Heat rays proceed in straight lines and the in-

—The lamp of the Hefner standard con tensity of the rays radiated from any one source, varies inversely as the square of their distance sists e entially of a cylindrical base for from the source. The rate at which a hotter holding the amyl-acetate which is drawn body radiates heat, and at which a colder body up through a German silver tube by means absorbs heat, depends upon the state of the surfaces of the bodies as well as on their tempera of a specially prepared wick. tures. The rates of radiation and absorption are increased by darkne and roughne of the sur- | Hefner Standard.—The unit of light faces of the bodies, and diminished by smooth adopted as standard in Germany. It is ne and polish. The oiling of smoothly polished castings, as of cylinder heads of steam engines, the light given by the Hefner-Alteneck more than doubles the lo by radiation, while it amyl acetate lamp. does not seriously affect rough surfaces.