IX.] POROSITY OF ROCKS. 165 



the exhausted receiver of an air-pump, by which process a large 

 portion of air was removed. The exhaustion of the receiver was 

 several times repeated, at intervals, until the portions of rock were 

 as nearly as possible saturated, and bubbles ceased to escape on 

 further exhaustion. They were then removed, carefully wiped 

 with blotting-paper, and again weighed, first in air, and then 

 in water. These three weighings furnish the data necessary for 

 determining, 



I. The specific gravity of the mass, or the apparent specific gravity, 



compared with water as unity. 

 II. The specific gravity of the particles, or real specific gravity. 



III. The volume of water absorbed by 100 volumes of the rock. 



IV. The weight of water absorbed by 100 parts by weight of the rock. 



The loss in weight of the saturated rock when weighed in water, 

 being equal to that of the volume of water displaced by the mass, 

 enables us to determine the specific gravity of the latter ; while this 

 loss in weight, less the weight of the water absorbed by the mass, 

 gives the true volume of water displaced by its particles, and hence 

 the means of determining their specific gravity. The division, by 

 the volume of water displaced, of the amount of water absorbed, 

 gives the absorption by volume ; and the division of the weight 

 of the water absorbed by that of the dry mass, the absorption by 

 weight : 



a = the weight of the dry rock. 



b = the weight of water which the rock can absorb. 



c = the loss of weight, in water, of the saturated rock. 



We have then the following equations : 



I. c : a :: 1.000 : x specific gravity of the mass, or apparent specific 



gravity, water being 1.000. 



II. c- b : a :: 1.000 : x = specific gravity of the particles, or real specific 

 gravity, water being 1.000. 



III. c : b :: 100 : x = volume of water absorbed by 100 volumes of the 



rock. 



IV. a : b :: 100 : x = weight of water absorbed by 100 parts by weight 



of the rock. 



From these the following table has been calculated, the results 

 given under the last four columns corresponding to the four equa- 

 tions above : * 



* A similar series of results will be found in a report to the British House 

 of Commons, in 1839, by Messrs. Barry, Delabeche, and Smith, made with 



