or TRITURATION, the act of breaking or comminuting a solid body, and reducing it into powder. See PULVERISATION and LEVIGATION.
The painters colours are grinded 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 desired 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 excitation of heat from friction. The steel very soon becomes ignited when the friction is performed on a dry stone; and even when immeried 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. grindstone from Newcastle of a fine grit, 10 inches in diameter, with a block of mahogany to be employed with 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, in which the emery might be lodged. The face of the stone was left smooth, with a trough under it to hold 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 soon 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 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 in a state of fluidity; but when tallow instead of oil was employed, most of the heat was used 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.
This discovery may yet be of considerable importance, for which we are indebted to the ingenuity of the learned editor of the Journal which bears his name, a performance which is much esteemed upon the continent as well as at home, by every man of literature and science.