In the horizontal operation of dysprosium‐thallium and scandium‐sodium iodide lamps, the internal surface of the upper portion of the arc tube is etched, causing precipitation on the side and lower portions. This precipitation produces spheroidal outgrowths on the side internal surface and closely bunched fibers on the lower internal surface. These deposits are examined by electron beam microanalysis and micro x‐ray diffraction analysis, and are construed to be amorphous SiO2. The transport of silica is found to occur also in a furnace experiment, where dysprosium iodide is heated in an evacuated silica tube held at 750–1150 °C.