The remarkable development of molecular biology has had its counterpart in an impressive growth of a segment of biology that might be described as atomic biology. The past several decades have witnessed an explosive growth in our knowledge of the many elements that are essential for life and maintenance of plants and animals. These essential elements include the bulk elements (hydro- gen, carbon, nitrogen, oxygen, and sulfur), the macrominerals (sodium, potas- sium, calcium, magnesium, chloride, and phosphorus), and the trace elements. This last group includes the ultra trace elements and...