The Precipitation of Calcium and Magnesium from Sea Water.*

ApPARENTLYcalcium carbonate is precipitated in certain parts of the ocean by processes which are inorganic in so far as that the calcium does not first form a true constituent of organisms (Clarke, 1920, p. 128). The conditions governing solubility of calcium in sea-water have been reviewed by Johnston and Williamson (1916) with the conclusion that surface layers of the ocean are approximately saturated, and that slight natural changes, particularly in carbon dioxide tension, might suffice to cause precipitation. The molar concentration of magnesium in sea-water is about five times that of calcium. In marine sediments magnesium carbonate is a constituent which varies in proportion according to the organic remains producing them (Olarke and Wheeler, 1922). It is a frequent constituent of shells, but much less abundant than calcium. The mode of formation of magnesium-containingdepositsis often obscure. . Magnesium hydroxide is precipitated from sea-water by addition of small amounts of alkali. In fact, both magnesium and calcium are in a delicate equilibrium where slight changes in alkalinity and carbon dioxide tension may cause precipitation. The delicacy of these equilibria and the nearness of their points of maximum sensitivity to natural conditions make calcium and magnesium particularly subject to changes induced by organisms. Likewise they are two of the most important elements biologically, both in amount and specific effect. It is therefore significant to examine the solubility conditions for both elements together, and to determine the conditions for their precipitation.