To express this appearance in dyeing, the French make use of the word "virer," to turn from one side to the other; they thus very adroitly convey an idea which others attempt to express by terms indicating the component hues. 533. The effect produced with litmus is one of the most known and striking of this kind. This colouring substance is tendered red-blue by means of alkalis. The red-blue is very readily changed to red-yellow by means of acids, and again returns to its first state by again employing alkalis. The question whether a culminating point is to be discovered and arrested by nice experiments, is left to those who are practised in these operations. Dyeing, especially scarlet-dyeing, might afford a variety of examples of this fluctuation. XLI. PASSAGE THROUGH THE WHOLE SCALE. 534. 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. 535. A passage of this kind, regular and evident to the senses, from yellow through red to blue, is apparent in the colouring of steel. 536. The metals may be arrested at various points of the colorific circle by various degrees and kinds of oxydation. 537. 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. 538. 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. 539. From all this it appears that a certain chasm exists in nature between yellow and blue, the opposite characters of which, it is true, may be done away atomically by due immixture, and, thus combined, to green; but the true reconciliation between yellow and blue, it seems, only takes place by means of red. 540. The process, however, which appears unattainable in inorganic substances, we shall find to be possible when we turn our attention to organic productions; for in these, the passage through the whole circle from yellow, through green and blue, to red, really takes place. XLII. INVERSION. 541.