FIG. 2.

ing his authority, we shall designate them hereafter exclusively by this word.

The rude diagrams before you will help me to make clear the difference between the two suppositions I have made. In the first (Fig. 1), we assume that the material of this cubic inch is uniformly expanded through the cubic foot. In the other (Fig. 2), we have. in both volumes a definite number of molecules, the only difference being that these dots, which we have used to represent the molecules, are more widely separated in the one case than in the other. Now, which of these suppositions is the more probable? Let us submit the question to the test of experiment.

We have here a glass globe, provided with the necessary mountings—a stop-cock, a pressure-gauge, and a thermometer—and which we will assume has a capacity. of one cubic foot. Into this globe we will first

pour one

INTERSPACES IN VAPORS.

17

cubic inch of water, and, in order to reduce the conditions to the simplest possible, we will connect the globe with our air-pump, and exhaust the air, although, as it will soon appear, this is not necessary for the success of our experiment. Exposing, next, the globe to the temperature of boiling water, all the liquid will evaporate, and we shall have our vessel filled with ordinary steam. If, now, that cubic foot of space is really packed close with the material we call water-if there is no break in the continuity of the aqueous mass—we should expect that the vapor would fill the space, to the exclusion of every thing else, or, at least, would fill it with a certain degree of energy which must be overcome before any other vapor could be forced in. Now, what is the case? The stop-cock of the globe is so arranged that we can introduce into it an additional quantity of any liquid on which we desire to experiment, without otherwise opening the vessel. If, then, by this means, we add more water, the additional quantity thus added will not evaporate, provided that the temperature remains at the boiling-point. Let us next, however, add a quantity of alcohol, and what do we find? Why, not only that this immediately evaporates, but we find that just as much alcoholvapor will form as if no steam were present. The presence of the steam does not interfere in the least degree with the expansion of liquid alcohol into alcohol-vapor. The only difference which we observe is, that the alcohol expands more slowly into the aqueous vapor than it would into a vacuum. If, now that the globe is filled with aqueous vapor and alcoholvapor at one and the same time, each acting, in all respects, as if it occupied the space alone, we add a quantity of ether, we shall have the same phenomena re

peated. The ether will expand and fill the space with its vapor, and the globe will hold just as much ether-vapor as if neither of the other two were present; and so we might go on, as far as we know, indefinitely. There is not here a chemical union between the several vapors, and we cannot in any sense regard the space as filled with a compound of the three. It contains all three at the same time, each acting as if it were the sole occupant of the space; and that this is the real condition of things we have the most unquestionable evidence.

You know, for example, that a vapor or gas exerts a certain very considerable pressure against the walls of the containing vessel. Now, each of these vapors exerts its own pressure, and just the same pressure as if it occupied the space alone, so that the total pressure is exactly the sum of the three partial pressures.

Evidently, then, no vapor completely fills the space which it occupies, although equally distributed through it; and we can give no satisfactory explanation of the phenomena of evaporation except on the assumption that each substance is an aggregate of particles, or units, which, by the action of heat, become widely separated from each other, leaving very large intermolecular spaces, within which the particles of an almost indefinite number of other vapors may find place. Pass now to another class of facts, illustrating the same point.

The three liquids, water, alcohol, and ether, are expanded by heat like other forms of matter, but there is a striking circumstance connected with these phenomena, to which I wish to direct your observation. I have, therefore, filled three perfectly similar thermometerbulb tubes, each with one of those liquids. The tubes are mounted in a glass cell standing before the con

UNEQUAL EXPANSION IN LIQUIDS.

19

denser of a magic lantern, and you see their images projected on the screen. You also notice that the liquids (which have been colored to make them visible) all stand at the same height; and, since both the bulbs and the tubes are of the same dimensions, the relative change in volume of the inclosed liquids will be indicated by the rise or fall of the liquid columns in the tubes. We will now fill the cell with warm water, and notice that, as soon as the heat begins to penetrate the liquids, the three columns begin to rise, indicating an increase of volume; but notice how unequal is the expansion. The ether in the right-hand tube expands more than the alcohol in the centre, and that again far more than the water on the left. What is true of these three liquids is true in general of all liquids. Each has its own rate of expansion, and the amount in any case does not appear to depend on any peculiar physical state or condition of the liquid, but is connected with the nature of the substance, although, in what way, we are as yet wholly ignorant.

But you may ask: What is there remarkable in this? Why should we not expect that the rate of expansion would differ with different substances? Certainly, there is no reason to be surprised at such a fact. But, then, the remarkable circumstance connected with this class of phenomena has yet to be stated.

Raise the temperature of these liquids to a point a little above that of boiling water, and we shall convert all three substances into vapor. We thus obtain three gases, and, on heating these aëriform bodies to a still higher temperature, we shall find that, in this new condition, they expand far more rapidly than in the liquid state. But we shall also find that the influence of the nature of the substance on the phenomenon has wholly

disappeared, and that, in the aëriform condition, these substances, and in general all substances, expand at the same rate under like conditions.

Why, now, this difference between the two states of matter? If the material fills space as completely in the aëriform as it does in the liquid condition, then we cannot conceive why the nature of the substance should not have the same influence on the phenomena of expansion in both cases. If, however, matter is an aggregate of definite small masses or molecules, which, while comparatively close together in the liquid state, become widely separated when the liquids are converted into vapor, then it is obvious that the action of the particles on each other, which might be considerable in the first state, would become less and less as the molecules were separated, until at last it was inappreciable; and if, further, as Avogadro's law assumes, the number of these particles in a given space is the same for all gases under the same conditions, then it is equally obvious that, there being no action between the particles, all vapors may be regarded as aggregates of the same number of isolated particles similarly placed, and we should expect that the action of heat on such similar masses would be the same.

Thus these phenomena of heat almost force upon us the conviction that the various forms of matter we see around us do not completely fill the spaces which they appear to occupy, but consist of isolated particles separated by comparatively wide intervals. There are many other facts which might be cited in support of the same conclusion; and among these two, which are more especially worthy of your attention, because they aid us in forming some conception of the size of the molecules themselves.