COHO BAT I ON

15,784 to 17,850 lbs.; oak, from 8,889 to 12008 lbs.; teak, from 14,062 to 15,405 lbs.; pear, from 8,834 to ll,.5371bs., tiC. {Barlow on the Strength of Materials, 1837.) The metals differ more widely from each other in their cohesive strength than the several species of wood or vegetable fibres. According to the experiments of Mr. George Rennie in 1817, the cohesive power of a rod an inch square of different metals, in pounds avoirdupois, with the corresponding length in feet, is as follows: — Cast steel - - 134,2.56 lbs. — 39,4.55 feet. Swedish malleable iron 72,064 — 19,740 English ditto - - 55,872 — 16,938 Cast iron - - - 19,096 — 6,110 Cast copper - - 19,072 — 5,003 Yellow bra - - 17,9.58 — 5,180 Cast tin - - - 4,736 — 1,496 Cast lead - - - 1,824 — 348 It is difficult to measure directly the cohesion of fluids (and it is very considerably affected by the temperature); but an approximation may be derived from the magnitude of drops, and the thickne of liquid sheets, heaped upon a horizontal surface. In this view, says Profe or Leslie, let us trace the formation of a drop of water as it slowly collects at the end of a capillary siphon. The mutual attraction of the particles always rounds the under part of the pendant fluid, which continues to lengthen till its accumulating weight begins to overcome the cohesion of the particles. But this force being 75 grains for each horizontal square inch, while a cubical inch of water weighs 252* grains, must correspond to the pull of a cylinder of -18 mch high, which will influence also the breadth of the pendant liquid. Beyond this limit a separation will ensue, when the cylinder merges into a sphere a little wider, or about 2-lOths of an inch diameter. The cohesion among the particles of alcohol and of sulphuric acid being respectively the fifth part of 215 and 460 grains, the weight of a cubic inch of each of these fluids, a drop of them should measure "17 of an inch in diameter. The cohesive force of quicksilver at the ordinary temperature amounts to 312 grains on each horizontal inch, while a cubic inch of it weighs 3424 grains; the drop must separate when its mean depth approaches to the 9- 100th part of an inch. {Elements of Nat. Philosopkv, p. 369.) "With respect to the ultimate agent by which the effects of cohesion are produced, it is remarked by Dr. T.Young, that if it is allowable to seek for any other agent than a fundamental property of matter, appearances extremely similar might be derived from the pre ure of a universal medium of great elasticity. But all suppositions founded on such analogies must be considered as merely conjectural; and our knowledge of every thing which relates to the intimate constitution of matter, partly from the intricacy of the subject, and partly for want of sufficient experiments, is at present in a state of great uncertainty and imperfection. ( Young's Lectures on Nat. Philosophy, vol. i. p. 630.) See Strength of Materials. [s. 257]