A respiratory humidification device comprising: a casing comprising a casing top and a casing bottom that cooperate to enclose an interior space, wherein the casing bottom defines an underside portion of the casing that is formed from thermally conductive material to conduct heat into the interior space from a heating component positioned external to and underneath the casing to heat water within the casing; a separator wall within the interior space that extends downward from the casing top and toward the casing bottom to divide the interior space into a filling chamber and humidification chamber, wherein a bottom edge of the separator wall leaves a space between the separator wall and the casing bottom that defines a main passage between the filling chamber and the humidification chamber that allows the water to flow atop the casing bottom between the filling chamber and the humidification chamber, and that enables a level of the water within at least one of the filling chamber or the humidification chamber to rise high enough to block the main passage to prevent respiratory gas from passing therethrough; a valve and a first float within the filling chamber that cooperate to control a flow of the water into the filling chamber from a water source external to the casing to prevent the level of the water within at least the filling chamber from rising higher than a predetermined threshold level; a gas inlet formed through the casing top to enable entry of a main flow of the respiratory gas into the humidification chamber from a medical device external to the casing; a gas outlet formed through the casing top to enable the main flow of respiratory gas to proceed through the humidification chamber in a direction from the gas inlet to the gas outlet, and to be output from the gas outlet to a patient after humidification of the respiratory gas; wherein at least one portion of an outer wall of the casing top and at least one portion of the separator wall are configured to cooperate with a surface of the water within the humidification chamber to define an elongate tube-like pathway between the gas inlet and the gas outlet through which at least one portion of the main flow of the respiratory gas is to proceed along a path that extends generally parallel to the surface of the water within the humidification chamber to enable humidification of the respiratory gas with the water within the humidification chamber; and characterised in that respiratory humidification device further comprises: a heat spreader within the humidification chamber, wherein a lower portion of the heat spreader is positioned to extend downward into the water within the humidification chamber to absorb heat from the water within the humidification chamber, and an upper portion of the heat spreader is positioned to extend upward from the surface of the water within the humidification chamber and into the path of the at least one portion of the main flow of the respiratory gas to radiate the absorbed heat into the respiratory gas to enhance the humidification of the respiratory gas within the humidification chamber. a casing comprising a casing top and a casing bottom that cooperate to enclose an interior space, wherein the casing bottom defines an underside portion of the casing that is formed from thermally conductive material to conduct heat into the interior space from a heating component positioned external to and underneath the casing to heat water within the casing; a separator wall within the interior space that extends downward from the casing top and toward the casing bottom to divide the interior space into a filling chamber and humidification chamber, wherein a bottom edge of the separator wall leaves a space between the separator wall and the casing bottom that defines a main passage between the filling chamber and the humidification chamber that allows the water to flow atop the casing bottom between the filling chamber and the humidification chamber, and that enables a level of the water within at least one of the filling chamber or the humidification chamber to rise high enough to block the main passage to prevent respiratory gas from passing therethrough; a valve and a first float within the filling chamber that cooperate to control a flow of the water into the filling chamber from a water source external to the casing to prevent the level of the water within at least the filling chamber from rising higher than a predetermined threshold level; a gas inlet formed through the casing top to enable entry of a main flow of the respiratory gas into the humidification chamber from a medical device external to the casing; a gas outlet formed through the casing top to enable the main flow of respiratory gas to proceed through the humidification chamber in a direction from the gas inlet to the gas outlet, and to be output from the gas outlet to a patient after humidification of the respiratory gas; wherein at least one portion of an outer wall of the casing top and at least one portion of the separator wall are configured to cooperate with a surface of the water within the humidification chamber to define an elongate tube-like pathway between the gas inlet and the gas outlet through which at least one portion of the main flow of the respiratory gas is to proceed along a path that extends generally parallel to the surface of the water within the humidification chamber to enable humidification of the respiratory gas with the water within the humidification chamber; and