Hollow bricks are often enamelled for partitions or facings of ordinary walls; as in the case of solid enamelled bricks, we will postpone discussion of these until the chapter on Faience.

Fig. 312.

Fig. 313.

Fig. 314.

Fig. 315.

Figs. 310 to 315. Hollow Bricks of Special Shapes.

Fig. 316. Arch of Hollow Bricks.

## Dimensions And Shapes

The dimensions of hollow bricks are much more varied than those of solid bricks, as is shown by the following table giving current sizes.

 Holes. .045 x .15 x.22 2 or 3 .045 X .16 X .22 2 or 3 .055 x .11 x .22 2 or 4 (Fig. 272) .065 X.II X.22 4 or 6 (Fig. 270) .065 x .16 x .22 2, 4, or 6 .07 X .15 X .22 " .08 X .l6 x .22 4 or 6 .10 X .12 X .21 6 or 9 .11 X .11 X .22 9 (Fig. 277)
 Holes. .045 x .15 x .3 2 or 3 .045 x .16 x .3 „ (Fig. 276) .07 x .16 x .3 4 or 6 .07 X .21 X .3 " .08 x .16 x .3 " (Fig. 278) .10 x .11 x .3 6 or 9 .11 x .11 x .3 " .11 x .16 x.3 6,9, or 12 .11 x .22 x .3 "

Besides the above-mentioned bricks, some are also made of special shape (Figs. 285, 286, 287), .22 m. broad, of length varying by centimetres from 50 to 80, and of thickness varying from 5 to 11 centimetres. They are used to fill up intervals between the I-shaped irons of floorings (Fig. 286) or to raise partitions (Fig. 315). When the I-shaped iron pieces have a considerable height, special hollow products called "hourdis" are substituted for these bricks. Those most used are the Laporte "hourdis," so called from the name of their inventor (Figs. 288, 289). Their length is .33, and their thickness depends upon the height of the I-shaped iron bars. Their upper surface is placed on a level with the latter.

## Qualities

These are the same as those required for solid bricks, that is to say, that when used in facings they should have a uniform colour and sufficient hardness, that they should not be liable to crack, and should be of regular shape. The colour and resistance of the inside need not be so carefully supervised.

According to the experiments of Herve Mangon, the resistance offered by hollow bricks to crushing presents a curious difference according as the pressure is parallel or perpendicular to the direction of the holes. Thus, whereas a mean weight of 24 kilogrammes per square centimetre applied to a brick laid flat is sufficient to crush it, a weight of about 100 kilogrammes is required to crush the same bricks placed on end. Deducting the hollows formed by the holes, the pressure necessary to crush the bricks is, per square centimetre of the solid parts, respectively from 80 to 150 kilogrammes. That is to say, that a hollow brick under a crushing pressure perpendicular to the holes offers almost as much resistance as a solid brick. This difference of resistance is similar to that noticed in the case of wood, which is less resistant with the grain than across the grain. It is also a valuable indication to the builder, who should take it into consideration whenever he uses hollow bricks before exercising strong pressure.

For the official tests, see the end of Part I.