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lamps in tropical regions, although in temperate latitudes it is a permanent solid. In the combustion of a fixed oil in lamps, the oil undergoes destructive distillation, and at the burning point is resolved into a gaseous mixture. The comparative viscosity of all fixed oils renders it necessary to adopt some device supplementary to the capillary action of the wick for maintaining at the level of the burner a supply of oil sufficient to support uniform combustion. Again, the lubricant properties of fixed oils make it practicable to adopt various mechanical devices to regulate the supply of fuel to the burner, and otherwise control illumination.
The mineral oils, on the other hand, are, as sold, mixtures of various volatile hydrocarbons which give off inflammable vapours at comparatively low temperatures, and in consumption in lamps they come to the burning point in the condition of vapour. With highly volatile oils, and the use of imperfectly fitted lamps, though not with proper oil and fittings, there is some danger of explosion; there is also a risk that with imperfect combustion deleterious gases may be diffused through an apartment. Mineral oils possess such a high degree of limpidity that the suction of the wick alone is generally sufficient to bring the necessary supply of fuel to the burner.
The qualities of a lamp are judged of by the brilliance, steadiness, and uniformity of light it yields in proportion to the quantity of oil it consumes; by the convenient position of the light in relation to the equal illumination of the space it has to light; by the form, portability, and convenience of the lamp itself; and by the simplicity and economy of its construction, regard being had to efficiency. The chief points to consider in connexion with the structure of lamps are (1) the means of supplying oil to the burner and of regulating that supply, (2) the form and arrangement of the wick or medium over which the flame is supported, (3) the regulation and control of the currents of air in the lamp which support combustion, and (4) the position of the oil reservoir in relation to the dissemination of the light and the stability of the lamp itself.
The simple form which was used down to the end of the 18th century, and which as a "cruisie" continued in common use in Scotland till the middle of this century, illustrates the most elementary and most imperfect arrangement of a limp. Here, as in the lamps of antiquity, the oil vessel lies immediately behind the burning point of the wick, with which the oil is about level when the reservoir is full. The wick is a round soft cord or fibrous mass. Such a lamp has no merit but simplicity. The light is thrown only forward and to the sides, the back being entirely in shadow. The wick, being a round solid mass, takes up oil equally at the centre and circumference; but to the outer edges of the flame only is there any access of air; consequently combustion in the centre is imperfect, resulting in a smoky unsteady flame, and a discharge into the atmosphere of the acrid products of destructive distillation. Further, as the level of the oil sinks in the reservoir, the wick has to feed the flame from a greater distance by mere capillary force, and, the supply thus diminishing, the light decreases in proportion.
Since the time when inventors first began to better the primitive lamp, just one hundred years ago, the improvements in lamp construction have been enormous; the forms and modifications of invention bearing on lamps have been innumerable, and many excellent devices which did good service have been superseded by others simpler and more efficient. Notice can here be taken only of such inventions as developed new principles and features of originality.
The first improvement was in wicks and burners. In 1783 Leger of Paris devised a flat band or ribbon wick and burner, which produced a broad thin flame with no core, so that all parts of the oil supply were brought into intimate contact with the air, and perfect combustion and a steady flame were secured. The deficiencies of the flat wick flame were that the light was comparatively thin and impoverished, and that the parts of a room facing the thin ends of the flame were badly illuminated. To some extent these evils were overcome by the adoption of a curved form of burner, which in the end led up to the burner invented by Ami Argand of Paris, and patented in England in 1784. In its simplest form the Argand burner consists of two concentric tubes or cylinders, between which the tubular wick is contained. The inner tube is open throughout, and to it a current of air passes from below, and, being carried upwards by the draught of the flame, atmospheric oxygen for combustion is supplied as well to the inner circumference as to the outer side of the flame, whence the name "double current burner" which it frequently receives. An adequate and controllable flow of air to the interior of the Argand burner having been secured, it remained to devise some means by which the current supplied to the outer circumference of the flame could be strengthened and regulated. This Argand secured by means of a chimney, which was made at first of sheet iron and suspended over the flame; but that device was quickly abandoned in favour of a glass chimney which rested on a perforated gallery placed a little below the level of the burner. Subsequent experience suggested the formation of a shoulder or constriction on the chimney at a point a little above the level of the flame, whereby the air current is directed inward against the external surface of the flame, thus materially improving the combustion. Argand's original burner is the parent form of innumerable modifications all more or less complex in their adaptations.
A typical example of the burner and chimney is represented in fig. 1, in which the burner is composed of three tubes, d, f, g. The tube g is soldered to the bottom of the tube d, just above o, and the interval between the outer surface of the tube g and the inner surface of the tube d is an annular cylindrical cavity closed at bottom, containing the cylindrical cotton wick immersed in oil. The wick is fixed to the wick tube ki, which is capable of being moved spirally; within the annular cavity is also the tube f, which is capable of being moved round, and serves to elevate and depress the wick. P is a cup that screws on the bottom of the tube d, and serves to receive the superfluous oil that drops down from the wick along the inner surface of the tube g. The air enters through the holes o, o, and passes up through the tube g to maintain the combustion in the interior of the circular flame. The air which maintains the combustion on the exterior part of the wick enters through the holes m, with which rn is perforated. When the air in the chimney is rarefied by the heat of the flame, the surrounding heavier air, entering the lower part of the chimney, passes upward with a rapid current, to restore the equilibrium. RG is the cylindrical glass chimney with a shoulder or constriction at R, G. The oil flows from a side reservoir, and occupies the cavity between the tubes g and d. The part ki is a short tube, which receives the circular wick, and slides spirally on the tube g, by means of a pin working in the hollow spiral groove on the exterior surface of g. The wick-tube has also a catch, which works in a perpendicular slit in the tube f; and, by turning the tube f, the wick-tube will be raised or lowered, for which purpose a ring, or gallery, rn, fits on the tube d, and receives the glass chimney KG; a wire S is attached to the tube f,
