For practical and economic reasons the painter adds gel medium to his colours.从经济实用的角度出发，画家通常在颜料中加入凝胶调色剂。Heating boils off the liquid in the gel.加热可以去掉凝胶里的水分。The gel hardens quickly.凝胶很快就变硬了。The molecules of a gel disaggregate to form a sol.凝胶分子解聚而成溶胶。