John Aitken was the author of a number of important pioneering discoveries "On Dust, Fogs and Clouds" (the title of an 1880 article he penned). As early as 1874, Aitken had concluded that when water vapour in the atmosphere condenses, it must condense on some solid particle, and thus, without the presence of dust and other aerosol particles in the air, there would be no formation of fog, clouds, or rain. In 1884, he concluded that the brilliant colours often seen in the sunset are due to the refraction of light by dust particles in the upper atmosphere.