But by no process can we break up these gases; they may be subjected to the fiercest heat, and they will be as ready to form water again as ever, without the loss of a single particle. We picture, then, their elementary parts as themselves incapable of division by any of the forces at our command, or that have ever been in operation in the universe around us.

6. For if we take hydrogen from the bowels of the earth, where for thousands of years it has slumbered unmolested, and if we take it from the bosom of oxygen, in whose embrace it may have been for ages, and with which as water it may have lashed the shores of ocean or swept across the fields of air ten thousand times, it is precisely the same in all its properties.

Whatever may have been the history of the hydrogen and oxygen atoms, however many transformations and changes of partnership they may have had, the molecules of the one are as perfect to-day as centuries of centuries ago. Change of their positions is all that any natural or artificial process can effect; and by no changes can they be in the least worn or destroyed.

"Atoms-Molecules."

7. Substances that cannot be broken up, or whose molecules are indivisible, are called elementary substances, simple or chemical elements: and their molecules are called atoms (i.e. indivisible parts). Thus all molecules are not atoms, though all atoms are molecules.*

A molecule of common salt is the smallest possible portion of the substance salt, but it consists of one molecule of chlorine, tied up with one of sodium, each of which, so far as we know, is an atom.

8. Every known substance has been found by chemists to be composed of one or more of sixty-five different kinds of matter, or elements. These chemical elements are such as gold, silver, platinum, copper, iron, oxygen, hydrogen, nitrogen, chlorine, carbon, sulphur, phosphorus, &c.

So far as at present known, no amount of heat will resolve any

*Atom (from à and teμveiv, which cannot be cut or divided) and molecule (from moles, a mass, molecula, a little mass) are not synonymous. As applied to elementary substances an atom represents the smallest quantity of an element which can enter into combination, while a molecule represents the smallest amount of it which can exist in the free or uncombined state.

5

Impenetrability of Matter.

one of these, and no kind of alchemy will transmute the one into the other. An atom of gold is altogether different from one of copper or iron, different in size, and weight, and shape; and each atom has an independent imperishable existence.

9. How sixty-five kinds of matter should, by variously combining, form the endless diversity of things and appearances which our globe presents, is not without analogy. All the words, all the literature of the English tongue, is formed out of twenty-four letters, and all the letters of the multitude of tongues on the face of the earth are not more in number than the chemical elements. Even more wonderful is the fact that all the words of the English, or any other language, may now be signalled along a telegraph wire by combinations of only two different signals, a long and a shor“

one.

10. Though the complete discussion of the facts upon which the molecular theory of matter is based, belongs to chemistry rather than natural philosophy, yet there are many physical arguments that favour this view, derived from several of the departments of natural phenomena. These will be considered in a subsequent portion of the volume, as they are not of a sufficiently elementary character to be introduced in this place.

"Matter extended or impenetrable."

11. The very simplest idea that we have of substance is that it is extended, or occupies space. No two portions of it can occupy the same spot at the same instant.

Though it is usual to give this as one of the universal properties of matter, and call it impenetrability, this implies really no more than is contained in the very notion of matter.

When we drive a nail in a door, the particles of the nail do not penetrate the particles of the door. We merely push the latter aside and put the nail where the wood was an instant before.

So pushing a bottle, mouth down, into water will never fill it; as the bottle is not empty but contains air, and there is no way for the air to escape so that the water may take its place. In a vacuum it would fill as readily with its mouth down, as with its mouth up.

12. It is of course the atoms or molecules that are really impenetrable. If we take two measures of hydrogen, and one of oxygen, and apply heat, the gases will rush together with an explosion, and Occupy only two measures in their new form of water vapour. The penetration is however only apparent. Between the hydrogen

Gravitation and Weight.

7

molecules there is more than room for the oxygen atoms, and the two together are contented with the original accommodation of the hydrogen.

"All matter gravitates."

13. There is another property which constantly and inseparably accompanies matter, and which we are equally powerless to control. This is the power of attraction or gravitation according to which all matter draws and is drawn by all other matter. Every particle is invested with it, and by its power the huge balls of matter that roll through space are bound together. So inalienable is this property of gravitation from matter that we estimate the mass or quantity of matter in any case by the weight; and in chemistry an increase or decrease of weight points to an addition or withdrawal of matter. Of the nature of the invisible cords by which this attraction or pulling takes place we are as yet ignorant, though we know the laws or mode according to which it operates. There are many familiar instances of attraction, or drawing of matter from a distance without any apparent medium to transmit the action; and it is just possible that they may all be but modifications of this universal attribute of matter.

"Weight is the resisted attraction of the Earth."

14. When we drop a stone we say it falls down by its weight; and a person on the opposite side of the globe would say the same, though there the stone really falls up, if here it falls down. People in New Zealand stand with their feet against ours, like flies on opposite sides of a pane of glass, and hence they are called our antipodes.

Weight therefore is merely the pulling or attraction of the earth on bodies at its surface, and is the conjoint effect of the gravitating power of all the parts which compose our globe.

It acts as if the whole power of the earth were condensed at its centre, and the vertical, or line in which a plummet hangs or a stone drops, points at each spot over the globe towards that

centre.

15. It is owing to this general attraction that our earth itself is a globe; all its parts being drawn towards each other, that is, toward a common centre, the entire mass assumes the spherical or rounded form. The sun, the moon, and the planets are also round, indicating the influence of the same law.

8

Gravitation of all Matter.

16. The drawing influence of our earth is not confined to its immediate vicinity, but reaches to the confines of the universe. It serves as a cord to keep the moon in its course around us; as the sun's attraction restrains our globe from flying off like a stone out of a sling.

In every case, however, we must remember that the pulling is mutual; the moon exerts the same pull on the earth that the earth does on it; only the earth being so much the larger is comparatively but little influenced.

17. If we suspend two balls of lead near to each other, the same power is at work between them as between these huge balls we call worlds. The power is inconceivably less because the discrepancy in the amount of matter is so great. Still, what we want in matter may be supplied to a certain extent by bringing the balls nearer to each other; and with delicate suspension and means of observa. tion we can detect an unmistakable influence.

Again, if in place of bringing one of the balls near to another of the same size, we can suspend it near to a huge mass, such as the precipitous side of a lofty mountain, the effect is not so insignificant ; there is a marked leaning of the suspended ball from the vertical towards the mountain. This was proved by Dr. Maskelyne from actual trial on the mountain Schehallion in Perthshire.

"Law of gravitation of all matter."

18. The conditions on which depends the intensity of gravitation. -as well as of light, magnetism, sound, or any other influence spreading uniformly from a centre-may be well illustrated by taking the case of light. Illuminating power is dependent, first on the extent of the light-giving source. If we double a gas flame we get double the amount of light. Two candles together will give twice the light of one of them at the same distance, and will cast twice as strong a shadow. But, again, we can see to read as clearly with one candle as with two, if the single flame be brought nearer us than the double flame. And one candle flame a yard off will give us the light not of two but of four similar flames two yards away, so that a decrease of distance more than compensates for a decrease of flame at the same rate. The reason of this is manifest from the following illustration.

19. A board a foot square, represented in fig. 1 by A B, placed at any distance from a candle at C, will just shadow a board, E D,

Laws of Gravitation.

9

of two feet square placed at double the distance, and one of three feet square, L K, placed at triple the distance. But ED will have four times as much surface as A B, because the former is both

twice as long and twice as broad as the latter; and the board, L K. of three feet square, will in like manner have nine times as much surface as A B. Thus the light that A B would catch will be spread over four times as much space at E D, nine times at L K ; and, consequently, it is only one-fourth as strong at double the distance, one-ninth at triple the distance, one-sixteenth at four times, and

so on.

So if we had a bell ringing at C, the amount of sound that would be caught by an ear-trumpet with an opening of, say, one square foot, placed at B, will have spread out, at double the distance, over four times the space; and the ear-trumpet there would catch only one-fourth of the sound it caught at B, and therefore the sound would be only one-fourth as strong.

In more technical language the law is expressed, "The intensity varies inversely as the square of the distance," that is to say, the intensity of light, sound, gravitation, &c., increases or decreases at the square of the rate that the distance decreases or increases.

20. Accordingly, what weighs a thousand pounds at the level of the sea weighs considerably less at the top of a mountain or when raised in a balloon, as is proved experimentally by a spring balance or other means.

Astronomical tests show that the amount of the earth's attraction on the moon is diminished according to the same law. The moon is about sixty times farther from the centre of the earth than we on its surface are; thus the force with which the moon is drawn to the earth is only about 1-3600th of its weight at the surface of our globe.

21. The gravitation existing between two masses or quantities of