25. =Tenacity.=--The power of holding together, termed tenacity, depends on the degree of attraction between the particles. By attraction I mean a disposition in particles to come together, this disposition being manifested in opposition to any force tending to draw them apart. I shall soon speak of this more particularly. Tenacity does not exist at all in gaseous substances. The particles of air and of steam, for example, show no disposition to cling together; that is, have no tenacity. This property is weak in liquids. It is only strong enough in water to enable its particles to hang together in the shape of a drop. It is strong in solids, enabling their particles not only to hold together in large quantities, but to hold up also heavy weights suspended to them. It is stronger in iron than in any other solid. It is stronger in wrought iron than in cast iron; and strongest of all in steel. 26. =Comparative Tenacity of Substances.=--Various metals and other substances have been tested in reference to their comparative tenacity. It was done in this way: Wires were made of the metals, all of the same size. Weights were suspended to them, and additions were made to the weights by little and little till the wires broke. The table underneath was made by placing against each metal the greatest weight that its wire would hold: Cast steel 134 pounds. Best wrought iron 70 pounds. Cast iron 19 pounds. Copper 19 pounds. Silver 11 pounds. Gold 9 pounds. Tin 5 pounds. Lead 2 pounds. Oak wood, tried in the same way, was found to hold up 12 pounds, one more pound than silver. Some animal substances have great tenacity, as the thread of the silk-worm, hair, wool, and the ligaments and tendons of our bodies and of other animals. 27. =Value of Tenacious Substances.=--"The gradual discovery," says Dr. Arnot, "of substances possessed of strong tenacity, and which man could yet easily mould and apply to his purposes, has been of great importance to his progress in the arts of life. The place of the hempen cordage of European navies is still held in China by twisted canes and strips of bamboo; and even the hempen cable of Europe, so great an improvement on former usage, is now rapidly giving way to the more complete and commodious security of the iron chain--of which the material to our remote ancestors existed only as useless stone or earth. And what a magnificent spectacle is it, at the present day, to behold chains of tenacious iron stretched high across a channel of the ocean, as at the Menai Strait between Anglesea and England, and supporting an admirable bridge-road of safety, along which crowded processions may pour, regardless of the deep below, or of the storm; while ships there, with sails full-spread, pursue their course unmolesting and unmolested."