One of the most remarkable features in the recent progress of Astronomy is the way in which it has shown that the greatest and the smallest things in Nature are not at the two extremities of a longcontinued upward slope, but are mingled in the closest intimacy. Astronomy illustrates the phenomena of electricity in a Comet, of heat in the Sun, of light in a Star or a Planet, of gaseous radiation in a Nebula, upon a scale which is immensely great, by means of the violent encounters or collisions, or (in plain English) by the knocks of most minute atoms and molecules.

In Astronomy we have to do with the greatest things in Nature. The Sun is 90 millions of miles distant from the Earth, and of a bulk l} million times as great. The planet Neptune is 30 times as far away as the Sun. The nearest to us, so far as we know, of all the stars (a Centauri) is well-nigh 10,000 times as distant as the planet Neptune; while beyond it are hundreds of millions of stars further and yet further off. Some that can be just detected are probably 10,000 times as remote as a Centauri, or, in other words, 3,000 million times as far away as the Earth is from the Sun.

But the light that comes across those distances, and reveals those far-away orbs, reaches us through movements and vibrations due to molecules far smaller than any microscope can reveal. That light shakes the minute molecules of a photographic plate placed in the focus of a telescope, and leaves behind the record of its knocks. It vibrates in the bright lines of Solar and Stellar Spectra. In a no less wonderful way molecular knocks—most minute, but most numerous—transmit and maintain the heat of the Sun and of the Stars.

But it may be asked : What are molecules and atoms? Can we affirm their existence ? Can we measure their size or detect their action? Can we count them, or determine the number and energy of their knocks, if they are so minute ?

An atom literally means that which cannot be cut. According to the Atomic Theory of the constitution of matter, all bodies are supposed to be made up of atoms. An atom, therefore, represents the smallest possible quantity of any elementary body, a quantity incapable of subdivision, if indeed such a conception of indivisibility is possible.

A molecule literally means a little mass, and is considered to be an aggregation of a certain number of atoms; in general, of atoms of different elements, but in some cases, it may be, of atoms of a like kind. Molecules are held to form the ultimate constituent particles of a compound body. The molecules of such a body cannot be divided if it is to retain its nature as a compound. They will, however, be resolved into constituent atoms, if the compound body be resolved, by some process or other, into its constituent elements. So long, for instance, as water is water, its molecules each consist of two atoms of hydrogen joined to one of oxygen. But, if a volume of water be resolved, by heat or electricity, into two separate volumes of oxygen and hydrogen, each molecule of the water is thereby resolved into its constituent atoms. All the atoms of oxygen go together to make up the total volume of oxygen, and all those of hydrogen to form the total volume of hydrogen, obtained from the given volume of water. So also in other similar cases.

There must be a certain maximum limit of size for the molecules in any compound body and for the atoms which compose them. If we could take a drop of water and divide it into two equal parts, and repeat the process with each half, again and again, a time would come when we should at last be forced to divide a molecule, and break it up into its atoms. Those atoms would be oxygen or hydrogen, but they would no longer be water. Sooner or later, according to what the size of a molecule may be, this would occur, otherwise water would not be the compound that it is.

The hypothesis that all bodies are made up of ultimate atoms, and that, in each compound body, a certain regular number of the atoms of its components are combined into molecules, is accepted, because it explains so many of the simpler and of the most complicated phenomena of chemistry and of other kindred sciences. Nevertheless molecules or atoms are believed to be of a diameter from 250 to 500 times too small for the most powerful microscope to reveal them.

To attempt to measure, in any way, the size of particles so minute might almost seem to be hopeless. The measurement has nevertheless been made, not perhaps very accurately, but with a remarkable amount of success, compared with the difficulty of the problem. For instance, a soap bubble has been formed, in which the film was proved to have a thickness less than www of an inch. Pure water

| Last year, in his Inaugural Address to the British Association, Lord Salisbury remarked : What the atom of each element is, whether it is a movement, or a thing, or a vortex, or a point having inertia; whether there is any limit to its divisibility, and, if so, how that limit is imposed .... all these questions remain surrounded by a darkness as profound as ever.'

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would not have held together to form such a bubble. But the admixture of a small proportion of soap gave it the requisite tenacity. Hence it was concluded, that, in any little cube of water, measuring less than wwe of an inch in the length of its side, there was at least one molecule of soap occupying only a small part of that little volume. How minute, therefore, a molecule of soap must be ! It has, in fact, been calculated that, in such a case, it would be less than 1206.00, of an inch in diameter.

If, however, further experiments are performed, determining the tension overcome and the heat produced in expanding such a bubble (which tension and heat depend upon the number of molecules in the thickness of its film), a diameter is indicated for the molecules of water decidedly, but not greatly, less than of an inch. This very minute, although finite, divisibility of matter has been in some degree confirmed by the sense of taste, or colour, or smell, in cases of extreme dilution; and very decisively by the spectroscopic analysis of the light of a flame, when there has been a quantity of sodium, or of other substances, vaporised in it so small that it would take several million times as much to weigh a single grain.

It has also been shown by Lord Kelvin that a certain amount of electrical action, involving the generation of heat, occurs when zinc and copper are brought into contact, which heat would be greater the more numerous the atoms in any given quantity of the metals. And, from the observed amount of heat produced when zinc and copper are used to form that alloy which we term brass, he has concluded a that the constituent atoms of copper, or zinc, cannot be much, if at all, less than ...wo of an inch in diameter, but that they may be considerably larger. This gives an approximation to a minimum value for the size of an atom.

Careful calculations as to the effect of the molecules of a prism, in dispersing into a lengthened spectrum the variously coloured component undulations of a ray of white light passed through it, further confirm the above statements.

It may be assumed from these, as well as from other lines of investigation, that the diameter of the ultimate molecules, or atoms, of bodies very probably lies somewhere between 06.00 of an inch and 7.0.0,000 of an inch. They cannot well be much larger, or much smaller. And if it be said that there is a considerable difference between these two sizes, the answer is that it matters little whether we can state a certain limit, or one a hundred times smaller, in comparison with the achievement of having determined such limits at all. The actual range of possible size, just stated, is almost as nothing compared with that which might have seemed to be probable.

But there remains still to be mentioned one more instance of

? Popular Lectures and Addresses, vol. i. p. 173.

molecular action which has been investigated with even greater fulness. It is one which is intimately connected with recent Astronomy, and one which brings us into the closest relation to those knocks of which we have already made mention.

It is the kinetic, i.e. the movement, theory of gases, involving the distinction between the solid, liquid, and gaseous states of matter. In the solid state of matter, the atoms, or molecules, cannot be moved about amongst one another without the expenditure of considerable force to overcome the cohesion which holds them together. In the liquid state, while they still resist being torn apart, they are so far in a less restrained condition that they can be easily moved round one another. In the gaseous state they are quite free from cohesion, and are believed to be flying about in all directions with immense velocity, constantly knocking against each other, or against any surface within which a gas is contained.

Upon this supposition all the phenomena of gases can be explained. Heat expands a gas in making the molecules move more violently. Pressure heats a gas by affording additional energy to them. Expansion cools a gas when the molecules use up their energy in expanding it. A gas presses upon any containing surface by means of the knocks of its molecules. If a skin filled with gas be placed under the cover ofan air-pump, and the surrounding air be exhausted, then the gas within the skin will swell it out. Why so ? Because of the energy of the knocks of the molecules of the gas inside. Those molecules are constantly flying about and hitting the inner surface of the skin, but their knocks are not now counterbalanced (as they were before the air-pump was worked) by the knocks of the air-molecules outside. Once more, if a gas be compressed, then (apart from any alteration in its temperature) it is found that every time the space occupied by it is halved its pressure upon the containing surface is doubled. Why so ? Because the same number of molecules are in one half of the previous space, and therefore their knocks upon any part of the bounding surface are twice as frequent as before.

All this confirms the theory of the incessant movement of the molecules of gases; while those molecules must be within the limits of size already stated. But it may next be asked: At what speed, or speeds, do their movements take place within the volume of any mass of gas ? Can their velocities be determined ? Yes! So far as regards their average speed in any given gas. That average speed must be such as will enable the molecules of a given volume of gas to produce by their knocks the pressure actually experienced by the surface which contains the gas. It is also possible, by observing the rate at which two volumes of gas, allowed to intermingle, are diffused into one another, to determine how far the molecules of any given gas move between their successive knocks against each other. We cannot describe such investigations more fully here.

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it suffice to say that they indicate that the molecules or atoms of each individual gas have their own special average rate of motion. To those of hydrogen, for example, which, owing to its light density, move with especial rapidity, a speed is assigned of about 6,000 feet per second, or seventy miles per minute, at the zero temperature of the Centigrade thermometer-a velocity about six times as great as the average speed of a cannon ball. These gaseous molecules are so numerous that the most careful mathematical and physical calculations indicate that, under ordinary temperature and pressure, every molecule of hydrogen undergoes about 18,000 millions of knocks from other molecules in every successive second.

In the Earth's atmosphere, the molecules of oxygen, one of its two principal components, move, upon an average, with about one-fourth of the speed of those of hydrogen, and inflict proportionally fewer knocks upon one another. Those of nitrogen, which forms its other chief component, move with a speed a little greater than those of oxygen. In the vapour of water the speed is about one-third greater than in oxygen.

We assume, then, that all gases are composed of atoms or molecules, of which there are millions of millions of millions in a cubic inch. These myriads of mites are ever flying about with intense velocities. Each knocks against, or encounters, its fellows, it may be 5,000 millions of times, it may be 20,000 millions of times, in a second. By the energy of these knocks heat is evolved, or pressure produced upon any surface which bounds or restrains the gas.

But what have these knocks to do with Astronomy? We shall presently show their relation to the maintenance of the Sun's temperature. There is, however, another interesting question connected with them, which we will now mention. It has been asked : May not the great velocities of these molecules in the gases which form a planet's atmosphere enable them to run away from any such planet, so that either the whole of its atmosphere, or certain constituent gases belonging to it, may thus be gradually lost ?

The answer must depend upon the power of attraction of the planet, at a given distance from its centre, as compared with the velocity of any molecule there situated. If a particle were placed at rest at a certain distance from an attracting globe, it would begin to move towards the globe, with constantly increasing speed, until it should reach its surface. On reaching the globe its velocity would depend partly upon the mass of the attracting body. That velocity would also be greater, the further off the point from which it started. But, however far away that point might be, mathematical calculations prove that the velocity, when the particle should reach the surface of the globe, could never exceed a certain limit of value. In the case of the Sun, the Earth, Mars, and the Moon, those limiting velocities would be respec

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