IGNITION, in chemistry, is that illumi nation, or emission of light, produced in bodies by exposing them to a high tem perature, and which is not accompanied by any other chemical change in them. It may be distinguished from combustion, a process in which there is also the emis sion of light and heat. Combustion is the result, not of mere increase of tempara ture in the body wh ch suffers it, but of the chemical action of the air, or of a prin ciple which the air contains: hence com bustible substances are alone suspectible of it, and when the process has ceased, the body is no longer combustible. Igni tion is an effect of the operation of caloric alone ; it is wholly independent of the air ; all bodies, at least solid and liquid substances, are equally susceptible of it, and if it has ceased, from a reduction of temperature, it may be renewed by the temperature being again raised. The point of temperature at which the first stage of ignition takes place, or at which bodies arrive at a red heat, appears to be the same in all, and is supposed to be about 800° of Fahrenheit. Byraising the temperature, the illumination becomes brighter, and the red light acquires a mix ture of yellow rays. At length, by still
increasing it, we come to the white heat, which is the highest state of ignition. Ae riform fluids are not brought into a state of illumination by heat. The phenomena are produced not only by the application of heat, but likewise by friction and at trition.
JIB, in naval affairs, the foremost sail of a ship, being a large stay-sail, extended from the outer end of the bowsprit, pro longed by the jib-boom, towards the fore top-mast-head. In cutters and sloops, the jib is on the bowsprit, and extends towards the lower mast-head. The jib is a sail of great command with any side wind, but especially when the ship is close-hauled, or has the wind upon her beam ; and its effort in turning her head to leeward is very powerful, and of great utility, parti cularly when the ship is working through a narrow channel. Jib-boom is a continu ation of the bowsprit forward, being run out from the extremity in a similar man ner to a top-mast on a lower-mast, and serving to extend the bottom of the jibs and the stay of the fore-top-gallant-mast.