Take mercury and dissolve it in moderately diluted nitric acid by means of heat, taking care, however, that there be always an excess of metallic mercury remaining; decant the solution and pour it into a solution of sulpho-cyanide of ammonium or potassium, which may be bought at a good drug store, or of a dealer in chemicals. Equal weights of both will answer. A precipitate will fall to the bottom of the beaker or jar, which is to be collected on a filter and washed two or three times with water, when it is put in a warm place to dry. Take for every pound of this material one ounce of gum tragacanth which has been soaked in hot water. When the gum is completely softened it is to be transferred to a mortar, and the pulverized and dried precipitate gradually mixed with it by means of a little water, so as to present a somewhat dry pill mass, from which by hand pellets of the desired size are formed, put on a piece of glass, and dried again; they are then ready for use.