252 STUDIES FOR STUDENTS 
absorption is available the calculations of the size of the pores 
are liable to be less accurate. 
For scientific purposes, other than determining the quality of 
a stone for building, the porosity, and not the ratio of absorption 
is the factor sought after. The porosity test will result in a 
higher percentage than the ratio of absorption, and may there- 
fore meet with disfavor among quarrymen. However, when it 
becomes known that the value of a stone cannot be estimated 
from its porosity except when the size of the pore spaces is known, 
objections will cease. In all cases it is thought that the porosity 
should be determined in preference to the ratio of absorption. 
Weight per cubic foot of stone —The weight of stone when it is 
first quarried, depends upon its specific gravity, the amount of 
pore space, and the water content. For a given stone, the only 
fluctuating element is the water content. In the more porous 
rocks this will vary at different seasons of the year and will 
depend upon the thoroughness with which the rock has been 
seasoned. Any determination of the weight per cubic foot of 
a stone which includes an indefinite quantity of interstitial water 
is unscientific and unsatisfactory. Determinations thus made 
depend upon a number of conditions, changes in any one of 
which will give a different result. The only constant weight is 
that of the dry stone. 
The commercial weight of a stone may be obtained in two 
ways. First, by weighing directly a known volume of the stone 
which has been thoroughly dried at a temperature of in@ Cos 
second, by computation from the data obtained in determining 
the porosity. By the second method, the weight of a cubic foot 
of stone can be obtained by multiplying the weight of a cubic 
foot of water by the specific gravity proper of the stone and sub- 
tracting therefrom the weight of a mass of stone, equal in vol- 
ume to the pore space of the given rock and of the same specific 
gravity. 
A simpler method would be to compute the apparent specific 
gravity as directed above and multiply by 62.5, which should 
give the same result. 
