Several hydrated forms of the oxide are known, and a colloidal variety may be obtained by the dialysis of a strong hydrochloric acid solution of sodium molybdate.
Still, according to Murray and Irvine, finely divided colloidal clay is to be found in all parts of the ocean however remote from land, though in very small amount, and there is less in tropical than in cooler waters.
Schneider and others, have proved the existence of " colloidal silver "; similar forms of the metals gold, copper, and of the platinum metals have been described.
The silica, in the form of diatom or radiolarian skeletons, is eventually deposited on the ocean floor after the death of the organisms. Most of the fine colloidal clay is, however, deposited as river-sludges when the fresh water carrying it mixes with denser sea-water.
The colloidal particles are electrically charged and become discharged by the ions of sodium, magnesium and calcium present in the sea-water.