The most serious disadvantages of the ordinary types of regenerative furnaces are due to the considerable dimensions of the regenerative apparatus, necessitating a costly form of construction and occupying a large space, while the necessity of periodically reversing the valves so as to secure the alternation in the flow of outgoing and incoming gases requires special attention on the part of the men engaged in operating the furnace, as well as the construction and maintenance of valves under conditions of heat and dirt that are not favourable to the life of mechanical appliances. It is claimed that all these disadvantages are overcome to a considerable extent in one or other of the various forms of furnace known as “recuperative.” In these furnaces there is no alternation of flow, and the regenerator chambers are replaced by the “recuperators.” These consist of a large number of small flues or pipes passing through a built-up mass of fire-brick in two directions at right-angles to one another; through the pipes running in one direction the waste gases pass out to the chimney, while the incoming gas and air pass through the other set of pipes. A transference of heat between the two currents of gas takes place by the conductivity of the fire-brick, and thus the outgoing gases are continuously cooled while the ingoing gases are heated--the transference of heat being somewhat similar to that which takes place in the surface condenser of a steam engine. Theoretically this is a much simpler arrangement than that of separate regenerator chambers, and to some extent it is found preferable in practice, but there are certain disadvantages associated with the system which arise principally from the peculiar nature of the material--fire-brick--of which the recuperators must be constructed. In the first place, the heat-conductivity of fire-brick is not very high, so that, in order to secure efficiency, the recuperators must be large, and while the individual pipes must be of small diameter, their area as a whole must be large enough to allow the gases to pass through somewhat slowly. Next, owing to the tendency of fire-brick to warp, shrink and crack under the prolonged effects of high temperatures, it becomes difficult to prevent leakage of gases from one set of pipes into the other. If this occurs to a moderate extent its only effect will be to allow some of the combustible gas to pass direct to the chimney, and at the same time a dilution of the gases entering the furnace by an addition of products of combustion from the waste-gas flues. This, of course, will materially reduce the efficiency of the furnace and require a higher fuel consumption if the temperature of the furnace is to be maintained at its proper level. If, however, the leakage should become more serious, a disastrous explosion might easily result, particularly if the nature of the leakage were such as to allow the incoming gas and air to mix in the flues.