There is proof enough of this. A picture, unless it be painted with very little oil indeed, will become, in a few days after being painted, greasy--it will not take water on the surface--in fact, "secca teme acqua" will not bear water. If, in this state, the surface be lightly rubbed over with common sand and water, this greasiness will be removed, and the surface will not only be clean, but beautiful; this greasiness will, however, in a day or two come again. If the process of sanding be repeated, _until the greasiness does not_ come again, we conjecture that we have done for the picture what time, but a long time, might do--we have removed _all_ the impurity of the oil. We believe that pictures after that do not undergo further change, and if the paint be tolerably hard, may be varnished--and that they will become much sooner hard; for it is more than probable that this greasiness in the oil is the main cause of retarding the drying. We have followed this practice many years, and always with the same results. It is surprising how soon after painting you may sand--even coarse red sand will not remove paint, that is yet tacky--it much remedies the "colori olcesi." The translator lays much stress in the preface upon the importance of white grounds. In the olden time, it appears, that when they were not of gold, they were white; and Leonardo da Vinci thus lays down _his_ precept--"Sempre a quelli colori che vuoi che habino belleza preparerai primo il campo _candidissimo_, e questo dico de' colori che sono transparenti, perche a quelli che non sono transparenti non giova campo chiaro." And yet Leonardo is said to have painted occasionally on the canvass without any other priming than a coat of glue. His pictures so painted are said to be durable, and worthy his great name. We should have doubted if Titian did always paint on a white ground--and should fix upon the "Peter Martyr" as the subject of doubt. It is said to have been the practice of Correggio; if so, he did not always derive the benefit from the ground which white grounds are said to confer, for his painting is so generally solid, and the transparency so much the effect of his glazing, that there seems to be no reason why he should have given the preference. It is said the Flemish School used white grounds--probably Rubens did so generally, not all other painters. Teniers used a light drab, and, if we were to judge from some of his skies, painted upon it when that thinly coloured ground was wet. Unless a great body of colour be used, even in the most transparent painting, white grounds are apt to give a weakness and flimsiness. Gaspar Poussin, and perhaps generally, Nicolo, painted on red grounds; the former probably often upon a vermilion ground, though most commonly on one of a deeper tone; the advantage of this, in landscapes, such as his, is evident. There is no colour so good as red to set off greens; and in fact, to make tints appear green, that on