1. Apparatus for humidifying and heating air to be breathed by a patient undergoing artificial respiration, comprising humidification and heating means to heat air to a suitable temperature which does not exceed, under steady operating conditions, the maximum desired temperature at which the air is desired to be admitted into the patient''s body, and to substantially saturate the air with water vapor at that output temperature; and further heating means to further heat the substantially saturated air from said output temperature, the air heated by said further heating means being carried in use through auxiliary apparatus into the patient''s body, the construction and arrangement being such that in use the increase in temperature of the air to be breathed which is imparted by said further heating means at the output thereof is selected to be equal to or to exceed the temperature drop which occurs in the air so heated in its passage between said output of said further heating means and the patient''s body and in any event said temperature increase is adJusted to insure that the air is delivered to the patient at no more than said maximum desired temperature; said air humidifier and heater comprising a body portion forming a water container adapted to be pressurized and including sealable water inlet means to enable water to be introduced from time to time in use; a water heater to heat water placed in said water container in use; a heat conductive plate arranged as a spiral coil with suitably spaced turns and located within said water container; a water absorbent material disposed along the spiral passage formed by said heat conductive plate; firs and second air passages through said body portion communicating with the interior of said body portion, at least the entrance of the outlet air passage being above water level and one of said air passages communicating with the outside core, and the other with the inside core, of said spiral passage, said water absorbent material being partially immersed in water, said spirally coiled heat conductive plate arranged with its axis substantially vertical in use and also being partially immersed in said water to provide a lower seal to said spiral passage, the top of said spirally coiled heat conductive plate being substantially in sealing engagement with said body portion so that air passing in whichever of said first or second air passages is the inlet passage can only leave via the other air passage and after passing through said spiral passage during which it gains an increased heat and water vapor content.