The earth's core is contained in an oxide bottle, the mantle. With regard to any dissolved oxygen, therefore, the outer core (OC) is in a reactive relationship to its container. The mantle contains only trivial amounts of sulfur compared to its solubility in the liquid OC, so the bottle is effectively inert to sulfur. This difference affects whether the diluent convects freely away from the freezing inner core (sulfur) or moves under constraint of saturation at the interface with the container (oxygen). The motion of oxygen is limited by diffusion; it requires double freezing and hence release of latent heat at both top and bottom of the OC, or else (if the top of the OC is a heat sink) no escape of heat to the mantle. The motion of sulfur allows rapid transport of latent heat away from the inner core boundary by compositional convection and implies major heat loss from the core. The velocity of compositional convection (due to sulfur) may be as much as 104 times that of diffusion of oxygen, so sulfur in the outer core should be far more important in driving the dynamo than oxygen.