a well-known instrument of steel for cutting and abrading metal, ivory, wood, &c. Files are of various forms, sizes, and cuts, according to the uses to which they are to be applied. When the surface is cut in transverse furrows by a straight sharp-edged chisel and mallet, the instrument is properly called a file; but when it is raised by a triangular punch, it is termed a rasp. This last kind is chiefly employed for rubbing wood and horn. The larger kinds of files are made of blistered steel; but the small and fine files of cast steel. Various ingenious machines have been contrived for cutting the teeth of files, but these have not hitherto succeeded so well as to supersede file-making by the hand. After the file has been cut it must undergo the process of tempering. This is said to be well effected in the following manner: A saturated solution of common salt, stiffened to the consistence of cream with ale grounds, or with any cheap farinaceous matter, such as bean-flour (some use well-dried chimney-soot), is to be spread over the file, in order to preserve its surface from oxidation during the process; it is then uniformly heated in a coke or charcoal fire to a cherry-red colour; and on its removal from the fire it is to be suddenly quenched in cold and pure spring water. It is subsequently cleaned with charcoal and a rag; after which it is laid up in wheat bran to preserve it from rust. When the file is intended to cut iron or steel, it is found preferable to substitute animal carbon for the grounds or farinaceous matter mentioned above. This will give even to iron a superficial hardness sufficient for any kind of file.