An open-ended, 500 mm long borosilicate glass tube, ideal for the ‘diffusion of gases experiment’. Cotton wool soaked in concentrated ammonia solution is placed in one end of the tube and cotton wool with concentrated hydrochloric acid is placed in the other end. Both ends are stoppered with bungs, to contain the fumes. The gases diffuse along the tube from both ends, and where they meet a white ring of ammonium chloride forms. Tube ends have been fired (smoothed), for safety purposes. Supplied without stoppers (33 mm or 35 mm solid stoppers required).