An automatic device, actuated by temperature changes, designed to control the gas supply to the burner(s) in order to maintain temperatures between predetermined limits and in which the thermal actuating element is an integral part of the device: (1) graduating thermostat, a thermostat in which the motion of the valve is approximately in direct proportion to the effective motion of the thermal element induced by temperature change; (2) snap-acting thermostat, a thermostat in which the thermostatic valve travels instantly from the closed to the open position, and vice versa.