TENACITY. This property is a result of the attractions and repulsions which act within the insensible spaces supposed to exist be tween the particles of bodies ; it is con sequently different in different materials, and in the same material it varies with the state of the body with respect to temperature and other circumstances. The particles of liquids adhere to ,one another, and generally to those of solid bodies, by attractive forces which decrease very rapidly ; and, at insensible distances from the supposed places of contact, the adhesion entirely disappears. It is on account of the small distance to which the attraction of the fluid molecules extend, and to the freedom with which the particles move on one another, that fluids appear to have so little tenacity ; but from the weight which it supports in glass tubes, Dr. Robison has estimated that the mutual attractions of the particles of water on a surface equal to one square inch must far exceed 190 lbs.
Though, when a piece of metal is fractured, the parts will not by simple adjunction adhere together; yet, in some cases, by hammering them upon one another, so many points on their surfaces may be brought within the limits to libich the force of cohesion extends, that they will acquire a tenacity equal to that which the metal had in its natural state.
The .tenacity of wood is much greater in the direction of the length of its fibres than in the transverse direction, the fibres being united by a substance having little cohesive power. With respect to metals, the processes of forging and wire drawing increase their tenacity in the longitudinal direction ; the augmentation of friction and lateral cohesion, arising from the particles being forced to gether in the transverse direction, more than compensates for the diminution of the attrac tion which may result from the particles being forded or drawn farther asunder longitudinally. Copper and iron have their tenacity more than doubled; while gold, silver, brass, and lead have it more than tripled, by those metals being drawn into wire.