A mechanical process, in which certain effects are produced by the attrition of two surfaces. The process of grinding is of extensive use in various mechanical arts; and great differences exist in the mode of conducting it according to the purposes for which it is employed, which are very varied; thus, in grinding corn, the object is to reduce the grain to an impalpable powder; in grinding lenses, it is to give them a certain figure and polish; cocks and valves are ground into their seats to promote intimate contact; colours are ground to promote the intimate mixture of the colouring matter with the oil; and cutlery and tools are ground, to impart to them a sharp edge. The latter operation, as is well known, is performed by applying the articles to be ground to the periphery of a cylindrical stone of a rough, gritty texture, revolving with great rapidity; and to reduce as much as possible the heat caused by the friction of the two surfaces, the stone is mounted over a trough containing water.
Some curious experiments are detailed in Nicholson's Journal upon this point, from which it appears that tallow is much more effective than water in keeping the temperature low; for in trying to grind down the teeth of a file with the grindstone immersed in water, the file soon became too hot to hold, and the teeth were scarcely touched, but by applying a tallow candle to a dry grindstone as it revolved, so as to give an even coating of tallow, he was enabled to grind down the teeth rapidly, and the temperature of the file was scarcely raised until the tallow became melted. This effect Mr. Nicholson attributes to the heat absorbed or rendered latent in bringing a solid substance into a fluid state.