Bunsen's water-bath with regulator, c. 1850.

Bunsen's water-bath with regulator, c. 1850.

The prolonged heating of substances at moderate but constant temperatures was difficult to achieve with the older types of heating device (furnaces, sand trays and water baths for example). The water bath (the "double saucepan") had been known to the Alexandrian alchemists, but its design had not greatly improved. Bunsen's form introduced a constant level device which made it possible to keep a constant boiling water temperature for an indefinite period. The bath is connected to a reservoir, with a float valve which cuts off a supply of water. When water is lost by evaporation, the float sinks and admits water. The float rises and cuts off the supply at the pre-determined level.