CAPILLAR'ITY. That branch of physics which considers the properties of liquid surfaces. The fundamental property of such surfaces is their tendency to contract. This is shown by the fact that a liquid surface always assumes the smallest area compatible with the existing condi tions. Thus falling drops of liquids are spheri cal; and it is known from geometry that the area of the surface of a sphere is less than that of any other solid of an equal volume. If a soap bubble is not detached from the pipe. it will con tract when the mouth of the one who blows the bubble is removed. .Again, it requires work to blow a bubble; and this proves that there is a force opposing the increase in area of the bubble. It should be noted that when a soap bubble is blown, it is not a question of stretching the surface, but of making more surface by forc ing some of the liquid from the interior out to the surface. (The liquid film finally becomes so thin that this is impossible, and then the sur face may be stretched.) This contracting power of a liquid surface ex plains the phenomena observed when tubes of fine bore are partially lowered into a liquid. (The word 'capillarity' is derived from this fact, be cause these tubes must have been comparable with the size of a hair, the Latin word for which is capillus.) If the material of the tube is such that it is 'wet' by the liquid, i.e. if when dipped into liquid and then withdrawn there is a liquid film sticking to it (e.g. glass and water), it will be observed that, if the tube is first immersed in the liquid and then placed vertical, only dipping into the surface. the level of the liquid in the tube is higher than that outside by an amount which varies inversely as the radius of the bore. The surface of water inside the tube is like the inside of the finger of a glove, being, a lining of the upper portion of the tube and in cluding the top of the liquid column. This surface con tracts, rounding off the corners so as to be concave up ward, and drawing the column of liq uid up the tube, until stopped by the action of grav ity on the portion of liquid above the general level. Il
lustrations of this capillary action are given by the use of blotting-paper, the action of a lump of sugar on water, the action of wicks in lamps and candles, etc.
If, on the other hand, the solid is one which is not wet by the liquid (e.g. glass and mercury), the level of the liquid inside the tube will, under similar conditions, be lower than that of the level outside. In this case the surface of the liquid in the tube is like the outside of the finger of a glove; and, as it contracts, it rounds off the corners, makes the surface convex upward. and draws the level down. The depression will be found to vary inversely as the radius. The fact that the smaller the radius of a surface, so much the greater is the contracting power, is shown also by another experiment : a tube for blowing soap-bubbles is so made that two bubbles can be blown at one time on opposite ends of a connecting tube; if one bubble is blown larger than the other, and if the bubbles are then left to themselves, it will be observed that the smaller increases in size, blowing out the larger one. This proves that the pressure produced inside the smaller bubble by its contraction is greater than that in the larger. It is seen, therefore, that the pressure varies inversely as the radius; and to start a bubble ab initio, i.e. with a radius in finitely small, would require an infinitely great pressure. In fact, it is observed that bubbles of vapor in a boiling liquid or of gas in aerated liquids nearly always have a minute nucleus of dissolved gas to begin on. The presence of a solid with sharp points also facilitates the forma tion of bubbles, because the liquid surface can start around them. Similarly, the pressure in side a liquid drop varies inversely as the radius; and to start a drop from an infinitely small radius presupposes an infinite pressure. Thus drops of liquid are always condensed around some nucleus, such as a particle of dust or the points of a solid. Drops of rain have, in general, bits of dust inside; dew is formed on rough ob jects more quickly than on smooth ones, etc.