While an ink is in effect no more than a solution of dye, watercolour is a dispersion of solid, finely ground, particles of coloured pigments that remain in suspension through the effect of Brownian motion – the random movement of microscopic particles when suspended in liquids or gases, caused by the impact of the collision with each other and the molecules of fluid with which they are mixed. The most important characteristic of watercolour is its brilliancy, a result of its translucent nature, which allows the white surface of the paper to shine through. To extend its range of possible effects, artists often incorporated opaque pigments and gouaches in their watercolours. (James Thornhill, Design for a Ceiling; William Blake, Cain Fleeing from the Wrath of God and J M W Turner, Simplon Pass). Watercolours are sold as cakes of dry paint or as liquid in tubes, to which water is added. The paint can be applied in various techniques such as wet-on-wet and wet-on-dry to obtain different effects.