Mercury exists primarily in the mercuric and mercurous forms as a number of complexes with varying water solubilities. Mercury is strongly sorbed to humic materials. Selenium occurs in nature usually in the sulfide ores of the heavy metals and constitutes about 0.09 ppm of the earth's crust. It is the most strongly enriched element in coal, being present as an organoselenium compound, a chelated species, or as an adsorbed element. Selenium is used extensively in the manufacture and production of glass, pigments, rubber, metal alloys, textiles, petroleum, medical therapeutic agents, and photographic emulsions.
Selenium dioxide is the most widely used selenium compound in industry.
The toxicity of selenium depends on whether it is in the biologically active oxidized form. In alkaline soils and oxidizing conditions, selenium may be oxidized sufficiently to maintain the availability of its biologically active form, and cause plant uptake of the metal to be increased. In acidic or neutral soils, it tends to remain relatively insoluble and the amount of biologically available selenium should steadily decrease. Silver nitrate, the most important silver compound, is used extensively in photography. Silver iodide is used in seeding clouds to produce rain.