Page:The New International Encyclopædia 1st ed. v. 04.djvu/211

* CAPILLARIES. 113 CAPILLABITY. of distribution might be mentioned. These va- rious arrangements have l)een discovered by the microscopic examination of tissues that have been injected and stained with colored tiuids. The circulation of the blood through the capil- laries may be readily seen under the microscope in the web between the toes of the hind foot of the frog, in the tongue of that animal, in the tail or gills of the tadpole, in the wing of the bat, etc. See Blood-Vessels ; Digestion ; Secke- Tiox. 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 whiih 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 If the material of the tube is such that it is 'wet' by the liquid, i.e. if when dipped into liquid and then withdrawn there is a liquid film sticking to it (e.g. glass and water), it will be observed that, if the tube is first immersed in the liquid and then placed vertical,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 CAPILLAEV ACT,0^«JETWEEN <=^'^^^ cRpiU^ry HCtioli RTe 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 foimd 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 tlie other, and if the bubbles are then left to themselves, it will be obsen'ed 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 presstire 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 obsened 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. This contracting power of a liquid surface is greatly afl'ected by the introduction of impuri- ties into the surface and by changes in tempera- ture. Soapy water has less contracting power than ptire water: and the motions of bits of camphor or of sodium or potassium on the sur- face of water are explained by the unequal rates of pollution of the surface at various points of the solid and the consequent unequal alterations in the surface forces, which thus pull the solid bit around in a most random path. If the temperature is raised, the surface forces decrease, as is shown by the fact that the height to which water stands in a glass tube decreases as the temperature increases. These contracting tendencies of a liquid surface are due to the action of the minute particles of the surface ; there are evidently forces holding these particles together. The force acting across a line of unit length is called the 'surface-ten- sion,' and it may be proved that if there is a spherical surface of radius r, there will be a contracting pressure given by the formula p = 2T/r where T is the surface tension of the liquid. Thus, to keep a soap-bubble of radius r from contracting, it is necessary to blow into it with a pressure 4 T/r, because there are tico con- tracting surfaces in a soap-bubble. Further, if the liquid stands in a tube of small bore at a height h above the general level of the liquid, the hydrostatic pressure due to this height must be counterbalanced by the contracting pressure of the concave surface of the water in the tube. This hydrostatic pressure is pgh, where p is the