There are only two of these preparations medically used, representing the two chemical oxides, the protoxide, namely, and the deutoxide or peroxide. Some chemists consider the latter as the true protoxide, on account of its greater stability, and therefore call the former the suboxide, altering the equivalent of mercury accordingly from 202 to 101. This change is medically very unfortunate, as it introduces still further confusion into the nomenclature of important and powerful preparations, just beginning to rise out of a former confusion, which had been the cause of death in more than one instance. I shall adhere to the nomenclature recognized by the U.S. Pharmacopoeia. The deutoxide is the only one of these oxides now officinally recognized.