Bunsen's thermostat, 1867.
© Science Museum / Science & Society Picture Library
This thermostat asisted the determination of the specific gravity of vapours and gases by maintaining a steady temperature within an enclosure. Developed by the German chemist Robert Wilhelm Bunsen in 1867, it was the first really effective means of maintaining an enclosure at a fixed temperature for a long period. It consisted of a double-walled elliptical cylinder. Heat is transferred along copper rods from the hooded Bunsen burners on the outside. The rods are positioned at a region of the flame, where the heat is most uniform, in order that the temperature attained by the bar is unaffected by small variations in the height of the burner flame.