Hence these attractions are considered to result from what are termed molecular forces, or those special forces which the molecules of bodies exert on each other, and it is from the modifications of these internal forces that the three forms of matter, namely, solid, liquid, and gaseous, arise.

The evidence on which the hypothesis of the existence of these molecular forces and of the molecular constitution of matter, rests, is strictly speaking mathematical evidence of the highest kind. We know from the discoveries of Newton that there are facts, the existence of which would never have been suspected but from mathematical reasoning and calculation; and the labours of Laplace have led to similar results in the present instance. But the observed facts and known phenomena also furnish considerable presumption of the truth of this hypothesis, as we shall have repeated opportunities of remarking in the course of this work. Independently however of all hypothesis, there are certain properties inseparable from any of the three states in which bodies exist, and to which we shall proceed in the following chapter.

CHAP. II.

DIVISIBILITY—ATOMIC THEORY-COMPRESSIBILITY-POROSITY

ELASTICITY-DILATABILITY.

7. Divisibility.-All bodies which possess sensible extension may be divided into several parts, and these again may be subdivided into particles more or less small, and so on to an extreme degree of minuteness; but whether or not bodies are infinitely divisible is a question which, owing to the imperfection of our senses, cannot be determined by direct experiment. The argument in favour of the infinite divisibility of matter is derived from mathematical reasoning; for a line or surface admits of division without limit; hence, however small the particles of any body may become, each of them, since it possesses surface or extension, is, mathematically speaking, susceptible of still farther division. But though substances may not in the mathematical sense be infinitely divisible, they are divisible physically, with our present imperfect means, to an astonishing degree of minuteness, and we have certain evidence of a divisibility in which the particles exist in a state of subdivision infinitely less than the powers not only of our senses assisted by the finest instruments, but almost of our conceptions and imaginations. The ingenious Wollaston obtained, for astronomical purposes, platinum wire, the diameter of which did not exceed the 18,000th of an inch; and the thinness of the gold in the very fine gilt threads of embroidery is almost beyond belief. What can be more incredible,

C

and yet a more decisive proof of the minuteness of matter, than that derived from the sense of smelling? There is taken from certain animals a substance termed musk, which will continue for years to send forth a strong odour without any apparent diminution in bulk. If a grain of iron be dissolved in nitro-muriatic acid and mixed with 3137 pints of water, some portion of the iron may, by proper chemical tests, be detected in every part of the liquid. This experiment proves the grain of iron to have been divided into more than 24,000,000 of parts; and if the same quantity of iron were still farther diluted, its diffusion through the whole liquid might be proved by appropriate processes. But the vegetable kingdom presents us with a species of fungus, whose cellules must, according to Professor Lindley, be developed at the rate of more than 66,000,000 in a minute;† how can we conceive the minuteness of the matter which is almost instantaneously developed into these? But the animal kingdom affords the most striking instances of the divisibility of matter. Animalcules have been discovered whose magnitude is such that a million of them does not exceed a grain of sand. Their bodies are organized, they move about with exceeding rapidity, eat, drink, and derive nutrition, evidently exercising a digestive apparatus. They have also circulating fluids; if then a globule of their blood bears the same proportion to their whole bulk, as a globule of our blood bears to our magnitude, what power of imagination can conceive any adequate idea of its minuteness?

8. Atomic Theory.-The advocates of this theory suppose each body to be composed of ultimate particles or atoms, which are infinitely hard and indivisible. The evidence for this theory rests on the discoveries of modern chemistry, and especially on the researches of Dalton, from which it appears that substances, by their combining in simple and invariable proportions, give rise to the varieties of com

*Turner's Chemistry, p. 2. + Prout's Bridgewater, p. 24.

pounds; thus the composition of bodies appears to be fixed and invariable, and every compound substance, so long as it retains its characteristic properties, always consists of the same elements united together, in the same proportion. Sulphuric acid, for example, is always composed of sixteen parts of sulphur, and twenty-four of oxygen; water is formed of one part of hydrogen, and eight of oxygen; and were these elements united in any other proportion, some new compounds, differing from sulphuric acid and water, would be the product. And the same is true of all other substances, however complicated, and at whatever period produced; thus marble, whether formed ages ago by the hand of nature, or recently by the chemical geologist,* is always composed of the same elementary substances. Such being the observed fact in the constitution of all compounds, the assumption that all bodies are composed of ultimate atoms, the weight of which is different in different kinds of matter, serves at once to explain all the phenomena of chemical union; and this mode of reasoning is, in the present case, almost decisive, since the phenomena do not appear explicable on any other supposition.† The phenomena of crystallization, as we shall see presently, furnish very strong evidence in favour of the preceding theory.

In connexion with this and the preceding article, we cannot refrain from quoting the opinion of the immortal Newton, in whose destiny it was to exhibit, or at least to indicate, the principal phenomena of the universe. After a brief review of the different energies which are in action,‡ he says, 'All these things being considered, it seems probable to me, that God, in the beginning, formed matter in solid, massy, hard, impenetrable, moveable particles; of such sizes and figures, and with such other properties, and in such proportion to space, as most conduced to the end for which he formed them, and that these primitive parti

* Sir James Hall.

+ Turner's Chemistry, p. 225. Optics. Book III. Qu. 31.

cles being solids are incomparably harder than any porous bodies compounded of them; even so very hard as never to wear or break in pieces; no ordinary power being able to divide what God himself made one, in the first creation. While the particles continue entire, they may compose bodies of one and the same nature and texture in all ages; but should they wear away or break in pieces, the nature of things depending on them would be changed. Water and earth composed of old worn particles and fragments of particles, would not be of the same nature and texture now with water and earth composed of entire particles in the beginning. And therefore that nature may be lasting, the changes of corporeal things are to be placed only in the various separations and new associations and motions of these permanent particles; compound bodies being apt to break, not in the midst of solid particles, but where those particles are laid together and only touch in a few points.'

9. Compressibility.-The term compressibility is used to express the property which all bodies have in some degree, of being reduced to a less apparent size or volume. Every one is sensible of the existence of this quality in sponge or Indian rubber, and the diminution of the apparent bulk of most substances is familiar to every one. The apparent volume is essentially different from the real volume, which is the space that the elementary particles would exactly occupy, supposing them to be in actual contact. A substance so constituted would be perfectly hard, and consequently absolutely incompressible; that is, it could not be reduced to a smaller size. No such substance is however known to exist in nature. The diamond and manufactured steel, gold and platinum, water and mercury, have all an apparent volume, which admits of a diminution the same in kind, though widely different in degree, as the sponge and Indian rubber. When a substance is compressed, we have the same quantity of matter in a less space; this is expressed by saying, that the density of the body is increased. Thus one substance is more dense than another