may be considered as composed of ultimate atoms. These are in every case less than 10,000,000 of an inch in diameter, so that by a simple calculation it may be proved that a cubic inch of them would, if extended on a level surface so that they may touch but not overlap each other, cover an area of 218,166 square feet, or more than five acres of ground.*

3. The minute atoms composing masses of matter may be, and often are, chemically compound, although physically simple; thus a piece of marble may be divided into its ultimate molecules, each consisting of carbonate of lime, and here physical analysis stops; but by chemical analysis we can separate each of these atoms into carbonic acid and lime, the former being again chemically divisible into carbon and oxygen, and the latter into calcium and oxygen. In physics, therefore, an atom is regarded as simple when it cannot be further divided without separating its chemical elements.

4. Atoms are held together by means of a force denominated attraction, the firmness of their union being modified by the presence of an opposing force, termed repulsion, and upon the preponderance of one of these forces over the other depend all the physical properties of matter, known as hardness, softness, fluidity, &c. Attraction and repulsion are mutually exerted from the centres of each atom with an intensity, decreasing with the squares of their distance (12). If the mutual attraction of atoms be so considerable as to prevent a sharp body, as a knife, being inserted between them, the mass is said to be hard; but if so feeble as to permit their ready separation, the resulting mass is soft; and a fluid or gaseous body results when the intensity of the mutual attraction between the atoms is so far diminished, as to allow any substance to be moved between them without experiencing any considerable resistance.

* Mitscherlich, Lehrbuch der Chemie, p. 384.

EXTENSION OF MASSES.

7

Thus the various states in which matter exists, as solid, viscous, liquid, or gaseous, merely depend upon the varying intensity of the molecular forces of attraction and repulsion. These several states are readily convertible into each other by various mechanical means, and by alterations of temperature; thus, water at 32° and mercury at 72° lower, are solids, the one being transparent, the other opaque (424). At ordinary temperatures both are liquids, whilst at 212° water, and at 670° mercury, become vapours or gases, both being transparent; these several changes depending merely on the greater separation of their atoms by the addition of caloric. The density of matter in any of its three states is measured by the quantity contained in a given bulk, and is expressed by its specific gravity or specific weight, as compared with some body taken as a standard; thus, if a given bulk of water contain 1000 atoms of matter, a similar bulk of platinum will contain about 23,000; of copper nearly 9000, of iron 8000, and of glass about 3000: these several numbers being identical with the specific weight or gravity of the respective substances.

Masses of matter moreover possess several properties which may be considered as accessary, all depending upon the different degrees of intensity with which the physical atoms are mutually tied together. Among the more important of these may be ranked, Divisibility, Flexibility, Tenacity, Brittleness, Elasticity, &c.

5. Divisibility or Extension of Masses. This character may be considered as well illustrating the extreme, and almost inconceivable minuteness of physical atoms; depending upon the immense, although finite number of parts into which a mass may be divided. Thus, an imperceptibly small portion of strychnia will render a whole pint of water bitter, and a single grain of the ammoniacal hyposulphite of silver will render intensely sweet 32,000 grains of water; one grain of iodide of potassium dissolved in 480,000 of water, when mixed with a little starch, will tint every drop of

the fluid blue on the addition of a solution of chlorine. In all these cases, we have at once evidence of the extreme minuteness of atoms furnished by the divisibility of the masses or aggregation of atoms by means of solution. When animal or vegetable substances are burnt, they are neither consumed nor destroyed, their atoms are merely divided or separated from each other to form new combinations. Excellent illustrations of the same property are met with in many processes of art; a single pound of wool will furnish a piece of yarn 100 miles in length. Gold under the hammer is reduced to such a state of tenuity, that 360,000 of the leaves produced would, if piled on each other, only equal the thickness of an inch. Even this is far exceeded in the art of the wire-drawer, who, in the most economical mode of preparing gilded silver wire, extends two ounces of gold over a length of 1,351,900 feet, or rather more than 768 miles.

6. Flexibility. When any substance is capable of being bent in any given manner within moderate limits, by the application of sufficient force, it is said to be flexible; for a body to possess this property it is necessary that the attraction existing between one portion of its atoms should be capable of being partially overcome, and that between

A

C

another portion proportionably inOB creased: thus, if ABCD represent O two rows of atoms situated at their normal distance from each other, on applying force sufficient to flex the whole into the curved form EFGH, the arc Er will be larger than the arc GH, and, consequently its atoms will occupy a larger and those of the lesser arc a smaller space than they did when in the rectilineal figure. Lead, gold, annealed copper, soft iron, wax, &c., are examples of flexible bodies.

H

7. Tenacity. This character is dependent upon the

intensity of attractive force existing between atoms being sufficient to oppose their separation, to such an extent as to cause the rupture, or fracture of the whole mass, consequently, all flexible, ductile, and malleable bodies are tenacious; although many substances possess the latter property without any of the former. Tenacity varies extremely in different substances: metals afford the best examples of it; thus, a piece of steel wire of given diameter is capable of supporting without fracture 39,000 feet, or seven miles and a half of its own length. Wire of different metals of the same diameter, require different weights to overcome the mutual attraction of their component atoms, as shown in the following table, the figures representing the number of pounds weight required to break wires one inch in diameter of the metals enumerated.

8. Brittleness. This is obviously the converse of the last character; it points out that condition of a substance where the attraction between its molecules is capable of being overcome by a comparatively slight force. Very hard bodies are often extremely brittle; thus, a piece of glass will scratch the surface of polished steel, and yet is the most brittle of substances, unless spun into exceedingly fine threads. This property is frequently acquired during the hardening of bodies, when their atoms are brought nearer each other's

repulsive influence; thus soft steel is tenacious, yet a hard knife-edge is as brittle as glass, cast-iron is extremely brittle, and bar-iron is the toughest substance in nature.

9. Elasticity. A body is said to be elastic when, after being bent in any direction, it spontaneously recovers its former shape on the force which had altered its figure being removed; all elastic bodies must be so constituted as to allow a certain number of their atoms to be brought, at least momentarily, nearer each other than they previously were. If the body be a metallic rod, then, on being bent (see last figure) in the curved form EGHF, it will have a tendency to assume its primitive rectilinear form on the removal of the coercing force, in consequence of the exertion of two forces, viz. attraction between the partially-separated atoms on the outside, and repulsion between the unnaturally approximated atoms on the inside of the curve; the rod will obey these forces, and after a few oscillatory or vibratory movements will, if perfectly elastic, recover its primitive form. In this case, the change of form which brought into action the elasticity of the body is very obvious, from the curve produced by its flexure; sometimes this change of figure, even in the most perfectly elastic bodies, is not evident to the eye, on account of their figure; still such change does demonstrably take place. Thus a ball of ivory is elastic, and this property causes it to rebound from the floor when forcibly thrown upon it, its figure, on its impact, becoming altered and compressed, nor does it again become spherical until after it has

for some instants been an ellipsoid, of which the greater diameter is successively horizontal and vertical, as shown by the dotted curves in the marginal figure. Different elastic bodies vary extremely in the extent to which they will yield without

rupture; thus caoutchouc will yield considerably, and will afterwards very nearly regain its former shape, unless it has been stretched for some time. Glass threads, steel springs,