When it is found to escape in sufficient quantities from the burners at F, set it on fire, remove the heater, and a beautiful bright flame will be supported by its own heat as long as there is oil in A; it may be found necessary to replace the first heater by a second, when the lamp is used for the first time, to expel more effectually the atmospheric air from the generator and tubes The principle of this lamp is the same as that of Mr. Blackadder's (from which probably the idea was taken), viz. to decompose the oil by causing it to pass over an incombustible substance heated to redness; but the arrangements are move complex and not so efficient. Besides the objection to the det-ached heater, from the trouble of heating so large a mass, in comparison with the incombustible wicks in Mr. Blackadder's plan, the decomposing vessel being of metal, will be found far inferior in effect to glass tubes or similar substances of inferior conducting power, and in a short time would soon become incapable of decomposing the oil, as it is found necessary at the Oil Gas Works to introduce into the retorts pieces of broken bricks, coke, etc, or plates of iron, which are renewed daily.
The decomposing chamber D, and the circular rim E, both of which require to be situated over the flame of the lamp, are also highly objectionable, not only as cumbrous and unsightly appendages, but on account of the dark and extensive shadow which they would throw upon the ceiling. The size of wick must be proportioned to the degreeof light the lamp is required to afford, but with the ordinary wicks, composed of cotton yarn slightly twisted, if the diameter be much increased, sufficient air does not arrive at the central part or the flame to cause the entire combustion of the fuel, and the lamp consequently burns with much smoke, and a deposit of carbonaceous matter upon the wick takes place; it is therefore found preferable to use two or more small wicks instead of one larger one. Count Rumford, whose experiments upon warming and lighting have produced such great improvements in these two branches of domestic economy, invented a lamp, the wick of which is formed of a kind of broad tape wove for the purpose; and, as a reading lamp, it is equal to any, whilst at the same time its construction is extremely simple: but the greatest improvement yet made in lamps, is the Argand lamp, so named from their inventor, M. Argand, of Geneva. The distinguishing feature of these lamps is that the wick is hollow or tubular, and the wick-holder is so constructed as to allow a passage for the air through the centre of the flame, as well as on the exterior, by which means every particle of the oil is decomposed and burned, and a most brilliant flame is produced, free from smoke or smell.
When oil of the best quality is used, lamps of this description are found infinitely superior to all others for all situations where they do not require to be moved about, and are now manufactured in an endless variety of the most tasteful and elegant forms, and with various additional contrivances for regulating the height of the flame, the flow of oil, doing away with shadow of the oil vessel, etc. The annexed cut represents a very common and simple description of Argand lamp, adapted either to stand upon a bracket, or hang against a wall. In these lamps the oil surrounding the wick is maintained constantly at the same level, by a contrivance similar in principle to the bird fountain. a is the oil reservoir or fountain, closed at the top, but having an aperture at bottom fitted with a conical or button valve. The reservoir fits loosely into an outer case 6, so as to allow free admission for the air between the two; c is the neck by which the oil flows into the wick-holder d, which is composed of two concentric tubes joined together at the bottom by a circular plate, having an aperture in its centre for the passage of the air to the centre of the flame, equal to the aperture of the interior tube; e is the circular wick, fixed upon a ring, which can be raised or lowered at pleasure, by a contrivance which will require a separate diagram for its illustration. f is a glass chimney, to cause a more rapid current of air, and g is a ground glass shade, to equalize the light and soften the glare.
To charge the lamp, the vessel a is withdrawn from the casing b, and, being inverted, the valve falls inwards, leaving the aperture open by which the oil is to be introduced. When the reservoir is filled, the aperture is closed by pulling up the wire, or tail, projecting from the valve, and the reservoir may be returned to its erect position without any escape of oil. Upon replacing it within the casing b, before it quite reaches the shoulder of the case by which it is supported, the tail of the valve rests upon the bottom of the case, and the valve is pushed inwards, upon which the oil flows from the reservoir into the casing and wick-holder, until it touches the bottom of the reservoir, which descends a little below the level of the wick-holder, when the admission of the air into the reservoir being prevented by the oil in the casing b covering the aperture, no more oil can escape. Upon lighting the lamp, as the oil rises to the wick by its capillary attraction, the level falls in the casing b, and the aperture in the vessel a becoming uncovered, the air enters and expels a fresh portion of oil, until the level of the oil rises in the case and closes the aperture; and thus, during the time the lamp continues lighted, the oil in the casing and wick-holder is constantly maintained at the same level: a small cup h is screwed on below the wick-holder to receive any oil which may chance to overflow; care being taken that the cup shall be so far below the circular aperture of the wick-holder as not to impede the passage of the air to the flame.
We shall now proceed to describe the means by which the wick is raised or lowered in order to regulate the flame. The adjoining figure represents a section of the wick-holder, except that part of the internal tube is shown entire in order to exhibit more clearly a spiral groove which makes two or three turns round it. a is the wick, the lower end of which is drawn over a small metal ring b, which has a small stud c, projecting each way, the internal end entering the spiral groove on the surface of the centre tube, and the external end passing through a longitudinal slit or groove, extending the whole length of a tube d, which is soldered into a ring, or collar, resting upon the top of the external tube of the wick-holder; from this ring, or collar, proceed the bent wires e e, which support the rim f, upon which the chimney g rests. Now, upon considering the figure, it will be seen that on turning the rim f, the tube d, which is connected with it by the bent wires e e, will also be turned round, and will carry with it the ring b, by its stud c; and as one end of the stud is engaged in the spiral groove, it will, in turning round, either ascend or descend along the groove, according to the direction in which it is turned: h is the tube by which the oil flows to the wick, and k is the cup to receive any overflowings.