Before we dismiss this subject, we observe [Pg 240] that it may be well worth while to make certain delicate experiments as to how far light and air exhibit their action in the removal of colour. It might be possible to expose coloured substances to the light under glass bells, without air, or filled with common or particular kinds of air. The colours might be those of known fugacity, and it might be observed whether any of the volatilized colour attached itself to the glass or was otherwise perceptible as a deposit or precipitate; whether, again, in such a case, this appearance would be perfectly like that which had gradually ceased to be visible, or whether it had suffered any change. Skilful experimentalists might devise various contrivances with a view to such researches. 602. Having thus first considered the operations of nature as subservient to our proposes, we add a few observations on the modes in which they act against us. 603. The art of painting is so circumstanced that the most beautiful results of mind and labour are altered and destroyed in various ways by time. Hence great pains have been always taken to find durable pigments, and so to unite them with each other and with their ground, that their permanency might be further insured. The technical history of the schools of painting affords sufficient information on this point. 604. We may here, too, mention a minor art, to which, in relation to dyeing, we are much indebted, namely, the weaving of tapestry. As the manufacturers were enabled to imitate the most delicate shades of pictures, and hence often brought the most variously coloured materials together, it was soon observed that the colours were not all equally durable, but that some faded from the tapestry more quickly than others. Hence the most diligent efforts were made to ensure an equal permanency to all the colours and their gradations. This object was especially promoted in France, under Colbert, whose regulations to this effect constitute an epoch in the history of dyeing. The gay dye which only aimed at a transient beauty, was practised by a particular guild. On the other hand, great pains were taken to define the technical processes which promised durability. And thus, after considering the artificial extraction, the evanescence, and the perishable nature of brilliant appearances of colour, we are again returned to the desideratum of permanency. XLIX. NOMENCLATURE. 605. After what has been adduced respecting the origin, the increase, and the affinity of colours, we may be better enabled to judge what nomenclature would be desirable in future, and what might be retained of that hitherto in use. 606.