We may here in conclusion observe, that glazing derives its effect from treating the prepared colour underneath as a light ground. By this operation colours may have the effect of being mixed to the eye, may be enhanced, and may acquire what is called tone; but they thus necessarily become darker. PIGMENTS. 911. We receive these from the hands of the chemist and the investigator of nature. Much has been recorded respecting colouring substances, which is familiar to all by means of the press. But such directions require to be revised from time to time. The master meanwhile communicates his experience in these matters to his scholar, and artists generally to each other. 912. Those pigments which according to their nature are the most permanent, are naturally much sought after, but the mode of employing them also contributes much to the duration of a picture. The fewest possible colouring materials are to be employed, and the simplest methods of using them cannot be sufficiently recommended. 913. For from the multitude of pigments colouring has suffered much. Every pigment has its peculiar nature as regards its effect on the eye; besides this it has its peculiar quality, requiring a corresponding technical method in its application. The former circumstance is a reason why harmony is more difficult of attainment with many materials than with few, the latter, why chemical action and re-action may take place among the colouring substances. 914. We may refer, besides, to some false tendencies which the artists suffer themselves to be led away with. Painters are always looking for new colouring substances, and believe when such a substance is discovered that they have made an advance in the art. They have a great curiosity to know the practical methods of the old masters, and lose much time in the search. Towards the end of the last century we were thus long tormented with wax-painting. Others turn their attention to the discovery of new methods, through which nothing new is accomplished; for, after all, it is the feeling of the artist only that informs every kind of technical process. ALLEGORICAL, SYMBOLICAL, MYSTICAL APPLICATION OF COLOUR. 915. It has been circumstantially shown above, that every colour produces a distinct impression on the mind, and thus addresses at once the eye and feelings. Hence it follows that colour may be employed for certain moral and æsthetic ends. 916. Such an application, coinciding entirely with nature, might be called symbolical, since the colour would be employed in conformity with its effect, and would at once express its meaning. If, for example, pure red were assumed to designate majesty, there can be no doubt that this would be admitted to be a just and expressive symbol. All this has been already sufficiently entered into. 917.