GRINDING, or Trituration, the act of breaking or comminuting a solid body, and reducing it into powder.

Grinding is also used to indicate the rubbing or wearing off the irregular parts of the surface of a body, and reducing it to the destined figure, whatever that may be.

Grinding, in 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; whilst, on the other hand, if the water be poured on the stone from above by means of a cock, the quantity will be too small to preserve a sufficiently Iðw temperature. But let the quantity of water be ever so great, if the instrument to be sharpened have not its point or edge so held as to meet the stream, it will almost inevitably be rendered softer.

To remedy these defects in the common mode of grinding, [7:10:778]Mr Nicholson made an experiment with a grindstone from Newcastle, of a fine grit, ten inches in diameter, with a block of mahogany, having emery on the face of it. The grindstone and block were fixed on an axis, to be applied occasionally between the centres of a strong lathe. Both were cylindrical, and of the same diameter; the wood was grooved in opposite directions, that the emery might be lodged therein. The face of. the stone was left smooth, with a trough under it to contain the water. The cylinder of wood was faced with emery and oil, and the stone was used with water. A file was the instrument ground, and it was proposed to efface all the teeth. The mechanism of the lathe produced the rotation, by which the grinding apparatus made five revolutions in a second. The operation of the stone was slow, and the workman found inconvenience from the water in the trough being soon exhausted; but the emery cylinder cut rather faster. The friction operated by quick changes on the whole surface of the file, yet it soon became too hot to be held conveniently by the uncovered hand; and even when it was held with a cloth, such was the rapid increase of heat as to decompose the oil, which emitted an empyreumatic odour. When the stone became dry, the file was tried on the face of it, which soon became blue, and then nearly red hot. After this both cylinders were covered with tallow, and emery was sprinkled upon the wood cylinder, when the same instrument was again held to the stone in rapid motion. The friction at first was scarcely apparent; but the pressure of the tool soon fused the tallow, and the stone cut very fast. When the tool after some time began to be a little heated, it was removed to a new zone of the cylinder, by which means the temperature was diminished. Similar effects accompanied the use of the wooden cylinder.

When oil was used upon the cylinder of wood, the heat occasioned by the friction raised the temperature of the instrument and of the oil; but when tallow instead of oil was employed, most of the heat was absorbed in fusing that substance. The increased capacity of the melted tallow absorbed this heat, which became latent, and did not raise the temperature; and when the tallow already melted began to grow hot, as well as the tool, the employing another zone of consistent tallow reduced the temperature.