The gas on leaving the producer passes along fire-brick flues or passages to the furnace proper; the path which it is now caused to take varies somewhat according to the arrangement of the furnace in question. Modern gas-fired furnaces usually belong to one of two distinct types according to the manner in which the heat of the escaping products of combustion is utilised for heating the incoming gas and air; these two types are known as the “regenerative” and the “recuperative” respectively. In regenerative furnaces the hot products of combustion, after leaving the furnace chamber proper, and before reaching the chimney, pass through chambers which are loosely stacked with fire-bricks; these chambers absorb the heat of the escaping gases, and thus rapidly become hot. As soon as a sufficiently high temperature is attained in these chambers or “regenerators,” the path of the gas-currents is altered; the escaping products of combustion are made to pass through, and thus to heat a second set of regenerating chambers, while the incoming gas and air are drawn through the heated regenerator chambers before entering the furnace chamber proper. The incoming gas and air are thus heated, absorbing in turn the heat stored in the brickwork of the regenerators. It is evident that two sets of such regenerators are sufficient, the one set undergoing the heating process at the hands of the escaping products of combustion, while the other set is giving up its heat to the incoming gas and air; when this process has gone far enough, it is only necessary to interchange the two sets of chambers, by the operation of suitable valves, and this series of alternations may be continued indefinitely. The arrangement is shown diagrammatically in Fig. 3. In recuperative furnaces the same principle is utilised in a somewhat different manner; the outgoing products of combustion pass through tubular channels formed in fire-clay blocks, while the ingoing gas and air pass around the outside of these same blocks; the heat of the outgoing gases is thus transferred to the incoming gases by the process of conduction through the fire-clay walls of the recuperator tubes. The relative merits of the two systems have been hotly contested; the regenerative system has the advantage of avoiding all reliance on the heat conductivity of fire-clay, while it also avoids the somewhat complicated special tubular blocks required for the other system; on the other hand, the recuperative system avoids the necessity for all “reversing” valves and their regular periodical working, while it also occupies somewhat less space. Temperatures sufficiently high for all glass-melting purposes can be attained by both means.