Gold colloid. A sample flask containing a sample of a gold colloid made by Michael Faraday in 1857. A gold colloid is a suspension of microscopic particles of metallic gold in water. He used phosphorus to reduce a solution of gold chloride, as the metal precipitates out it forms particles of the order 100 nanometres in diameter. The colour of the colloid is determined by the particle size. This sample is on display at the Royal Institution, London.