All modern tank furnaces work on the principle of the recovery of heat from the heated products of combustion as they leave the furnace, and the return of this heat to the furnace by utilising it to pre-heat the incoming gas and air; but the means employed to effect the application of this “regenerative” principle differ considerably in various types of plant. Perhaps the most widely-used form of furnace is the direct descendant of the original Siemens regenerative furnace, in which four regenerator chambers are provided with means for reversing the flow of gas and air in such a way that each pair of chambers serves alternately to absorb the heat of the outgoing gases and subsequently to return this heat to the incoming air that passes through one, and the incoming gas that passes through the other of these chambers. In these furnaces, the regenerator chambers themselves are generally placed underneath the melting furnace, and they are built of fire-brick and filled with loosely-stacked fire-bricks, whose function it is to absorb or deliver the heat. In the most modern type of furnaces of this class, the gas-regenerators are omitted entirely, the air only being pre-heated by means of regenerators, while the gas enters the furnace direct from the producer, thus carrying with it the heat generated in the producer during the gasification of the fuel. While this arrangement is undoubtedly economical, it has the serious disadvantage, especially in the manufacture of sheet-glass, that the gas, rushing direct from the producer into the furnace, carries with it a great deal of dust and ash, which it has no opportunity of depositing, as in the older types of furnace, in long flues.