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two acids, the same effect often takes place; sulphate of baryta is less soluble than nitrate, and this than hydrochlorate of baryta, &c. But on the other hand, the almost insoluble sulphite and carbonate of baryta are decomposed by nitric acid.
Bergman deduced from his analyses of various salts the two following laws which are directly opposed to Berthollet's theory: (1.) An acid has the greatest affinity for that base of which it takes up the greatest quantity. (2.) A base has likewise the greatest affinity for that acid of which the greatest quantity is required to neutralize the base.
Kirwan, from the results of his own analyses of salts, adopted the first of Bergman's laws; but with respect to the affinity of a base for different acids, he on the contrary laid down the law, that a base has the greatest affinity for that acid of which it takes up the smallest quantity.
All these laws could only have acquired an appearance of validity from the fact, that but a small number of acids and bases were examined in relation to them, and moreover in an inaccurate manner. Adopting the more exact quantitative relations at present known, and placing all the salifiable bases,-including those of the heavy metallic oxides—in the sulphuric acid column, and at the acids in the lime column, it will be plainly seen that decomposition by simple affinity has no fixed relation to proportional quantity. Moreover we now know that the latter depends on the atomic weight of bodies. If then the strength of affinity were determined by relative quantity, the former would bear a simple proportion, direct or inverse, to the atomic weight. Accordingly to Berthollet's law, the affinity of hydrogen for other bodies should be the greatest of all, since hydrogen has the smallest atomic weight, and therefore the smallest quantity of it suffices to saturate other bodies; the affinity of iodine on the contrary ought to be less than that of most other bodies, sulphur for example, since 126 parts of iodine are necessary to saturate a quantity of metal for which 16 parts of sulphur are sufficient; nevertheless iodine, though more volatile than sulphur, decomposes the metallic sulphurets.
General Laws, by which the Magnitude or Strength of Affinity is
regulated. 1. With regard to the same two bodies. If A combines with different quantities of B, it holds the first quantity of B with greater force than the second, the second with greater force than the third, and so on. This law holds good without exception.
Carbonic acid, C O’, in contact with hydrogen gas, zinc, iron, &c. at a red heat, gives up only 1 atom of oxygen, the other remaining-in consequence of the superior affinity of the carbon-united with it in the form of carbonic oxide, C O. Brown peroxide of lead, Pb 02, is reduced at a low red heat to red lead, Pb3 0', oxygen gas being evolved; the red lead at a stronger red heat gives up more oxygen and is converted into the yellow oxide, Pb 0; and this will not part with its one atom of oxygen even at the most intense heat, but sublimes unchanged.
Apparent exceptions: Many combustible bodies abstract oxygen from nitrous oxide, N 0, at a lower temperature than from nitric oxide, N 0?, This anomaly must be attributed to the hindrances which the gaseous state offers, in various degrees, to chemical action. The fact that nitric oxide is deprived of its second atom of oxygen by alkaline sulphites, and converted into nitrous oxide which suffers no further alteration from the action of the same salts, proves the correctness of the law.—Nitric acid, N Oʻ, does not give up its oxygen to many bodies so easily as hyponitric acid, N 0! But nitric acid, precisely on account of the small aftinity of the nitrogen for the last atom of oxygen, is not known to exist in the separate state, but only in combination with water or salifiable bases. In aqueous nitric acid the great affinity of the water for the acid opposes, to a certain extent, the transfer of the oxygen to other bodies. So likewise hyperchlorate of potash, KO, CI 0, is less easily decomposed by heat and combustible bodies than chlorate of potash, KO, CI O', although it contains two more atoms of oxygen. But hyperchloric acid is just on account of this greater quantity of oxygen a stronger acid than chloric acid, and the consequent greater affinity of potash for hyperchloric acid renders its decomposition more difficult.
2. With regard to different bodies. (a.) Simple substances exhibit the strongest affinities for each other, e, g. oxygen, chlorine, bromine, iodine, &c. towards most other elements. Then follow compounds of the first order, e.g. acids and salifiable bases. The affinities of compounds of the second order, salts, for instance, are much weaker. In proportion as the affinities of the elements are satisfied by combination, their tendency to form further combinations diminishes and ultimately ceases.
b. The more opposite bodies are in their physical properties, the greater for the most part is their affinity. Thus, metals being similar bodies have generally but little affinity for one another, but great affinity for oxygen, chlorine, bromine, iodine, sulphur, and other non-metallic bodies; in a similar manner, acids have little affinity for other acids or bases for other bases, but the affinity between acids and bases is very strong
Columns of Affinity. The following are a few columns of affinity drawn up principally from decompositions by simple affinity and from analogical reasoning. From the difficulty of the subject they must be considered as merely rough approximations. The bodies separated only by a comma are those whose relative position is as yet undecided; after each semicolon are placed substances whose affinity is decidedly weaker.
Orygen: K; Na, L?; Ba, Sr, Ca, Mg, Ce, Y, G, Al, Th, Zr, Si; Ti, Ta, W, V, Cr, Mn; CO, H; Mo; Zn; Fe; Cd; Ni; Co; Sn; U; Ta 0%; Mn 0; Mo 0); Ti0; P; Pb; Bi; Sb; PO?; S; Cu; Mo O’; As; N; Sn O HCI; S 0?; W 0; N 0; Se; Pt Ir; Fe 0 S 0?; Hg; Te; Os; R; Pd; Ag; K 0; Ba 0; As 0?; I: NO'; Cr? 0?, V203; Au; Br; CI; F; 1 0; CI 05; HO.
Chlorine: K and the other alkaline metals; metals which are the bases of the earths; Ti; Zn; Fe; Cd; Co; Sn; C 0; H; P; Pb; Bi; Sb; S; Hg; As; Ag; Sn CI; Hg? Cl; Pd; Pt; Au; I; Br; 0.
Fluorine, Bromine, and Iodine are similar in their relations to chlorine.
Sulphur: 0; K and the other alkaline metals; Zn, Fe; Sn; Cu; Cl; H; C; Pb; Bi; Sb; Hg; Ag; Pt; CuPS; Mo S; Au.
Phosphorus: 0; Cl; Br; 1; K; Zn; S; H.
Metals: 0; F; Cl; Bs; Í; Se; P; H. It is true that many metallic oxides and fluorides are decomposed at a red heat by chlorine; but in such cases the affinity of heat for the more volatile substances, oxygen and fluorine, must be taken into account. Nevertheless the affinity of the noble metals for chlorine seems to be greater than for oxygen. On the whole it appears that different metals have different orders of affinity.
Sulphuric acid: Ba 0); Sr 0; KO; Na O; L O? Ca 0; Mg 0; Pb 0; NH; Fe 0; Zn 0; Ni 0; Co 0; Cu 0; Al 0; Fe: 0. Сотр. Shnaubert (Unters. der Verwandtsch, 57); Karsten (Scher. 5, 583); GayLussac (Amm. Chim. 89, 21.)
Hydrochloric acid: 4Pb 0; K 0; Na 0; Ba 0; Sr 0; Ca 0; Mg 0; NH; Co 0; Ni0; Hg 0; Ce 0; Zn 0; Mn 0; Fe 0; U 0; Au O'?; Cu 0; Cu 0; Sn 0; G0; Al? 0?; U? 0; Cr? 0°; Fe 0"; Sn 0?; Bi? 0, Sb 03. Comp. Schnaubert; Anfrye and Darcet (A. Gehl, 3, 325), GayLussac, a. a. 0., and especially Persoz (Ann. Chim. Phys. 58, 180, also J. Pr. Chem. 6, 32). Persoz places the suboxide of copper above the protoxide, without however resting his supposition on experiment, and perhaps incorrectly. With regard to 4Pb O, vid. page 139.
Nitric Acid: K 0; Na 0; L02; Ba 0; Sr 0; 6Pb 0; Ca 0; Mg 0; NH”; Ag 0; Co 0; Ni 0; Ce 0; Zn 0; Mn 0; Cd 0; (6Pb 0, N 0"); Cu 0, GO; Al? 0?; U? 0; Cr 0'; Hg2 0; Hg 0; Fe’0; Bio 0%; according to Schnaubert, Gay-Lussac, Anfrye and Darcet, and particularly according to Persoz. Here also the place assigned by Persoz to suboxide of mercury above the protoxide is to be doubted; according to Proust, (A. Gehl, i, 525) protoxide (red oxide) of mercury stands likewise above protoxide of copper, and according to Schnaubert, protoxide of manganese precedes protoxide of nickel.
For phosphoric acid the order appears to be as follows: Baryta, strontia, lime, potash, and soda; and for oxalic acid: Lime, baryta, strontia, magnesia, potash, soda, ammonia.
In the revision of these columns of acids particular attention should be paid to the formation of basic salts.
Potash: 2 At. Mo 0}; 2S 0; 2Cr 03; 2 At. oxalic acid: 2 At. tartaric acid; S 0'; Se 0; N 09; I O'; cl O'; HF; H CI; P O'; As 05; I O'; Br 0%; CIO; H Br; Cr 0%; P O'; HI; Se Oo; N 0°; Mn O'; Mu Oo; B 0°; CO?; As 09; H Se; HS; H Cy. This column is as yet very uncertain, and requires many corrections and additions.
Other bases exhibit orders of affinity more or less resembling that of potash.
VI. ORIGIN AND NATURE OF THE POENOMENA OF AFFINITY.
1. Atomic Hypothesis. According to the Atomic or Corpuscular Theory, matter is an original essence, and consists of certain very small parts called Atoms, Molecules, or Particles, arranged, not in absolute contact, but with Intervals or Pores between them; so that bodies, which to the eye appear perfectly continuous, like a piece of glass or metal, must be regarded, not as being completely filled with matter, but as aggregates of atoms and empty spaces. In chemical combination the heterogeneous atoms arrange themselves close to each other, but without penetration-juxtaposition takes place and the aggregate of the so formed compound atoms, with the intervals or pores between them, constitutes the new compound. The ancient and modern atomic theories are distinguished from one another, according to the force which is supposed to act in bringing about the juxtaposition of the heterogeneous atoms.
A, ANCIENT ATOMIC THEORY. In this theory, the atoms are supposed to be actuated, not by any attractive force, but by a motion existing from all eternity-by virtue of which these atoms, invisible from their smallness, are continually falling through infinite space, but not in exactly parallel lines,—so that they sometimes come in contact and become aggregated together in larger masses, like the earth and the other bodies of the universe. Those atoms which go on moving alone, and so come in contact with bodies on the surface of the earth, sometimes fall without effect through their poressometimes, again, they strike on the atoms of the bodies, and thrust them either against the earth or against one another, thereby producing the phenomena of gravitation, cohesion, adhesion, and affinity. (Leucippus, Democritus, Epicurus, Lucretius, Lesage.)
B. MODERN ATOMIC THEORY. According to this now almost universally preferred hyhothesis, the atoms are supposed to be imprest with innate forces which give rise to their mutual attraction, sometimes exhibited in the form of mechanical, sometimes in that of chemical force.
a. Constitution of Atoms. Atoms are not infinitely small, in the mathematical sense of the words, but bodies of determinate magnitude, which cannot be separated into smaller parts, either by mechanical or other forces. They are of definite weight, definite magnitude, and definite form; and these are constant in the atoms of the same substance, but may differ in those of different substances. It is probable, however, that the atoms of all bodies have the same density, so that the weights of two heterogeneous atoms are in direct proportion to their volumes—and that if the atoms could place themselves side by side without leaving spaces between them, all bodies would have the same specific gravity, viz., that of the atoms. But there must exist between them considerable intervals, of various magnitudes in different substances,—and these are filled up with heat, the principle of elasticity,—whereby the atoms, which, by virtue of their mutual attraction, would place themselves in actual contact, are kept at certain distances from one another. When bodies are compressed or expanded, the atoms themselves suffer neither contraction nor expansion, but the pores are narrowed or widened. That atoms must be extremely small, and considerably less than yoooooo line in diameter, is proved by the microscopal investigations of Ehrenberg. (Pogg. 24, 35.)
Respecting the Form of Atoms, two views are principally entertained.
According to one of these hypotheses, atoms have the same form as the fragments obtained by splitting a crystallized body in the direction of its planes of cleavage. 'Antimony, which may be cleft in directions parallel to the faces of an acute rhombohedron, is resolved by this mode of division into similar rhombohedrons of continually smaller and smaller dimensions; and if we conceive the cleavage to be carried to the utmost possible limit, the smallest rhombohedrons thus obtained will be the atoms of antimony. The atoms of a body which may be cleft parallel to the faces of a cube will have the form of a cube; a substance which may be cleft according to the faces of a six-sided prism, will be ultimately reducible to triangular prisms. Cleavage according to the faces of a regular octohedron will give both regular octohedrons and tetrahedrons; hence one of these forms must belong to the atoms of a body so constituted, diamond for example. According to this view, atoms must have the form either of a parallelopiped (rhonbohedron, cube, square, rectangular, or rhombic prism),—or of a triangular prism,—or of a tetrahedron or octohedron, regular or irregular. This theory certainly affords the easiest explanation of the crystalline form and determinate cleavage of simple substances; but it gives no explanation of amorphism-it does not accord well with dimorphism—and the more easily it explains the crystalline form of simple substances, the less is it adapted to account for that of compounds; for the juxtaposition of two or more atoms of different forms must produce a compound atom of very complicated figure.
Greater probability attaches, therefore, to the second theory, which has been particularly developed by Ampère (Ann. Chim. 90, 43). According to this theory, all atoms have a spherical form; and in the first place, these spherical atoms, by arranging themselves in various numbers and at various angles, produce aggregates possessing one or other of the forms which may be obtained by cleavage; these aggregates may be called crystalline molecules. Thus, 4 such spheres forming a base, and 4 placed perpendicularly over them, may produce a cube; so likewise may 3 layers of spheres, each containing 9, arranged in a square. Two or 4 such layers would give a flattened or elongated square prism; 2 or more rectangular layers of 6, 8, 12, or more spheres, placed one above the other, might produce a rectangular prism; 9 or 16 spheres arranged on a plane in the form of a rhombus, and 3 or 4 such layers one above another, would form a rhombohedron; 3 spheres below and 1 above, or 6 at the bottom, 3 above them, and 1 at the top, a tetrahedron; 3 spheres below and 3 above, a triangular prism, and so on. These crystalline molecules, formed at the earliest stage of crystallization, afterwards unite themselves, by attracting each other chiefly at their surfaces, into larger crystalline masses, separable in the directions according to which the janction has taken place—and thus planes of cleavage are determined. It is true that this theory leaves it at present unexplained, why the spheres should, according to the nature of the bodies to which they belong, unite themselves in different numbers and at different angles, whose magnitude is constant in each particular substance—and that thus sometimes one crystalline molecule, sometimes another, should be formed. But, on the other hand, it gives the best explanation of dimorphism and amorpbism. When from viscosity in the liquid, or a too rapid passage from the fluid to the solid state, the atoms cannot first unite themselves in crystalline molecules in the manner just described, they all remain at equal distances from each other; consequently neither regular cleavage nor crystalline form can exist; that is to say, the body is in the amorphous state. Dimorphism arises when the atoms, according to temperatore or other circumstances, unite themselves in different numbers and at different angles, into crystalline molecules of different forms, which, therefore, by their approximation, must produce crystals of different shape and cleavage. The compound atoms of compounds are aggregates of two or more spheres, and are capable of uniting to form crystalline molecules in the same manner as simple atoms.
With respect to ponderable fluids, it is supposed, according to the atomie theory, that each individual atom (whatever may be its form) is surrounded by a sphere of heat, which takes up but a small space in