The first excitation and gradual increase of colour take place more on the plus than on the minus side. So, also, in passing through the whole scale, colour exhibits itself most on the plus side.
A passage of this kind, regular and evident to the senses, from yellow through red to blue, is apparent in the colouring of steel.
The metals may be arrested at various points of the colorific circle by various degrees and kinds of oxydation.
As they also appear green, a question arises whether chemists know any instance in the mineral kingdom of a constant transition from yellow, through green, to blue, and vice versâ. Oxyde of iron, melted with glass, produces first a green, and with a more powerful heat, a blue colour.
We may here observe of green generally, that it appears, especially in an atomic sense, and certainly in a pure state, when we mix blue and yellow: but, again, an impure and dirty yellow soon gives us the impression of green; yellow and black already produce green; this, however, is owing to the affinity between black and blue. An imperfect yellow, such as that of sulphur, gives us the impression of a greenish hue: thus, again, an imperfect blue appears green. The green of wine bottles arises, it appears, from an imperfect union of the oxyde of iron with the glass. If we produce a more complete union by greater heat, a beautiful blue-glass is the result.