Page:Vitruvius the Ten Books on Architecture.djvu/66

44 of half-bricks on the other, and they are bedded to the line on each face. The walls are bonded by alternate courses of the two

different kinds, and as the bricks are always laid so as to break joints, this lends strength and a not unattrac­tive appearance to both sides of such walls.

In the states of Maxilua and Callet, in Further Spain, as well as in Pitane in Asia Minor, there are bricks which, when fin­ished and dried, will float on being thrown into water. The reason why they can float seems to be that the clay of which they are made is like pumice-stone. So it is light, and also it does not, after being hardened by exposure to the air, take up or absorb liquid. So these bricks, being of this light and porous quality, and admitting no moisture into their texture, must by the laws of nature float in water, like pumice, no matter what their weight may be. They have therefore great advantages; for they are not heavy to use in building and, once made, they are not spoiled by bad weather.

1. walls of masonry the first question must be with regard to the sand, in order that it may be fit to mix into mortar and have no dirt in it. The kinds of pitsand are these: black, gray, red, and carbuncular. Of these the best will be found to be that which crackles when rubbed in the hand, while that which has much dirt in it will not be sharp enough. Again: throw some sand upon a white garment and then shake it out; if the garment is not soiled and no dirt adheres to it, the sand is suitable.

2. But if there are no sandpits from which it can be dug, then we must sift it out from river beds or from gravel or even from the sea beach. This kind, however, has these defects when used in