COHESION

(Lat. coha^reo, / hold together.) In Natural Philosophy, is 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 great, and, in fact, is that which causes solidity; in liquids it is le powerful, but still sufficiently manifest in the drops or globular form a umed by small quantities of water or mercury poured on a table. In the case of aeriform fluids it may be regarded as negative, the particles having a tendency to repel each other. The cohesive force of the elementery 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. Numerous 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. Wiien a bar of metal, a beam of wood, or a rope is stretched lengthwise, the tension which it bears, or the cohesive power evolved, is equal to the accumulated attraction of all the particles in any transverse section. The longitudinal distension which takes place before dis rupture is at first proportional to this attracr tion, 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 1000 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." {Leslie''s Nat. Philosophy/.) The following is a tabular view of the absolute cohesion of the principal kinds of timber employed in building and carpentry, showing the load which would rend a prism of an inch square, and the length of the prism which, if suspended, would be torn asunder by its own weight: — Teak - 12,915 lbs. — 36,049 feet Oak - 11,880 — 32,900 Sycamore - 9,630 — 35,800 Beech - 12,225 — 38,940 Ash - 14,130 — 42,080 Elm - 9,720 — 39,050 Memel fir - 9,540 — 40,500 Christian a deal - 12,346 — 55,500 Larch - 12,240 — 42,160 The above numbers must be regarded as only approximative, and representing the average cohesive force of the different kinds of wood specified; for the force differs greatly in different specimens of the same sort, and even in different parts of the same timber. Thus in the tables given by Profe or Barlow of experiments on the direct cohesion of different woods, we find the weight required to tear asunder a prism of an inch square varying as under: — Fir, from ll,000to 13,448 lbs.; ash, from R 2 [s. 256]