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 which 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. Few experiments have been made on the tenacity of wood perpendicularly to its grain, as it is called ; and from those of Mr. Emerson it appears to vary from one tenth to one-seventh of the tenacity in the other direction. When a strain takes placein the direction of the fibres, they become disengaged from one another, and thee lose the strength which arises from their lateral cohesion. They then become subject to separate strains; the weaker ones are first ruptured, and at length all give way, leaving an irregular surface of fracture. [Antiesiox.] 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 together in the transverse direction, more than compensates for the diminution of the attraction which may result from the particles being forced 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.
Mixed metals have, in general, greater tenacity than those which are simple : the tenacity varies with the different proportions in which the metals are mixed ; and the proportions which produce the greatest strength are different in different metals. The only experiments on this subject with which wo are acquainted are those of Musehenbroek ; and from these wo find that a compound of which 3 were gold and off the had a tenacity, or force of cohesion, more than double that of the gold or copper alone : brass, composed of copper and zinc, had a tenacity more than double that of the copper, and nearly twenty times as great as that of the zinc : a metal of which 3 were block-tin and 3 lead, had a strength more than double that of the tin ; and a mixture of which 3 were lead and 3 zinc, had a tenacity nearly double that of the zinc, and nearly five times as great as that of the lead alone.