512

Ascent of Liquids against. Gravity.

[Book V.

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

Numerous facts show, that when not pulled down by gravity, liquidsdiffuse themselves uniformly on substances with which they combine—asmuch upwards as downwards. Small drops of water or ink dashedagainst vertical sheets of paper equally extend themselves from thecentre. We are so much in the habit of contemplating fluids in masses,where gravitation greatly preponderates, that we overlook this propertyin them, or do not suspect its existence. The observation that waternever runs “ up hill” is proverbial, but it is not correct. Examples mightbe quoted, in which it prefers to ascend an inclined plane to going downone—to rise in a wet channel, than descend in a dry one. Take a drypiece of glass, or china, the blade of a knife, or the bottom of a saucer, oralmost any solid material, and dampen or slightly wet a part of it: placea drop of ink or water near the edge of the wetted part, then incline thesaucer so that the drop may be beneath, and make a channel of com-munication between them, by drawing with a pointed instrument a smallstreak of fluid from one to the other. The instant this is done, a cur-rent will set up with considerable velocity from the drop into the thinsheet above.

This effect takes place on wood and on metals, and even paper. Pen-men, who have their paper inclined towards them offen witness theexperiment in another form, especially when they make the bottom oftheir strokes thicker than the rest. The ink may then be seen to ascendfrom the bottom upon the removal of the pen. This takes place if thepaper be held vertically. Again, when a large drop of ink falls on abook, it is customary to shake out that which remains in the pen, and toplace the latter over the drop as in the act of writing; upon which a largeportion of the liquid enters the quill. This is then shaken, and the Opera-tion renewed. Here the principle of distribution again appears. Thereis a surplus below, and a deficiency (or less depth of it) above, and theliquid ascends to produce an equilibrium. Were the pen fully chargedwith ink before applied to the drop, it could take none from the latter.

Other examples of the ascent of liquids, and even of solids againstgravity are familiär to some classes of mechanics, but not to all. Whentwo sheets of tin plate are soldered together in an inclined position, smallpieces of solder laid near the lower edge of the joint are drawn up underthe face of the iron as soon as the fused mass touches them. Illustrationsof this occur in whatever position the joint may be. They are still morecommon in hard soldering, for copper and silversmiths commonly Chargetheir joints on the outside, so that the solder is below or next to the firewhen fused.

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