Lamp is a term somewhat loosely applied to various devices for artificially producing light or heat, but is more properly confined to those in which fluid fuel is burnt. A lamp consists essentially of a reservoir to contain the oil or spirit, a wick and wick-holder, and some means for ensuring an adequate supply of air to the flame. The lamp of the ancients was merely a shallow vessel, with a spout holding a solid round wick of fibrous material. This construction is retained in the case of spirit-lamps, but the supply of air is not sufficient for the complete burning of oil. An improvement was effected by the substitution of a flat for a round wick, as the flame was spread out and exposed more surface to the air. This flat wick was bent into a tube by Argand, who effected a further improvement by adding a chimney, which caused a current of air to impinge on the flame. This chimney was then contracted near the base of the flame, and a metal disc placed inside the flame, both of which deflected the air against it. Modern lamps have merely been improved in points of detail. Mineral oils (paraffin) are sufficiently limpid to rise in the wick by capillary attraction, but in the case of thick oils, such as colza, which were used until comparatively recently, some means must be provided to force the liquid up to the burner. The "Moderator" is the best of these: a piston, propelled by a spring, forces the oil upwards. In the "reading-lamp" the burner is about on a level with the bottom of the reservoir, the oil from which flows into the burner by gravity. Paraffin has practically superseded all other oils for burning. The cheaper forms of lamp have one flat wick, which, as in the case of almost all lamps, can be raised or lowered in a tube by means of toothed-wheels fitted on a spindle. A sheet-metal cap covers the wick-holder, and has an oval slit to direct the air-current against the flame. Better distribution of light is effected by having two wicks, as in duplex-lamps, and three or more wicks are used in lamps for optical lanterns. Several forms of tubular burners have been devised for paraffin-lamps, in some of which the air is supplied to the inner portion of the flame by a tube passing through the centre of the oil container. The chief points to be considered in the design of a lamp are: the chimney must be long enough to produce a strong current of air, which must be directed against the flame by a contraction of the chimney, or by properly-placed pieces of sheet-metal; the oil-holder must be as far as possible prevented from becoming heated by arranging that the air shall circulate round those parts which would conduct the heat downwards from the flame. Blow-lamps produce a non-luminous flame of great heat by combustion of the vapour produced by heating alcohol or benzolene. This is allowed to escape through a fine jet, and in the best forms burns in a Bunsen burner.