The forces of affinity, though also the same in kind, are still more numerous than those of sensible attraction; for instead of three, they amount to as many as there are heterogeneous bodies. The rate, indeed, at which they vary when the distance of the attracting bodies increases or diminishes, is probably the same in all, and so is also their variations as far as it regards the mass. But even when both of these circumstances, as far as we can estimate them, are the same, the affinity of two bodies for a third is not the same. Thus barytes has a stronger affinity for sulphuric acid than potash has: for if equal quantities of each be mixed with a small portion of sulphuric acid, the barytes seizes a much greater proportion of the acid than the potash does. This difference in intensity extends to particles of all bodies; for there are scarcely any two bodies whose particles have precisely the same affinity for a third; and scarcely any two bodies, the particles of each of which cohere together with exactly the same force.

It is this difference in intensity which constitutes the most important characteristic mark of affinity, and which explains the dif ferent decompositions and changes which one body occasions in others.

Thus it appears at first sight, that there are as many different affinities as there are bodies; and that affinity, instead of being one force like gravitation, which is always the same when the cir. cumstances are the same, consists of a variety of different forces, regulated, indeed, by the same kind of laws, but all of them dif ferent from each other. These affinities do not vary like magne. tism and electricity, though the mass continues the same, but are always of equal intensity when other circumstances are equal. Hence it is reasonable to conclude, that these affinities cannot, like magnetism and electricity, depend upon peculiar fluids, the quantity of which may vary; but that they are permanent forces, inherent in every atom of the attracting bodies.

12. It is very possible that this variation of intensity, which forms so remarkable a distinction between affinity and gravitation, may be only apparent and not real. For even in gravitation the intensity varies with the distance and the mass, and the same vari. ation holds in affinity. But as the attraction of affinity acts upon bodies situated at intensible distances from each other, it is evident that, strictly speaking, we have no means of ascertaining that distance; and consequently that it may vary without our discover.

ing the variation. But every such variation in distance must occasion a corresponding variation in the intensity of the attracting force. It may be, then, that barytes attracts sulphuric acid with greater intensity than potash, because the particles of barytes, when they act upon the acid, are at a smaller distance from it than the particles of the potash are.

But it may be asked, Why, if barytes, potash, and sulphuric acid, are all mixed together in water, the particles of potash do not approach as near the acid as those of the barytes, since they are both at liberty to act? To this it may be answered, that in all proba. bility they do approach each of them to the same apparent distance, (if the expression be allowed), but that, notwithstanding, their real distance may continue different. The particles of bodies, how minute soever we suppose them to be, cannot be destitute of magnitude. They must have a certain length, breadth, and thickness, and therefore must always possess some particular figure or other. These particles, indeed, are a great deal too mi. nute for us to detect their shape; but still it is certain that they must have some shape. Now it is very conceivable that the par. ticles of every particular body may have a shape peculiar to themselves, and differing from the shape of the particles of every other body. Thus the particles of sulphuric acid may have one shape, those of barytes another, and those of potash a third.

But if the particles of bodies have length, breadth, and thickness, we cannot avoid conceiving them as composed of au indeterminate number of still more minute particles or atoms. Now the affinity of two integrant particles for each other must be the sum of the attractions of all the atoms in each of these particles for all the atoms in the other but the sum of these attractions must depend upon the number of attracting atoms, and upon the distance of these atoms from each other respectively; and this dis tance must depend upon the figure of the particles. For it is obvious, that if two particles, one of which is a tetrahedron and the other a cube, and which contain the same number of atoms, be placed at the same relative distance from a third particle, the sum of the distances of all the atoms of the first particle from all the atoms of the third particle, will be less than the sum of the distances of all the atoms of the second particle from those of the third. Consequently, in this case, though the apparent distance of the particles be the same, their real distance is different; and

of course the cube will attract the third particle more strongly than the tetrahedron; that is, it will have a greater affinity for it than the tetrahedron.

But if the particles of bodies differ from each other in figure, they may differ also in density and in size: and this must also alter the absolute force of affinity, even when the distances and the figure of the attracting particles are the same. e. The first of these two circumstances, indeed, may be considered as a difference in the mass of the attracting bodies, and therefore may be detected by the weight of the aggregate; but the second, though also no less a variation in the mass, cannot be detected by any such method, though its effect upon the strength of affinity may be very con siderable.

There is no doubt that, upon the supposition that such differ. ences in the figure, density, and size of the attracting particles, really exist, and it is in the highest degree probable that they do exist, the variation in intensity which characterises chemical affi. nity may be accounted for, without supposing the intensity of affinity, as a force inherent in the ultimate particles or atoms of bodies, is really different. The same thing may be applied to electricity and magnetism. It is certainly possible, therefore, that attraction, both sensible and insensible, may not only vary at the same rate, and according to the same laws, but be absolutely the same force inherent in the atoms of matter, modified merely by the number and situation of the attracting atoms. This is certainly possible; and it must be allowed that it corresponds well with those notions of the simplicity of nature, in which we are accustomed to indulge ourselves. But the truth is, that we are by no means good judges of the simplicity of nature; we have but an imperfect glimpse here and there through the veil with which her operations are covered; and from the few points which we see, we are constantly forming conjectures concerning the whole of the machinery by which these operations are carried on. Superior beings smile at our theories as we smile at the reasonings of an infant; and were the veil which conceals the machine from our view to be suddenly withdrawn, we ourselves, in all proba. bility, would be equally astonished and confounded at the wide difference between our theories and conjectures, and the real powers by which the machinery of the universe is moved. Let us not therefore be too precipitate in drawing general conclusions; but

let us rather wait with patience till future discoveries enable us to advance farther; and satisfy ourselves in the mean time with arranging those laws of affinity which have been ascertained, without deciding whether it be the same force with gravitation, or a different one. [Thomson.

CHAP. XI.

ON CRYSTALLOGRAPHY,

THE word crystal (xpuolaλdos) originally signified ice; but it was afterwards applied by the ancients to crystallized silica, or rock crystal, because, as Pliny informs us, they considered that body as nothing else than water congealed by the action of cold. Chemists afterwards applied the word to all transparent bodies of a regular shape; and at present it is employed to denote, in general, the regular figure which bodies assume when their particles have full liberty to combine according to the laws of cohesion. These regular bodies occur very frequently in the mineral kingdom, and have long attracted attention on account of their great beauty and regularity. By far the greater number of the salts assume likewise a crystalline form; and as these substances are mostly soluble in water, we have it in our power to give the regular shape of crystals in some measure at pleasure.

1. Most solid bodies either occur in the state of crystals, or are capable of being made to assume that form. Now it has long been observed by chemists and mineralogists, that there is a par. ticular form which every individual substance always affects when it crystallizes this indeed is considered as one of the best marks for distinguishing one substance from another. Thus common salt is observed to assume the shape of a cube, and alum that of octahedron, consisting of two four-sided pyramids, applied base to base. Saltpetre affects the form of a six-sided prism; and sulphate of magnesia that of a four-sided prism; and carbonate of lime is of ten found in the state of a rhomboid. Not that every individual substance always uniformly crystallizes in the same form; for this

is liable to considerable variations according to the circumstances of the case but there are a certain number of forms peculiar to every substance, and the crystals of that substance, in every case, adopts one or other of these forms, and no other; and thus common salt, when crystallized, has always either the figure of a cube or octahedron, or some figure reducible to these.

2. As the particles of bodies must be at liberty to move before they crystallize, it is obvious that we cannot reduce any bodies to the state of crystals, except those which we are able to make fluid. Now there are two ways of rendering the bodies fluid, namely, solution in a liquid, and fusion by heat. These of course are the only methods of forming crystals in our power.

Solution is the common method of crystallizing salts. They are dissolved in the water: the water is slowly evaporated, the saline particles gradually approach each other, combine together, and form small crystals; which become constantly larger by the addition of other particles, till at last they fall by their gravity to the bottom of the vessel. It ought to be remarked, however, that there are two kinds of solution, each of which presents different phænomena of crystallization. Some salts dissolve in very small proportions in cold water, but are very solable in hot water; that is to say, water at the common temperature has little effect upon them, but water combined with caloric dissolves them readily. When hot water saturated with any of these salts cools, it be comes incapable of holding them in solution: the consequence of which is, that the saline particles gradually approach each other and crystallize. Sulphate of soda is a salt of this kind. To crys tallize such salts, nothing more is necessary than to saturate hot water with them, and set it by to cool. But were we to attempt to crystallize them by evaporating the hot water, we should not succeed; nothing could be procured but a shapeless mass. Many of the salts which follow this law of crystallization combine with a great deal of water; or, which is the same thing, many crystals formed in this manner contain a great power of crystallization.

There are other salts again which are nearly equally soluble in hot and cold water; common salt for instance. It is evident that such salts cannot be crystallized by cooling; but they crystallize very well by evaporating their solution while hot. These salts. generally contain but little water of crystallization.

There are many substances, however, neither soluble in water