sufficiently near to produce equilibrium between the attractive and repulsive forces, but not near enough to admit of any influence from their form, perfect mobility will exist among them, resulting from the similarity of their attractions, and they will offer great resistance when compressed, properties which characterize fluids, in which the repulsive principle is greater than in the gases. When the distance between the particles is still less, solids are formed in consequence of the preponderating force of cohesion; but the nature of their structure will vary, because, at such small distances, the power of the mutual attraction of the particles will depend upon their form, and will be modified by the sides they present to one another during their aggregation.

All the phenomena of capillary attraction depend upon the cohesion of the particles of matter. If a glass tube of extremely fine bore, such as a small thermometer tube, be plunged into a cup of water or alcohol, the liquid will immediately rise in the tube above the level of that in the cup, and the surface of the little column thus suspended will be concave. If the same tube be plunged into a cup full of mercury, the liquid will also rise in the tube, but it will never attain the level of that in the cup, and its surface will be convex. The elevation or depression of the same liquid in different tubes of the same matter is in the inverse ratio of their internal diameters, and altogether independent of their thickness. Whence it follows that the molecular action is insensible at sensible distances, and that it is only the thinnest possible film of the interior surface of the tubes that exerts a sensible action on the liquid. So much indeed is this the case, that, when tubes of the same bore are completely wet with water through

out their whole extent, mercury will rise to the same height in all of them, whatever be their thickness or density, because the minute coating of moisture is sufficient to remove the internal column of mercury beyond the sphere of attraction of the tube, and to supply the place of a tube by its own capillary attraction. The forces which produce the capillary phenomena are the reciprocal attraction of the tube and the liquid, and of the liquid particles to one another; and in order that the capillary column may be in equilibrio, the weight of that part of it which rises above or sinks below the level of the liquid in the cup must balance these forces.

The estimation of the action of the liquid is a difficult part of this problem. La Place, Dr. Young, and other mathematicians, have considered the liquid within the tube to be of uniform density; but Poisson, in one of those masterly productions in which he elucidates the most abstruse subjects, has recently proved that the phenomena of capillary attraction depend upon a rapid decrease in the density of the liquid column throughout an extremely small space at its surface. Every indefinitely thin layer of a liquid is compressed by the liquid above it, and supported by that below; its degree of condensation depends upon the magnitude of the compressing force, and as this force decreases rapidly towards the surface, where it vanishes, the density of the liquid decreases also. M. Poisson has shown that, when this force is omitted, the capillary surface becomes plane, and that the liquid in the tube will neither rise above nor sink below the level of that in the cup; but, in estimating the forces, it is also necessary to include the variation in the density of the capillary surface round the edges, from the attraction of the tube.

The direction of the resulting force determines the curvature of the surface of the capillary column. In order that a liquid may be in equilibrio, the force resulting from all the forces acting upon it must be perpendicular to the surface. Now, it appears that, as glass is more dense than water or alcohol, the resulting force will be inclined towards the interior side of the tube, therefore the surface of the liquid must be more elevated next the sides of the tube than in the centre, in order to be perpendicular to it, so that it will be concave, as in the thermometer. But as glass is less dense than mercury, the resulting force will be inclined from the interior side of the tube, so that the surface of the capillary column must be more depressed next the sides of the tube than in the centre, in order to be perpendicular to it, and is consequently convex, as may be perceived in the mercury of the barometer when rising. The absorption of moisture by sponges, sugar, salt, &c., are familiar examples of capillary attraction; indeed the pores of sugar are so minute, that there seems to be no limit to the ascent of the liquid. The phenomena arising from the force of cohesion are innumerable: the spherical form of rain-drops and shot, the rise of liquids between plane surfaces, the difficulty of detaching a plate of glass from the surface of water, the force with which two plane surfaces adhere when pressed together,—are all effects of cohesion, entirely independent of atmospheric pressure, and are included in the same analytical formulæ, which express all the circumstances accurately, although the law according to which the forces of cohesion and repulsion vary is unknown, except that they only extend to insensible distances.

The difference between the forces of cohesion and re

pulsion is called molecular force, and, when modified by the electrical state of the particles, is the general cause of chemical affinities, which only take place between particles of different kinds of matter, though not under all circumstances. Two substances may indeed be mixed, but they will not combine to form a third substance different from both, unless their component particles unite in definite proportions. That is to say-one volume of one the substances will unite with one volume of the other, or with two volumes, or with three, &c., so as to form a new substance, but in any other proportions it will only form a mixture of the two. For example, one atom of hydrogen gas will combine with eight atoms of oxygen, and form water; or it will unite with sixteen atoms of oxygen, and form deutoxide of hydrogen; but added to any other atom of oxygen, it will merely be a mixture of the two gases. This law of definite proportion, established by Dalton of Manchester, being universal, is one of the most important discoveries in physical science, and furnishes unhoped-for information with regard to the minute and secret operations of nature in the ultimate particles of matter, whose relative weights are thus made known. It would appear also that matter is not infinitely divisible, and Dr. Wollaston has shown that in all probability, the atmospheres of the sun and planets, as well as of the earth, consist of ultimate atoms, no longer divisible, and if so, that our atmosphere will only extend to that point where the terrestrial attraction is balanced by the elasticity of the air.

All substances may be compressed by a sufficient force, and are said to be more or less elastic according to the facility with which they regain their volume when the

pressure is removed, a property which depends upon the repulsive force of their particles. But the pressure may be so great as to bring the particles near enough to one another to come within the sphere of their cohesive force, and then an aeriform fluid may become a liquid, and a liquid a solid. Mr. Faraday has reduced some of the gases to a liquid state by very great compression; but although atmospheric air is capable of a great diminution of volume, it always retains its gaseous properties, which resume their primitive volume the instant the pressure is removed, in consequence of the elasticity occasioned by the mutual repulsion of its particles.

SECTION XVI.

The atmosphere is not homogeneous; it appears from analysis that, of 100 parts, 79 are azotic gas, and 21 oxygen, the great source of combustion and animal heat. Besides these, there are three or four parts of carbonic acid gas in 1000 parts of atmospheric air. These pro

portions are found to be the same at all heights hitherto attained by man. The air is an elastic fluid, resisting pressure in every direction, and is subject to the power of gravitation: for, as the space in the top of a tube of a barometer is a vacuum, the column of mercury suspended by the pressure of the atmosphere on the surface of the cistern is a measure of its weight; consequently, every variation in the density occasions a corresponding rise or fall in the barometrical column. The pressure of the atmosphere is about fifteen pounds on every square inch, so that the surface of the whole globe sustains a weight