Chap. 5.

Cohesion of Liquids.

513

one, for neither water nor any other liquid will thus rise except in channelsof the same substance as themselves. The effect does nottherefore appearto be due wholly to the material that sustains the liquid, but, to some ex-tent, to that force by which particles of matter congregate with their kindin preference to mingling with others. The aqueous vapor floating in theatmosphere moistens more or less the surfaces of all bodies. Glass tubesare coated with it; but if a capillary tube previous to its use was not thusprepared, it becomes so the instant one end is immersed in water—a.stream of vapor (though not obvious to sight) then passes thsough it: thewhole interior is thus coated with aqueous moleculss accumulating uponit at insensible distances from each other, and those adjacent to the surfaceof the liquid operate to solicit its ascent through the channel thus preparedfor it. The ascent of vapor under these circumstauces is unlimited, butthat of a liquid column is soon arrested. This however does not prove thatthe force excited is insufficient to raise liquids to great elevations, but thatit is the volume which determines the height. If the quantity be indefi-nitely small it will be raised indefinitely high. Experiments so far asthey have been made prove this ; but as the finest of artificial tubes are,when compared to nature’s, as a mast is to a needlo or a cable to a thread,the ascent of liquids ui them must uecessarily be very limited. As longas the liquid column can be sustained by adhesion to the sides of a tubeit will rise, but when the weight of the central parts (which not beingattached to the tube are sustained by cohesion alone) exceeds this force,the ascfent ceases.

The force with which particles of some fluids cohere is so energeticthat they present the singulär spectacle of liquid rods, pendent like iciclesor staläctites. When one of these rods is broken an interesting contestbetween gravitation and cohesion takes place, during which the figure ofthe pendent changes as one or the other of those forces prevails : itbecomes longer while the first predominates, shorter when the latterControls, and stationary when both are balanced. These phenomena maybe observed by letting a drop of molasses fall from the point of a knifeor a spoon. The globule descends to a considerable distance before it iswholly separated from the portion above, beCause a rod of the liquidcontinues to be formed that unites them. When this rod bheaks, the partsuspended from the mass above is drawn up : a thread over a foot in lengthis sometimes thus Contracted to less than ^ of an inch, strongly remind-wg one of the elasticity of caoutchouc.

Water rises to considerable heights through sand and other porousbodies—also through rags and threads of Cotton , &c. Oil ascends in thewicks of lamps. Capillary siphons formed of cotton wick are employedto supply oil to the journals and working parts of machinery. It is cus-tomary with stereotype founders to oil the faces of engraved wooden blocksprevious to taking casts from them. These blocks are of box, a speciesof wood whose texture is exceedingly close. We have often placed someof those used in the illustration of this work on receiving them from theengraver, into a dish containing oil to the depth of -J- inch, and have wit-nessed the appearance of the liquid at the top within half a minute, andfrequently in a quarter of one. Unlike water in glass tubes, the oil hererises entirely oüt of the tubes in the wood and collects in globules over theorifices.

From the infinite variety and importance of devices for raising liquidsthat are at Work in the animal and vegetable kingdoms and in generalnature, the wisdom displayed in their formation and movements, and theirwonderful effects, it would seem as if the Creator designed pa^ticularly