It is not always that ill drying is to be attributed to the pigments or vehicles, the states of the weather and atmosphere have great influence thereon. The direct rays of the sun are powerfully active in rendering oils and colours siccative, and were probably resorted to before dryers were--not always wisely--added to oils, particularly in the warm climate of Italy. The ground may also advance or retard drying, because some pigments united by mixing or glazing, become either more or less siccative by their conjunction. Many other accidental circumstances may likewise affect drying; and among these none is to be more guarded against by the artist than the presence of soap and alkali, too often left in the washing of his brushes, and which, besides other bad results, decompose and are decomposed by acetate of lead and most siccatives. In such cases desiccation is retarded, streaks and patches are formed on the painting, and the odium of ill drying falls upon some unlucky pigment. To free brushes from this disadvantage, they should be cleansed with linseed oil and turpentine. Dryers should be added to colours only at the time of using them, because they exercise their drying property while chemically combining with the oils employed, during which the latter become thick or fatten. Too much of the siccative will, as before noticed, often retard drying. The various affinities of pigments occasion each to have its more or less appropriate dryer; and it would be a matter of useful experience if the habits of every colour in this respect were ascertained. It is probable that siccatives of less power generally than the compounds of lead and copper might come into use in particular cases, such as the oxides of manganese, to which umber and the Cappagh browns owe their drying quality. To other good attributes of pigments, it would be well if we could in all cases add the property of being _innoxious_. As this, however, cannot be, and colours are by no means to be sacrificed on that account, cleanliness and avoiding the habit of putting the brush unnecessarily to the mouth, so common in water-painting, are sufficient guards against any possibly pernicious effects from the use of any pigment. No colour which is not imbibed by the stomach will in the slightest degree injure the health of the artist. PART III. * * * * * ON COLOURS AND PIGMENTS INDIVIDUALLY. CHAPTER VI. ON COLOURS AND PIGMENTS INDIVIDUALLY. Having briefly discussed the relations and attributes of colours and pigments generally, we come to their powers and properties individually--a subject pregnant with materials and of unlimited connexions, every substance in nature and art possessing colour, the first quality of pigments.