Gold sulfide colloid was prepared by mixing equal concentrations of a sulfur sol and a gold sol. The mixture was heated to 85 deg C for 15 minutes. The sulfur sol was prepared by dissolving solid sulfur in hydrazene then adding distilled water. This was followed by heating for 30 minutes at 75 deg C. The gold sol was prepared by reducing gold chloride with 1% sodium citrate. The mixture was heated and stirred at 85 deg C for 30 minutes.