The colouring of steel and the appearances analogous to it, might perhaps be easily deduced from the doctrine of the semi-opaque mediums. Polished steel reflects light powerfully: we may consider the colour produced by the heat as a slight degree of dimness: hence a bright yellow must immediately appear; this, as the dimness increases, must still appear deeper, more condensed, and redder, and at last pure and ruby-red. The colour has now reached the extreme point of depth, and if we suppose the same degree of semi-opacity still to continue, the dimness would now spread itself over a dark ground, first producing a violet, then a dark-blue, and at last a light-blue, and thus complete the series of the appearances. We will not assert that this mode of explanation will suffice in all cases; our object is rather to point out the road by which the all-comprehensive formula, the very key of the enigma, may be at last discovered.--Note S. PART III. CHEMICAL COLOURS. 486. We give this denomination to colours which we can produce, and more or less fix, in certain bodies; which we can render more intense, which we can again take away and communicate to other bodies, and to which, therefore, we ascribe a certain permanency: duration is their prevailing characteristic. 487. In this view the chemical colours were formerly distinguished with various epithets; they were called _colores proprii, corporei, materiales, veri, permanentes, fixi_. 488. In the preceding chapter we observed how the fluctuating and transient nature of the physical colours becomes gradually fixed, thus forming the natural transition to our present subject. 489. Colour becomes fixed in bodies more or less permanently; superficially, or thoroughly. 490. All bodies are susceptible of colour; it can either be excited, rendered intense, and gradually fixed in them, or at least communicated to them. XXXIV. CHEMICAL CONTRAST. 491. In the examination of coloured appearances we had occasion everywhere to take notice of a principle of contrast: so again, in approaching the precincts of chemistry, we find a chemical contrast of a remarkable nature. We speak here, with reference to our present purpose, only of that which is comprehended under the general names of acid and alkali. 492. We characterised the chromatic contrast, in conformity with all other physical contrasts as a _more_ and _less_; ascribing the _plus_ to the yellow side, the _minus_ to the blue; and we now find that these two divisions correspond with the chemical contrasts. The yellow and yellow-red affect the acids, the blue and blue-red the alkalis; thus the phenomena of chemical colours, although still necessarily mixed up with other considerations, admit of being traced with sufficient simplicity. 493.