Chap. 5.]

Forms of Drops.

511

gnats career on the surface of a pond as securely as on land. Some liquids are viscid, and may be drawn into threads; and even water may be stretched into sheets ere its substance be broken: bubbles produced during rains, and those pellicles sometimes formed over the mouths of small vials and the interstices of sieves are examples. Water, moreover, in common with other fluids, unites with some substances more readily than with others. It does not combine with oils, nor adhere to substances impregnated with grease. Hence umbrellas and water-proof dresses are made of oiled silk; and rain rolls off the backs of ducks and other aquatic birds without wetting them, because these fowls dress their feathers with an unctuous fluid which their bodies secrete.

When a vessel contains a liquid that readily unites with it, the liquid stands highest at the edges. Thus in cups of tea or tumblers of water, the fluid climbs up against the sides until it is considerably elevated above the general level. This is observable with milk in a pot, pitch in a cauldron, oil in cans, mercury in vessels lined with an amalgam; melted tin in tinned iron or copper vessels, and fused brass in an iron ladle whose interior has been coated with the alloy, as in the process of hard soldering. If, on the other hand, a liquid has no affinity for, or will not unite with the substance of which the vessel is made, an effect the reverse is produced; that is, the liquid is depressed at the sides, as when mercury is contained in a vessel of glass, wood, or earthen ware; or even in one of metal not lined with an amalgam, or with which the mercury cannot form one. The same thing occurs to fused brass, or lead or tin in crucibles, to water in greasy tubes or dishes, &c.

The same thing, in another form, occurs with drops of liquid. When water is sprinkled on a greasy surface, the particles remain separate however near to each other. By blowing against them, they may be rolled over the plate on which they rest without leaving any portion behind; but if the substance on which they are dropped combine readily with moisture their figure is changed; each becomes flattened by spreading, so that two adjacent drops quickly run together. A drop of oil or speck of grease makes a large stain on a lady's dress or a marble table. Quicksilver will not unite with marble, but a small portion dropped on a sheet of tin will spread over it like water on damp paper. A portion of tinmen's solder kept in fusion on clean plates of tin or lead spreads, and is absorbed in like manner. When ink is spilt upon unsized paper, the latter is stained to a considerable extent: round each drop a broad ring of moisture is formed; the darker and grosser particles remaining as a nucleus in the centre.

When

The different forms which drops assume when pendent from solid bodies, are governed by the parts with which they are in contact. water is sprinkled on a plate partly covered with grease, those particles that fall on the clean parts resemble very flat segments of spheres, while those on the greased parts are larger portions of smaller spheres; the liquid in these swelling out above the base on which they rest, in preference to extending itself like the others upon it. A drop hanging from the point of a wire is elongated vertically-if held between the finger and thumb, it may be stretched out horizontally. If suspended in a ring, its upper surface becomes hollow and its lower one convex, forming a species of liquid cup, and supported somewhat like the dishes which chemists hang over lamps in moveable rings of brass. A drop of liquid in a capillary tube is thus supported; the tube being nothing more than a deep ring.

The quantity of liquid contained in pendent drops varies with the

512

Ascent of Liquids against Gravity.

[Book V. extent of surface in contact with the supporting body. When one is ready to fall from an inclined object, as the bottom of a bucket or a tea cup, it may be retained by making the bottom approach nearer to a level; the fluid then spreads and holds by a larger surface. This is illustrated in the case of metals: tin-plate workers commonly take up solder on the face of their irons. The under sides of these instruments are tinned, and being placed upon the metal, a larger or smaller portion is melted and borne off at pleasure. An equal quantity of water may probably be thus suspended from a plane surface, as within a cylinder of the same

area.

66

Numerous facts show, that when not pulled down by gravity, liquids diffuse themselves uniformly on substances with which they combine-as much upwards as downwards. Small drops of water or ink dashed against vertical sheets of paper equally extend themselves from the centre. We are so much in the habit of contemplating fluids in masses, where gravitation greatly preponderates, that we overlook this property in them, or do not suspect its existence. The observation that water never runs up hill" is proverbial, but it is not correct. Examples might be quoted, in which it prefers to ascend an inclined plane to going down one-to rise in a wet channel, than descend in a dry one. Take a dry piece of glass, or china, the blade of a knife, or the bottom of a saucer, or almost any solid material, and dampen or slightly wet a part of it: place a drop of ink or water near the edge of the wetted part, then incline the saucer so that the drop may be beneath, and make a channel of communication between them, by drawing with a pointed instrument a small streak of fluid from one to the other. The instant this is done, a current will set up with considerable velocity from the drop into the thin sheet above.

This effect takes place on wood and on metals, and even paper. Penmen, who have their paper inclined towards them often witness the experiment in another form, especially when they make the bottom of their strokes thicker than the rest. The ink may then be seen to ascend from the bottom upon the removal of the pen. This takes place if the paper be held vertically. Again, when a large drop of ink falls on a book, it is customary to shake out that which remains in the pen, and to place the latter over the drop as in the act of writing; upon which a large portion of the liquid enters the quill. This is then shaken, and the operation renewed. Here the principle of distribution again appears. There is a surplus below, and a deficiency (or less depth of it) above, and the liquid ascends to produce an equilibrium. Were the pen fully charged with ink before applied to the drop, it could take none from the latter.

Other examples of the ascent of liquids, and even of solids against gravity are familiar to some classes of mechanics, but not to all. When two sheets of tin plate are soldered together in an inclined position, small pieces of solder laid near the lower edge of the joint are drawn up under the face of the iron as soon as the fused mass touches them. Illustrations of this occur in whatever position the joint may be. They are still more common in hard soldering, for copper and silversmiths commonly charge their joints on the outside, so that the solder is below or next to the fire when fused.

These experiments are all based on the same principles as the ascent of water in capillary tubes. We see that when a mass of liquid (wholly resting on a plane surface or enclosed in a cylinder) is connected by a short channel to a thin sheet of the same substance above, a part of the mass below will ascend. The channel it should be remembered is a fluid

Chap. 5.]

Cohesion of Liquids.

513

one, for neither water nor any other liquid will thus rise except in channels of the same substance as themselves. The effect does not therefore appear to be due wholly to the material that sustains the liquid, but, to some extent, to that force by which particles of matter congregate with their kind in preference to mingling with others. The aqueous vapor floating in the atmosphere moistens more or less the surfaces of all bodies. Glass tubes are coated with it; but if a capillary tube previous to its use was not thus prepared, it becomes so the instant one end is immersed in water—a stream of vapor (though not obvious to sight) then passes through it: the whole interior is thus coated with aqueous moleculæ accumulating upon it at insensible distances from each other, and those adjacent to the surface of the liquid operate to solicit its ascent through the channel thus prepared for it. The ascent of vapor under these circumstances is unlimited, but that of a liquid column is soon arrested. This however does not prove that the force excited is insufficient to raise liquids to great elevations, but that it is the volume which determines the height. If the quantity be indefinitely small it will be raised indefinitely high. Experiments so far as they have been made prove this; but as the finest of artificial tubes are, when compared to nature's, as a mast is to a needle or a cable to a thread, the ascent of liquids in them must necessarily be very limited. As long as the liquid column can be sustained by adhesion to the sides of a tube it will rise, but when the weight of the central parts (which not being attached to the tube are sustained by cohesion alone) exceeds this force, the ascent ceases.

The force with which particles of some fluids cohere is so energetic that they present the singular spectacle of liquid rods, pendent like icicles or stalactites. When one of these rods is broken an interesting contest between gravitation and cohesion takes place, during which the figure of the pendent changes as one or the other of those forces prevails: it becomes longer while the first predominates, shorter when the latter controls, and stationary when both are balanced. These phenomena may be observed by letting a drop of molasses fall from the point of a knife or a spoon. The globule descends to a considerable distance before it is wholly separated from the portion above, because a rod of the liquid continues to be formed that unites them. When this rod breaks, the part suspended from the mass above is drawn up: a thread over a foot in length is sometimes thus contracted to less than of an inch, strongly reminding one of the elasticity of caoutchouc.

Water rises to considerable heights through sand and other porous bodies also through rags and threads of cotton, &c. Oil ascends in the wicks of lamps. Capillary siphons formed of cotton wick are employed to supply oil to the journals and working parts of machinery. It is customary with stereotype founders to oil the faces of engraved wooden blocks previous to taking casts from them. These blocks are of box, a species of wood whose texture is exceedingly close. We have often placed some of those used in the illustration of this work on receiving them from the engraver, into a dish containing oil to the depth of inch, and have witnessed the appearance of the liquid at the top within half a minute, and frequently in a quarter of one. Unlike water in glass tubes, the oil here rises entirely out of the tubes in the wood and collects in globules over the orifices.

From the infinite variety and importance of devices for raising liquids that are at work in the animal and vegetable kingdoms and in general nature, the wisdom displayed in their formation and movements, and their wonderful effects, it would seem as if the Creator designed particularly

514

Siphons.

[Book V. to call man's attention to this department of knowledge, and to induce him to cultivate it.

Sources of hydraulic contrivances and of mechanical movements are endless in nature; and if machinists would but study in her school, she would lead them to the adoption of the best principles, and the most suitable modifications of them in every possible contingency.

CHAPTER VI.

SIPHONS-Mode of charging them-Principle on which their action depends-Cohesion of liquids— Siphons act in vacuo-Variety of siphons-Their antiquity-Of Eastern origin-Portrayed in the tombs at Thebes Mixed wines-Siphons in ancient Egyptian kitchens-Probably used at the feast at CanaTheir application by old jugglers-Siphons from Heron's Spiritalia-Tricks with liquids of different specific gravities-Fresh water dipped from the surface of the sea-Figures of Tantalus' cups-Tricks of old publicans-Magic pitcher-Goblet for unwelcome visiters-Tartar necromancy with cups-Roman baths-Siphons used by the ancients for tasting wine-Siphons, A. D. 1511—Figures of modern siphonsSucking tube-Valve siphon-Tin plate-Wirtemburg siphon-Argand's siphon-Chemists' siphons— Siphons by the author-Water conveyed over extensive grounds by siphons-Limit of the application of siphons known to ancient Plumbers-Error of Porta and other writers respecting siphons-Decaus-Siphons for discharging liquids at the bend-Ram siphon.

THE siphon, or as it is sometimes named the crane, is in its simplest form merely a tube bent so as to resemble an inverted letter U or V; and is employed to transfer liquids from one level to a lower one, in circumstances where natural or artificial obstructions prevent a straight pipe from being used; as when rocks or rising grounds intervene between a spring and the place where the water is required, or when the contents of casks and other vessels are to be withdrawn without making openings for the purpose in their bottom or sides. Thus farmers occasionally have water conveyed over hills to supply their barn-yards and dwellings; and portable siphons are in constant requisition with oil and liquor merchants, chemists and distillers. The two branches of a tube that constitute a siphon are commonly of unequal lengths, and named legs; the "short" or receiving leg, and the "long" or discharging one. The highest part where the legs are united is known as the apex or bend.

As liquids are raised in siphons by atmospheric pressure, the perpendicular length of the short leg, like the suction pipe of a pump, should never exceed 25 or 28 feet. To put siphons in operation, the air within them must be first expelled. Small ones are sometimes inverted and filled with a portion of the fluid to be decanted, but more frequently the liquid is drawn through the tube by sucking. Other devices for charging them will be noticed farther on.

The action of a siphon does not depend upon any inequality of atmospheric pressure, as some writers on natural philosophy have inadvertently intimated. In one popular work, it is said, "the pressure of the air is more diminished;" and in another, more "weakened or abated" over the discharging than over the receiving orifice; whereas, philosophically speaking, the reverse is the fact: for as the discharging end is nearer the earth, a deeper and consequently heavier column of atmosphere rests over

Chap. 6.]

Variety of their forms and materials.

515

it than over the other. Nor does the effect depend upon any difference in the actual lengths of the legs, for they are often in this respect the same; and sometimes the receiving one is much longer (in an oblique direction) than the other-not yet does their comparative diameters contribute to the results; for the short one may be much more capacious than the long one. It is the difference in the perpendicular length of the liquid columns within the legs that causes a siphon to act: the column in the

No. 234.

discharging leg must exceed in this respect that contained in the receiving one, or no action can take place. By examining the figure in the margin, it will be perceived that the column in the receiving leg extends only from the surface of the liquid in the vessel to the bend, whereas in the other it extends from the bend to the orifice. As the pressure of fluids is as their depth without regard to their volume, the hydrostatic equilibrium of the two columns is destroyed, when the longer one necessarily preponderates, upon which the vacuity left in the upper part of the tube is filled, by the atmosphere driving fresh portions up the other leg.

But siphons could not act at all were it not for that property of fluids by which their particles cling to each other. The tenacity of liquids may be considered like that of solids, only less intense; and thus it is when water flows through a siphon, the descending particles actually drag down those above them, somewhat like a chain or rope unequally suspended over a pulley, when the longer end pulls the shorter one after it. A siphon is in fact a contrivance by which liquid chains or ropes are thus made to act. But for the cohesion of liquids the contents of the discharging leg would drop out like sand, and no further effect would follow-the rope would be broken, and the separated parts fall asunder. The influence of cohesion in the action of siphons is proved by the fact that very short ones continue to operate when removed into a

vacuum.a

The tenacity, or what might almost be called the malleability of liquids, is beautifully exemplified in soap bubbles. These yield to impressions without breaking. They fall on and rebound from the floor like bladders or balls of india rubber. They shake in the wind, and their figures become altered like that of balloons tossed to and fro in the air: all this they often endure before their shells are broken by evaporation.

Siphons are exceedingly diversified in their forms, materials and uses. They are made of cylindrical and other shaped tubes, and both of uniform and irregular bore. The legs of some are parallel, while in others they meet at every angle-sometimes straight and often crooked-one may be larger than the other, or both may be alike; they also may be separate, one loosely slipping into or over the other. Instead of tubes siphons are sometimes formed by an arrangement of plates, and also by a combination of vases. This plastic property has occasioned their concealment in more various forms than Proteus ever assumed. Siphons are made of tin, copper, iron, silver, glass, lead, earthenware, leather, wood, canes; and (capillary ones) of paper, strips of cloth, threads of cotton, &c. Examples of their various forms and applications will be found noticed in the following historical sketch.

The origin of siphons like that of pumps is lost in antiquity. Some

For information on the action of siphons in vacuo, see Boyle's Works, by Shaw, vol. ii. 446. History and Memoirs of the French Academy, translated by Martin and Chambers, vol. iv. 374; and Desaguliers Exper. Philos. vol. ii. 168.