and magnesia, it dissolves the former in preference to the latter earth. The knowledge, of this fact, enables us to separate bodies when united, or to perform the process of decomposition. Thus if we add an aqueous solution of lime to a solution of magnesia in nitric acid, the latter earth is thrown down or precipitated, and the lime occupies its place in the acid. and 41. Upon this principle tables of attraction have been constructed, the substance whose affinities are to be represented being placed at the head of a column, and the bodies with which it combines beneath it, in the order of their respective attractions (see History of Chemistry, p. 78.;) thus the affinity of sulphuric acid for several bases would be shewn as follows: 30. In connexion with chemistry, the theory of crystallization opens Connexion of a new avenue to the science, and frequently enables us to ascertain erystallization directly, that which, independent of such aids, could only be arrived at by an indirect and circuitous route. We frequently read the chemical nature of substances in their mechanical forms. To the mineralogist, an intimate acquaintance with the crystalline forms and modifications of natural bodies is essentially requisite. Indeed, the theory of crystallization may be considered as one of the great suports of that useful branch of natural history, and it is to the indefatigable exertions of Haïy that much of its present perfection is to be referred. In the arts, the process of crystallization is turned to very valuable account, in the separation and purification of a variety of substances. Chemical at SECTION II. Heterogeneous Attraction, or Affinity. 31. We have hitherto considered Attraction as disposing the partitraction or affi- cles of bodies to adhere so as to form masses or aggregates; and, in nity. many instances, to arrange themselves according to peculiar laws, and to assume regular geometrical figures. We are now to regard this power as operating upon dissimilar particles; as presiding over the composition of bodies; and as producing their chemical varieties. This is CHEMICAL ATTRACTION, or AFFINITY. Result of this attraction. Gaseous 32. If, into a glass vessel, exhausted of air, be introduced some sulphur, and copper filings, and heat applied so as to melt the former, it will presently combine with the latter. We observe, as the results of this attraction between the sulphur and copper, 1. That the substance produced has not the intermediate properties of its elements, but that it presents new characters. 2. That much heat and light are evolved during the mutual action. 3. That sulphur and copper will unite in certain proportions only. 33. In liquids and gases, similar changes of properties may be exSolid products hibited, and, in many cases, a change of form or state results. Thus the combination of aëriform bodies produces a solid, as when muriatic and ammoniacal gases produce the salt called muriate of ammonia. The combustion of gunpowder offers a familiar instance of the conversion of solid into aëriform matter. Gases form a liquid, as when olefiant gas is mixed with chlorine. Solids also produce liquids, as is shown by triturating crystals of Glauber's salt with nitrate of ammonia : and in the action of concentrated solutions of muriate of lime and carbonate of potassa, liquids form a solid. Liquids produce gases, as when one part of nitric acid is mixed with two of alcohol an effervescence ensues, and aeriform matter is copiously evolved. Liquid. properties si The compound 34. In some cases of combination, the resulting compound differs mayhave some but little from its component parts, and their leading characters are still milar to its con- obvious in it. This is especially remarked in solutions of different stituents. substances in water and other fluids. Salt and sugar dissolved in water, retain their saline and sweet tastes, the only physical quality that is changed being that of cohesion. Distinct pre35. In other cases, the properties of the compound differ essential perties. CHEMICAL AFFINITY. ly from those of its component parts, and a series of new bodies, possessed of distinct and peculiar characters, are produced. Thus, when two volumes of nitric oxide gas are mixed with one of oxygen, an orange-coloured gas results, very sour, and soluble in water; whereas, the gases before mixture were colourless, tasteless, and insoluble in water. Such operations are not confined to art: Nature presents them on an extended scale; and, in connexion with the functions of life, renders them subservient to the most exalted purposes. 36. The new chemical powers, that bodies thus acquire in consequence of combination, are often extremely remarkable, and can only be learned by experiment. It frequently happens that inert bodies produce inert compounds, and that active substances remain active when combined; but the reverse often occurs: thus oxygen, sulphur, and water, in themselves tasteless and comparatively inert, produce sulphuric acid when chemically combined; and potassa, which is a powerful caustic, when combined with sulphuric acid, forms a salt possessed of little activity. : 37. The colours, the specific gravity, and the temperature of bodies are also commonly altered by chemical action. Thus the blue infusion produced by obtained by macerating violets in warm water is rendered red by acids, Other changes When chemical wholly destroyed by chlorine. ac opposed to green by alcalis, and its colour is equal parts of sulphuric acid and water are mixed, the resulting liquid tion. has a specific gravity much above the mean; the temperature is also Chemical ac much increased; and ignition frequently attends chemical action. (32.) tion takes ween 38. As chemical action takes place among the ultimate or constituent elementary elements of bodies, it must obviously be opposed by the cohesion of matters there their particles, and chemical attraction is often prevented by mechanic-simple coheal aggregation. A piece of the metal antimony, put into the gas called chlorine, is only slowly and superficially acted upon; but if the mecha-Attraction of nical aggregation be previously diminished, by reducing the metal to acts in opposi powder, it in that state rapidly unites with the gas, and burns the instant tion. that it is introduced. sion. aggregation Caloric pro motes Chem, 39. Heat increases the chemical energies of bodies. Its effects are min 40. Different bodies are possessed of different attractive powers, and 41. Upon this principle tables of attraction have been constructed, |