that quality of bodies on account of which they are denominated brittle, or which subjects them to be easily broken by pressure or percussion.

Brittle bodies are extremely hard; a very small percussion exerts a force on them equivalent to the greatest pressure, and thus may easily break them. This effect is particularly remarkable in glaiss suddenly cooled, the brittleness of which is thereby much increased. Tin, though in itself tough, gives a brittleness to all the other metals when mixed therewith. The brittleness of glaiss has been said to arise from the heterogeneity of the parts whereof it is composed, as salt and sand can never bind sufficiently together; but this cannot be the case; for the pure calces of metals, or any other simple substances, when vitrified, become brittle also. In timbers, brittleness seems to be connected with durability; the more brittle any sort of wood is, the more durable it is found. Thus oak is of very long duration: while beech and birch, as being tough, presently rot, and are of little service in building.