By a common lamp is understood one that feeds the wick with oil by capillary attraction, the column of oil being below the level of the flame. An argand lamp, on the contrary, has a column of oil considerably above the level of the flame and constantly pressing upwards to it like a fountain. The motion of a carriage has a tendency to make oil at times flow too rapidly and extinguish the flame, and sometimes to cause too sudden a rush of air up the central tube, which blows away the flame from the wick; and when these difficulties are overcome by ingenuity they become such complicated pieces of work that it is beyond the ordinary “gumption” of the servants to trim them properly, and if this be not done the object of the improvements is defeated.
The principle of constructing an argand lamp, so that it may regulate its own air draught, is set forth by Lord Cochrane in one of his patents. It is to divide the lamp into three chambers—one in the centre, which contains the reflectors and light, surrounded by the chimney glass, and is pierced with holes at the sides to permit the egress of the heated air and the ingress of the fresh air. From this chamber a tube or tubes communicate with the lower chamber, into which the air tube of the burner descends, and thus furnishes a regulated supply of air. It is evident by this process the air rushing in must be regulated by the air rushing out, and vice versâ.
The flame of the lamp is not produced by oil or tallow alone, it requires the oxygen of the air to mix with it in order to sustain combustion. This may be proved by putting a glass bowl over a candle, when, as soon as the oxygen is consumed, the light will be seen to go out, the bowl having the remaining constituent of air left in it, viz. nitrogen, which will neither support nor assist combustion. Herein consists the advantage of the argand lamp in furnishing atmospheric air in the centre of the flame. The flame arising from a thick wick is hollow, i.e. it is a film of light, like a bladder, and not continuous, the inner portion of the flame being filled with gas.
It is well known that if the flames of two candles be brought in contact they will produce a greater intensity of light than if burned separately. Upon this principle what are technically termed “cobblers’ candles” are made. For the same reason lamps are sometimes made to burn two or three wicks, placed just so far apart that the flames may come in contact. This is an approximation to the argand principle, by admitting air between them. But there is one difficulty attending them, viz. the regulating of all the wicks to an equal height, which would be considerable unless the lamp were so contrived that all could be regulated by one movement. If this difficulty can be overcome a very excellent lamp might be made by placing four wicks in a square.
CHAPTER XI.
PAINTING.
PRINCIPLES OF COLOURING IN PAINTING.
Colours are distinguished by artists as pure, broken, reduced, grey, dull, &c.
The pure colours consist of those which are called simple, or primary; these are red, yellow, and blue; and those which are formed from their mixture in pairs (binary compounds) are termed secondaries: such are orange, violet, green, &c.
Broken colours are formed by the mixture of black with the pure colours, from the highest to the deepest tone.
A normal colour is a colour in its integrity, unmixed with black, white, or any other colour.