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 o 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 redu

cible 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 (rhombohedron, 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 amorphism. 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 temperature 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 atomic 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

liquids, but so large a space in gases that the volume of the atoms is utterly insignificant in comparison with that of the calorific envelopes. This greater and more uniform separation of the atoms by the calorific envelopes is supposed to account for the mobility of fluids. In different elastic fluids the calorific envelopes have different volumes: if the volume of the envelope surrounding an atom of sulphur 1, the corresponding volumes in the other gases will be 3, 6, 9, 12, 18 or 24 (p. 55—67).

=

Of the correctness of the atomic theory, the following proof is adduced by Wollaston (Ann. Phil. 20, 251; Gilb. 72, 37). If matter were infinitely divisible, atmospheric air would by virtue of its elasticity expand into infinite space. The earth's atmosphere could not therefore have a definite limit, but must extend itself to the other heavenly bodies and form atmospheres around them, the density of which would be proportional to the mass and attractive power of these bodies. That no atmosphere is observed round the moon might perhaps be explained by the consideration that the lunar atmosphere, if it existed, must, on account of the small mass of the moon, be very rare and therefore imperceptible. But it may be astronomically demonstrated that the sun and Jupiter, whose masses are much larger than that of the earth, are likewise without atmospheres. Hence it follows that the air is not infinitely divisible, but that its atoms existing in the higher regions of the atmosphere do not separate from each other beyond that point at which their mutual repulsion is exactly balanced by their attraction towards the earth. Against this it may perhaps be alleged that even supposing the air to be infinitely divisible, its elasticity must at length be so much diminished by decrease of density that the earth's attraction will set bounds to the greater expansion which a further removal from the earth would involve. Moreover, if we admit, with Poisson and Dumas, that the uttermost parts of the air are, on account of the extreme cold there existing, in the solid or liquid state and surround the atmosphere in the form of snow-flakes of nitrogen and oxygen-the preceding demonstration will appear even less satisfactory.

b. Chemical Combination.

A chemical compound is produced, when one or more atoms of one substance arrange themselves in the most symmetrical manner possible by the side of one or more atoms of another substance, or of several other substances, and thus form a compound atom.-For the manner in which atoms arrange themselves one with another, vid. Gaudin (Bibl. univ. 52, 131). Atoms are always more inclined to unite in simple than in complex numbers, and the more intimate compounds of the inorganic kingdom generally exhibit simple numerical proportions; while in organic compounds formed under the influence of the vital force, very complicated proportions are met with. Compound atoms again unite with compound atoms of a different kind to form compounds of the second order; and the compound atoms of the second order thus formed, by combining with others of the same order, give rise to compound atoms and combinations of the third order, and so on. The mode of conceiving the formation of the less intimate compounds of variable constitution, e. g. solutions of acids, alkalis or salts in arbitrary quantities of water--whether in such cases these bodies first form compound atoms of definite constitution by combining with a small quantity of water, and these are afterwards surrounded by the remaining atoms of the liquid, or whether the mixture takes place in some other way,-must for the present remain undecided.

A chemical combination therefore is a mixture continued as it were to the extreme of intimacy; in a mixture properly so called, whole masses of atoms of the one body are laid side by side with those of the other— and these heterogeneous masses can be distinguished by the senses. But in chemical combination the individual heterogeneous atoms are laid side by side; and since atoms-even compound ones-are too small to be individually discernible, the eye perceives only the masses formed by the heaping together of these atoms by virtue of cohesion, and hence the chemical compound appears homogeneous. These masses may indeed be separated into smaller and smaller ones by mechanical force, but their compound atoms are not thereby resolved into simple atoms; only the cohesion is overcome which holds together the compound atoms, not the affinity by which the simple atoms are united into compound ones.

With regard to the innate force by which atoms are disposed to combine, three hypotheses have been laid down. By some, it is regarded as the same universal force of attraction which under different circumstances exhibits itself as gravitation, cohesion, and adhesion; by others, as an attractive force of a peculiar nature; by others again as electricity. First Hypothesis. Chemical combinations are produced by universal attraction.

Although Newton was the first who regarded chemical combination as the result of an attractive force, he nevertheless supposed that this force was different from universal attraction, and that it diminished according to the inverse cube of the distance. Buffon was the first to consider both these forces as identical. Since the force of universal attraction depends wholly on the mass of the attracting bodies and not at all upon their quality, while in chemical combination the latter is of the utmost importance, Buffon endeavoured to explain this difference by supposing that the centres of gravity of the atoms of heterogeneous substances might, in consequence of their difference of form, approach one another within different distances-and therefore, since the force of gravitation varies inversely as the square of the distance, the attraction between such bodies would vary in amount with the shape of their atoms.Bergman also attributed these differences between the action of gravitation and that of affinity to the different forms of the atoms and likewise to their relative position.-Guyton-Morveau perceived that to explain the great difference in the strength of affinity depending on the nature of the bodies concerned, on the hypothesis of a difference of form in their atoms, was mathematically impossible: but he was nevertheless inclined-since, according to his view, strength of adhesion and strength of affinity follow the same laws-to regard affinity as a particular manifestation of the gravitation of the atoms, and to hope that the peculiar characteristics of affinity would be explained by the discovery of new facts.

Berthollet's Theory. Universal attraction is probably the cause of chemical combination. Its action in this respect exhibits peculiar characters, because it is exerted, not on masses, but on molecules placed at extremely small distances from each other, and differing in form, cohesion and elasticity. All bodies have affinity for all others: but the affinity is not always manifested, because other forces, such as gravitation, cohesion and elasticity overcome it (p. 35).

Two bodies are, by virtue of their affinity, essentially capable of uniting in all proportions; the exceptions to this law are to be attributed to the cohesion and elasticity, partly of the simple substances themselves, partly of the compound. Thus, water dissolves only a certain quantity of salt,

because the cohesion of the salt ultimately balances the affinity: similarly, the elasticity of a gas prevents its absorption by water beyond a certain limit, and the elasticity of oxygen gas causes metals to combine with only a definite quantity of oxygen. Moreover, if a combination in certain definite proportions, e. g. that of 76'6 baryta and 40 sulphuric acid possesses very great cohesion, it separates from the state of solution in water, becomes thereby removed from the sphere of chemical action, and consequently takes up no more of the excess of baryta or sulphuric acid which may perhaps exist in the liquid. For a similar reason, hydrogen and oxygen combine only in the proportion which forms the most coherent compound, viz. water; (the more coherent peroxide of hydrogen was not known at that time). This state of greatest cohesion corresponds, in most combinations of acids and bases, with the proportion which produces the most complete neutralization: but in compounds of the more coherent substances oxalic acid and tartaric acid with ammonia, potash, and soda, it is found in the acid salt.

The smaller the quantity which any substance requires of another to produce neutralization, the greater is their natural affinity (p. 142). Since, for example, according to the later more exact determinations, 47.2 parts of potash require 40 of sulphuric and 54 of nitric acid to neutralize them, the affinities of sulphuric and nitric acid for potash are to one another as 54:40. But in the chemical actions of bodies, not only their force of affinity, but also their quantity must be taken into account. If we assume, according to what precedes, that the affinity of a molecule of potash for a molecule of sulphuric acid amounts to 54, and for a molecule of nitric acid to 40, and suppose that 1 molecule of sulphuric acid and 3 molecules of nitric acid act simultaneously on 1 molecule of potash,-the force with which the sulphuric acid tends to lay hold of the potash will be 1.54, and that exerted by the nitric acid 3.40 120. This product of the force of affinity into the quantity of the acting substance is called by Berthollet the Chemical Mass. Hence the chemical force of a body increases in direct proportion to its quantity; and a substance possessing but small affinity may, when its quantity is in excess, exert a stronger attraction on a third body than other substances possessing intrinsically greater affinity but present in smaller quantity.

=

When a body A comes in contact with two bodies B and C, both of which tend to combine with A, it does not combine exclusively with that one for which it has the greater affinity, not even when the quantity of the latter is sufficient for the complete saturation of A; neither does it combine exclusively with the one which acts with the greater chemical mass, but divides itself between the two in the proportion of their chemical masses. If, according to the preceding example, 1 molecule of sulphuric acid and 3 of nitric acid act on 1 of potash, the chemical mass of the sulphuric acid being 1.54 and that of the nitric acid 3. 40 = 120, of the quantity of potash present combines with the sulphuric acid, and with the nitric.

54

This law-that a body divides itself between two others which are endeavouring to lay hold of it, in the proportion of their chemical masses -is subject to exceptions, only when in such conflicts of affinity, a change in the state of aggregation of one of the acting bodies or one of the compounds, is produced by cohesion or elasticity, and these bodies are thus removed from the sphere of chemical action. In such cases A may combine exclusively with B or C. Hence a four following cases.