strengthening agent for metal halide particles in amounts from about 0 25 to about 5 mole percent preferably from about 1 0 to about 1 5 mole percent are preferred for use in metal halide lamps since the color rendition index and other spectral and electrical measurements of the lamp made from a strengthened particle are not significantly changed a further advantage in the use of silver iodide is that its vapor pressure is relatively low at the operating temperature of the metal halide arc tube a low vapor pressure will help to prevent the silver iodide from becoming an important part of the metal halide arc particularly the emission spectrum moreover it is known that silver and silver iodide are benign as to an attack on the silica arc tube of a metal halide lamp i e they do not devitrify or etch the walls of the arc tube because silver oxide ag 2 o is much less stable than silica sio 2 and the addition of silver metal to the arc tube dose does not accelerate the decomposition of iodides such as sodium iodide nai and neodymium iodide ndi 3 in the lamp see the chang european patent no 645 799 a1 and u s pat no 5 483 244 in one embodiment a small percentage by weight.