When a hand is held over a heated stove, heat is carried to the hand by convection and given up to the hand by conduction. But when the hand is held before the stove it is also heated, not by conduction, for fluids have little conducting power; not by convection, for convection currents are ascending. How then does the heat get to the hand ? The query comes to us with still greater force when we consider the transmission of the sun's heat to the earth, for the atmosphere can carry it by neither conduction nor convection. More important yet, how does the sun's heat reach the earth's atmosphere ? This heat passes through the atmosphere without heating it. If along a poker thrust into the fire the hand be moved toward the stove, the temperature increases. If a person ascend through the atmosphere toward the sun the temperature diminishes. We have here a wholly new set of thermal phenomena, heat passing through a substance and leaving the condition of that substance unchanged.