HARDNESS

, or Rigidity, that quality in bodies by which their parts so cohere as not to yield inward, or give way to an external impulse, without instantly going beyond the distance of their mutual attraction; and therefore are not subject to any motion in respect of each other, without breaking the body.

There were many fanciful opinions among the ancients concerning the cause of hardness; such as, heat, cold, dryness, the hooked figure of the particles of matter. The Cartesians make the Hardness of bodies to consist in rest, as that of soft and fluid ones in the motion of their particles.

Newton shews that the primary particles of all bodies, whether solid or fluid, are perfectly hard; and are not capable of being broken or divided by any power in nature. These particles, he maintains, are connected together by an attractive power; and according to the circumstances of this attraction, the body is either hard, or soft, or even fluid. If the particles be so disposed or fitted for each other, as to touch in large surfaces, the body will be hard; and the more so as those surfaces are the larger. If, on the contrary, they only touch in small surfaces, the body, by the weakness of the attraction, will remain soft.

At present, many philosophers think that Hardness consists in the absence or want of the action of the universal fluid, or elementary fire, among the particles of the body, or a deficiency of what is called latent heat; while on the contrary, fluidity, according to them, consists in the motion of the particles, in consequence of the action of that elementary fire.

Hardness appears to diminish the cohesion of bodies, in some degree, though their frangibility or brittleness does not by any means keep pace with their hardness. Thus, though glass be very hard and very brittle; yet flint is still harder, though less brittle. Among the metals, these two properties seem to be more connected, though even here the connection is by no means complete: for though steel be both the hardest and most brittle of all the metals; yet lead, which is the softest, is not the most ductile. Neither is Hardness connected with the specific gravity of bodies; for a diamond, the hardest substance in nature, is little more than half the weight of the lightest metal. And as little is it connected with the coldness, or electrical properties, or any other quality with which we are acquainted. Some bodies are rendered hard by cold, and others by different degrees of heat.

Mr. Quist and others have constructed tables of the Hardness of different substances. And the manner of constructing these tables, was by observing the order in which they were able to cut or make any impression upon one another. The following table, extracted from Magellan's edition of Cronstedt's Mineralogy, was taken from Quist, Bergman, and Kirwan. The first column shews the Hardness, and the second the specific gravity.|