If the same plates are plunged into mercury, there will be a depression according to an analagous law.

B

If two plates of glass, AB and AC, inclined to each other, as shown in Fig. 160, their line of junction being vertical, be plunged into any liquid which will moisten them, the liquid will rise between them. It will rise higher near the junction, the surface taking a curved

Fig. 160.

form, such that any section made by a plane through A, will be an equilateral hyperbola. This form of the elevated fluid conforms to the laws above explained.

If the line of junction of the two plates is horizontal, a small quantity of a liquid between them, which will moisten them, will assume the shape shown at A. If the liquid does. not moisten the plates, it will take the form shown at B.

Fig. 161.

Attraction and Repulsion of Floating Bodies. 183. If two small balls of wood, both of which can be moistened by water, or two small balls of wax, which cannot be moistened by water, be placed in a vessel of water, and brought so near each other that the surfaces of capillary elevation or depression interfere, the balls will attract each other and come together. If one ball of wood and one of wax be brought so near that the surfaces of capillary elevation and depression interfere, the bodies will repel each other and separate. If two needles be carefully oiled and laid upon the surface of a vessel of water, they will repel the water from their neighborhood, and float. If, whilst floating, they are brought sufficiently near to each other to permit the surfaces of capillary depression to interfere, the needles will immediately rush together. The reason of the needles floating is, that they repel the water, heaping it up on each side, thus forming a cavity in the surface; the needle is buoyed up by a force equal to the weight of the displaced fluid, and, when this exceeds the weight of the

needle, it will float. It is on this principle that certain insects move freely over the surface of a sheet of water; their feet are lubricated with an oily substance which repels the water from around them, producing a hollow around each foot, and giving rise to a buoyant effort greater than the weight of the insect.

The principle of mutual attraction between bodies, both of which repel water, or both of which attract it, accounts for the fact that small floating bodies have a tendency to collect in groups about the borders of the containing vessel. When the material of which the vessel is made, exercises a different capillary action from that of the floating particles, they will aggregate themselves at a distance from the surface of the vessel.

Applications of the Principles of Capillarity.

184. It is in consequence of capillary action that water rises to fill the pores of a sponge, or of a lump of sugar. The same principle, causes the oil to rise in the wick of a lamp, which is but a bundle of fibres very nearly in contact, leaving capillary interstices between them.

The siphon filter differs but little in principle from the wick of a lamp. It consists of a bundle of fibres like a lamp-wick, one end of which dips into a vessel of the liquid to be filtered, whilst the other hangs over the edge of the vessel. The liquid ascends the fibrous mass by the principle of capillary attraction, and continues to advance till it reaches the overhanging end, when, if this is lower than the upper surface of the liquid, the liquid will fall by drops from the end of the wick, the impurities being left behind.

The principle of capillary attraction is used for splitting rocks and raising weights. To employ this principle in cleaving mill-stones, as is done in France, the stone is first dressed to the form of a cylinder of the required diameter for the mill-stone. Grooves are then cut around it where the divisions are to take place, and into these grooves thoroughly dried wedges of willow-wood are driven. On being exposed to the action of moisture, the cells of the

wood absorb a large quantity of water, expand, and finally split the rock.

To raise a weight, let a thoroughly dry cord be fastened to the weight, and then stretched to a point above. If, now, the cord be moistened, the fibres will absorb the moisture, expanding laterally, the rope will be diminished in length, and the weight raised.

The principle of capillary attraction is also very extensively employed in metallurgy, in a process of purifying metals, called cupellation.

Endosmose and Exosmose.

185. The names endosmose and exosmose have been given to two currents flowing in a contrary direction between two liquids, when they are separated by a thin porous partition, either organic or inorganic. The discovery of this phenomena is due to M. DUTROCHET, Who called the flowing in, endosmose, and the flowing out, exosmose. The existence of the currents was established by means of an instrument, to which he gave the name endosmometre. This instrument consists of a long tube of glass, at one end of which is attached a membranous sack, secured by a tight ligature. If the sack is filled with gum water, a solution of sugar, albumen, or, in fact, with almost any solution denser than water, and then plunged into water, it is observed, after a time, that the fluid rises in the stem, and is depressed in the vessel, showing that water has entered the sack by passing through the pores. By applying suitable tests, it is also found, that a portion of the liquid in the sack has passed through the pores into the vessel.

If the operation

Two currents are thus established. be reversed, and the bladder and tube be filled with pure water, the liquid in the vessel will rise, whilst that in the tube falls. The phenomena of endosmose and exosmose are extremely various, and serve to explain a great variety of interesting facts in animal and vegetable physiology. The cause of the currents is the action of molecular forces exerted between the particles of the bodies employed.

CHAPTER VIII.

MECHANICS OF GASES AND VAPORS.

Gases and Vapors.

186. Gases and vapors are distinguished from other fluids, by their great compressibility, and correspondingly great elasticity. These fluids continually tend to occupy a greater space; this expansion goes on till counteracted by some extraneous force, as that of gravity, or the resistance offered by a containing vessel.

The force of expansion, which is common to all gases and vapors, is called their tension or elastic force. We shall take for the unit of this force at any point, the pressure which would be exerted upon a square inch of surface, were the pressure the same at every point of the square inch as at the point in question. If we denote this unit, by p, the area pressed, by a, and the entire pressure, by P, we shall have,

Most of the principles already demonstrated for liquids hold good for gases and vapors, but there are certain properties arising from elasticity which are peculiar to æriform fluids, some of which it is now proposed to investigate.

Atmospheric Air.

187. The gaseous fluid which envelops our globe, and extends on all sides to a distance of many miles, is called the atmosphere. It consists principally of nitrogen and oxygen, together with variable, but small portions of watery vapor and carbonic acid, all in a state of mixture. On an average, it is found by experiment that 1000 parts by volume of

atmospheric air, taken near the surface of the earth, consists

of about,

788 parts of nitrogen,

197 parts of oxygen,

14 parts of watery vapor,

1 part of carbonic acid.

The atmosphere may, physically speaking, be taken as a type of gases, for it is found by experiment that the laws regulating the density, expansibility, and elasticity, are the same for all gases and vapors, so long as they maintain a purely gaseous form. It is found, however, in the case of vapors, and of those gases which have been reduced to a liquid form, that the law changes just before actual lique

faction.

This change appears to be somewhat analagous to that observed when water passes from the liquid to the solid form. Although water does not actually freeze till reduced to a temperature of 32° Fah., it is found that it reaches its maximum density at about 38°.75, at which temperature the particles seem to commence arranging themselves according to some new laws, preparatory to taking the solid form.

Atmospheric Pressure.

B

188. If a tube, 35 or 36 inches long, open at one end and closed at the other, be filled with pure mercury, and inverted in a basin of the same, it is observed that the mercury will fall in the tube until the vertical distance from the surface of the mercury in the tube to that in the basin is about 30 inches. This column of mercury is sustained by the pressure of the atmosphere exerted upon the surface of the mercury in the basin, and transmitted through the fluid, according to the general law of transmission of pressures. The column of mercury sustained by the elasticity of the atmosphere is called the barometric column, because it is generally measured by an instrument called a barometer. In fact, the instrument just described, when

Fig. 162.