or TRITURATION, the act of breaking or comminuting a solid body, and reducing it into powder. See Pulverisation and Levigation.

The painters colours are ground on a marble or porphyry, either with oil or gum-water.

Grinding is also used for rubbing or wearing off the irregular parts of the surface of a body, and reducing it to the destined figure, whether that be flat, concave, or the like.

The grinding and polishing of glass is a considerable art; for which see Glass Grinding. For the grinding of optical glasses, see Optics, the Mechanical Part.

cutlery, is an operation universally understood, by which edge-tools are sharpened. According to the usual practice, this operation is attended with considerable inconvenience, occasioned by the extrication of heat from friction. The steel very soon becomes ignited when the friction is performed on a dry stone; and even when immersed in water, the operation must be slow, to prevent the water from being thrown off by the centrifugal force; and if the water is poured on the stone from above by means of a cock, the quantity will be too small to preserve a sufficiently low temperature. But let the quantity of water be ever so great, if the instrument to be sharpened has not its point or edge so held as to meet the stream, it will almost inevitably be made softer.

To remedy these defects in the common mode of grinding, Mr Nicholson made an experiment with a grindstone