the flames strike from the inside a symmetrical design is of course also possible when it is necessary or desirable for the window to resist the heat on both sides the window shown in fig1 comprises two glass sheets 1 2 made of hardened silicate soda glass which are separated by an inset 3 and mounted in a frame the frame consists of a rectangular tube 4 fastened into the splay of the wall it forms a groove with a flat iron 5 arranged on the internal face of the window a cushion frame 6 is placed on the external face the latter is provided with openings 7 shown in detail in fig2 thanks to which the edge of the glass sheet 1 is sufficiently heated in order to avoid the appearance of temperature gradients in its marginal region which might cause its destruction a heat insulating strip 8 consisting for example of asbestos is placed between the edge of the pane and frame 4 similarly an insulating strip 9 runs between the groove formed by flat iron 5 and glass sheet 2 and another insulating strip 10 runs between sheet 1 and the upper part of perforated cushion frame 6 the insulating strip 9 is covered along its rim with a layer of mastic 11 the insulating strip 10 can advantageously consist of a material which softens under the effect of heat and gives way when the window is deformed under the effect of fire materials such as alkaline silicates which foam under the effect of heat are useful for this purpose the listel 12 according to the invention consists of a u shaped section.