ARTIFICIAL STONE, OR CONCRETE 



work of the interior of the blocks, so arranged that the 

 walls will hold together, leaving an intervening air 

 space. At the necessary points of contact some of 

 these blocks are made with a layer of waterproof 

 composition, such as a mixture of cement, sand, and 

 hydrated lime, which is inserted during the process of 

 making the block in the same manner as the process 

 of facing. Buildings constructed of this form of block 

 will be strong as well as damp-proof. 



REINFORCED CONCRETE CONSTRUCTION 



We have seen that concrete, made into blocks and 

 laid up as the walls of buildings, forms an ideal fire- 

 proof material. Without some strengthening material, 

 however, such as a steel frame, it is open to the same 

 objections as stone or bricks for very high structures; 

 but the modern skyscraper is an example of what can 

 be done with it in combination. This same sky- 

 scraper, if built first of a skeleton of steel girders, and 

 filled in with brick or stone afterward, has many de- 

 fects. The steel in the structure has a different rate 

 of expansion from that of the walls, causing collapses 

 and catastrophes in conflagrations. For this reason 

 the fireproof skyscraper is almost as much feared by 

 firemen when its contents are burning as the older 

 forms of building, although the walls cannot actually 

 be burned. 



But perhaps the greatest enemy of the steel-frame 

 building is rust. Unprotected steel is a very perish- 

 able material, as building materials go. And while, 



