QUARRY MATERIALS OF NEW YORK 41 



tangular piece of the rock of known volume after drying to constant 

 weight. From that result, the weight per cubic foot is readily 

 calculated. 



Porosity. The determination of porosity is one of the most im- 

 portant physical tests. The pores of rocks admit moisture, and its 

 expansion on freezing exerts such pressure as may lead to disrup- 

 tion of the material. The scaling of some sandstones when exposed 

 to frost action is very noticeable. Furthermore, under equal con- 

 ditions porosity affords some indication as to the resistance stones 

 will offer to the solvent action of waters and vapors and to the 

 penetration of smoke, dust and other discoloring agencies. It has 

 been held by some writers that th^. porosity is an absolute measure 

 of the durability of stone; but chis is an overstatement of the 

 matter, since the size of the pores and their relations to each other, 

 that is, whether isolated or connected by capillary channels, has as 

 much, if not more, influence than the absolute porosity. 



The total pore space or porosity is readily calculated from the 

 determinations for specific gravity, according to the method already 

 described. The difference between the weights of the samples dry 

 and saturated gives the amount of water absorbed in the pores. By 

 multiplying this quantity by the specific gravity, we obtain an ex- 

 pression for the weight of rock required to fill the vacant pore 

 space. This, added to the dry weight, gives the total weight the 

 sample would have if there were no pore space. If the weight of 

 rock required to fill the pores is then divided by the latter and the 

 result multiplied 'by ioo, we have the porosity expressed in per- 

 centage of the volume of the sample. This method devised by 

 Buckley has been commonly followed in the reports on American 

 building stones. It has been used in the determinations made in 

 connection with the present report. 



German testing laboratories measure the porosity somewhat 

 differently by determining the specific gravity of the powdered rock 

 and the so-called " Raumgewicht " or density of the stone inclusive 

 of pores. The latter is found by dividing the weight of the sample 

 expressed in grams by the volume in cubic centimeters. The 

 difference of the two values divided by the specific gravity and the 

 result multiplied by ioo gives what is called the coefficient of 

 porosity. 1 



1 Consult Hirschwald, " Handbuch der bautechnischen Gesteinspriifung," 

 p. 109-10. 



