De Mayerne records the use of sand in purifying oils, as a communication from Mytens, painter to Charles I., _before the arrival of Vandyke_. "Coming from such a source," says Mr Eastlake, "it may be classed among the processes which were familiar to the Flemish and Dutch painters." The works of the Flemish and Dutch painters are undoubtedly those which the artists of the present day would desire to be the tests of vehicles and of colours. They can scarcely have, therefore, a more valuable document than this manuscript of De Mayerne, the friend of Vandyke. From this source there is much information with regard to colours. It has always been supposed that Rubens in particular was lavish in the use of Naples yellow. It was largely used by the Italian painters; but it is omitted in the list of colours of the Dutch and Flemish. Many yellows, which in oil alone will not stand, are, it seems, durable if protected by an oleo-resinous medium. After enumerating many other yellows, Mr Eastlake remarks--"There was, however, one substance, viz. gamboge, now undeservedly fallen into disuse in oil painting, which is superior to most, if not to all, of those above named; the colouring matter united with its resinous portion, which renders it more durable in oil painting, may be easily freed from mere gum. De Mayerne, it would seem on good grounds, pronounces in its favour; and his speculations respecting the best mode of using it are confirmed by modern authorities. Gamboge, he observes, furnishes a beautiful yellow, constant, unfading, and that works freely." We are not surprised to see another pigment commended; we have long used it, but believe it is unknown as a colour by the artists of the present day, though, we suspect, sold by colour-makers for common work as a cheap brown. It is common coal. De Mayerne says, "The shadows of flesh are well rendered by pit-coal, which should not be burned." It is also recommended by Van Mander, and by Norgate, "whose directions for oil painting correspond in all outward particulars with the Flemish methods." In some experiments recorded by Sir Joshua Reynolds--there are the words "Gamboge and oil--but no colour remains;" yet it should be observed that where it is protected it is most durable. We believe the Aloes Cavallino, spoken of in terms of commendation by Leonardo da Vinci, to be an excellent transparent colour--and well calculated to give great richness to browns and to greens. It is certainly very interesting to know the colours actually used by the best masters of bye-gone days,--but we must not forget that modern science may greatly have improved many, and produced others, and has surer grounds to pronounce on their permanency. Mr Field, in his Chromatography, has rendered a very great service to art.