CLAYS OF NEW YOKK 855 



tile which allow the water to percolate through them in less than 

 six hours should be rejected. 



For sewer pipes the water must be put under pressure, which is 

 done by closing the two ends of the pipe with plates of iron, the 

 joints being tightened by means of rubber bands around the edges. 

 The pressure is then applied by means of a piston till the manom- 

 eter shows the desired pressure, at which point it is allowed to 

 stand. If the pipe is impermeable, the manometer will remain 

 at that point, but if the pipe contain^ a flaw the liquid in the mano- 

 meter will fall and moisture will appear on the outside of the 

 pipe at the point where the flaw is. 



Breaking strength 



This test is made by allowing the stone to lie flat on two parallel 

 supports with sharp edges, while a third edge is caused to press on 

 the. upper surface halfway between the two supports. The 

 pressure required to break the stone is then measured. If the 

 upper surface of the stone is not perfectly flat, it can be made so 

 by laying on the upper surface two parallel cleats of portland 

 cement one cm wide. See " Paving brick ", p. 745. 



Hardness test 



The hardness of building material can be determined by means 

 of Moh's scale of hardness. This scale is made up of 10 different 

 minerals, of which each is harder than the preceding one in the 

 series, and softer than the succeeding one. The order, beginning 

 with the softest, is: 



1 Talc 



2 Gypsum 



3 Calcite 



4 Fluorite 



5 Apatite 



6 Orthoclase 



7 Quartz 



8 Topaz 



