This simple though important technical application suggested to me a condition of things under which the existence of so-called 'dry fog' would be possible. From our manufactories and domestic fires vast aggregate quantities of coal-tar and paraffin oil are daily distilled into the atmosphere, and, condensing upon, or attaching themselves to, the watery spherules of fog or cloud, must of necessity coat these latter with an oily film, which would in all probability, retard the evaporation of the water, …