NATURE, VOL I, January 27, 1870 Tyndall on Haze and Dust THE Friday Evening Discourse entitled "On Haze and Dust" delivered by Prof. Tyndall at the Royal Institution on January 21, 1870, is printed in full. Prof. Tyndall referred to experiments carried out at the Royal Institution, in which he passed a "powerfully condensed beam" of light through tubes originally exhausted and "optically empty", to which air from the laboratory was admitted after passing through potash and sulphuric acid. In every case, floating matter entered the tube, and revealed itself by scattering the light, showing the "conical track of the electric beam". If the air was heated before passing through the drying apparatus, the floating matter was burnt up. "It was therefore organic matter". Tyndall then tried different methods of heating the air stream. Imperfect combustion of the particles produced fine blue clouds in the experimental tube, which gave perfectly polarized light at right angles to the illuminating beam. He also reported experiments in which a heated body placed below a cylindrical beam of light produced wreaths of blackness like thick black smoke, due to air currents removing the scattering particles which made the beam visible.