Cohesion

[Fr. cohésion; Lat. con, and hæreo, hæsum, to stick, to cleave.) Act of sticking together; state of being united by natural attraction. - Connection; dependence; coherence; as, "natural cohesion of ideas." Locke. (Physics.) The force or attraction with which the particles of homogeneous bodies are kept attached to each other, or with which they resist separation. Cohesion is thus distinguished from adhesion: the latter term denoting the attractive force existing between two different bodies brought into contact, as a drop of water on a plate of gla ; or between two bodies of the same matter, as two lumps of lead when their smooth surfaces have been pre ed together. The three different forms which matter a umes-solid, liquid, and gaseous are determined by the degree of cohesive force existing among the elementary particles. In solids this force is greatest, and, in fact, is that which causes solidity; in liquids it is le powerful, but still sufficiently manifest in the drops or globular forms a umed by small quantities of water or mercury poured on a table. In the case of aëriform fluids, it may be regarded as negative, the particles having a tendency to repel each other. The cohesive force of the elementary particles of matter depends on the distances of the particles from each other; but of the law according to which its intensity increases or diminishes nothing is known, excepting that the force decreases rapidly as the distance increases, and vanishes altogether when the distance becomes so great as to be appreciable to the senses. It is a problem of very great importance to determine the cohesive power of the materials employed in mechanical structures. Many experiments have been made for this purpose; and their results have not only a practical utility, but throw much light on the constitution of bodies. When a bar of metal, a beam of wood, or a rope, is stretched lengthwise, the tension which it bears, or the cohesive power exerted, is equal to the accumulated attraction of all the particles in any transverse section. The longitudinal distention which takes place before disruption is at first proportional to this attraction, but afterwards increases in a more rapid progre ion. A bar of soft iron will stretch uniformly by continuing to append to it equal weights till it be loaded with half as much as it can bear; beyond that limit, however, its extension will become doubled by each addition of the eighth part of the disruptive force. Suppose the bar to be an inch square, and 1,000 inches in length; 36,000 lbs. avoirdupois will draw it out one inch, but 45,000 lbs. will stretch it 2 inches, 54,000 lbs. 4 inches, 63,000 lbs. 8 inches, and 72,000 lbs. 16 inches, where it would finally break.