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. The mixture of equal parts of red and yellow produces orange; mix equal portions of yellow and blue and we have green; equal parts of red and blue produce violet. These are called _secondary colours_. Three parts of red mixed with 1 part of blue produce violet-red; 3 parts of red and 1 of yellow produce a red-orange; 1 part of red to 3 of yellow produces orange-yellow; 3 parts of yellow to 1 of blue produce a light yellow-green; 1 part of yellow to 3 of blue produces a blue-green; 1 part of red to 3 of blue produces a light violet colour. All these are called _secondary hues_. Normal grey is black mixed with white in various proportions, producing numerous tones of pure grey.