THE PROPERTIES OF BUIEDING STONES, ETC. By 5) 
The method of obtaining the porosity which is ordinarily 
employed is as follows: The sample to be tested is heated at a 
temperature of 100° C, to drive off the moisture. After cooling, 
the sample is weighed, and then slowly immersed in distilled 
water. After bubbles cease to be given off, the sample is 
removed from the water and the surface quickly dried with 
bibulous paper, after which the specimen is again weighed. 
The difference in weight gives the increase due to the absorp- 
tion of water. This difference divided by the weight of the dry 
stone is taken as the ratio of absorption or porosity. 
Several errors are apparent in this method. The interstitial 
water is not easily expelled at a temperature of 100° C. To 
expel the moisture in a moderate length of time, the stone 
should be dried at a temperature of 110° C. Further, the samples 
cannot be completely saturated “by immersing in distilled 
water until bubbles cease to be given off.” Finally the method 
of computation gives the ratio of absorption and not the percen- 
tage of actual pore space or porosity. 
The porosity of a stone should be obtained in the following 
manner, using the determinations made in performing the 
specific gravity tests. The average weight of the dry sample 
should be subtracted from the weight of the saturated sample, to 
obtain the weight of the water absorbed. This, multiplied by 
the specific gravity of the stone, will give the weight of a 
quantity of stone equal in volume to the pore space and of the 
same specific gravity as the stone tested. This weight divided 
by the weight of the dry stone will be the porosity or actual 
percentage of pore space. 
The ratio of absorption can be obtained by dividing the 
weight of the absorbed water by the weight of the dry stone. 
In another part of this paper I have shown that neither the 
porosity or ratio of absorption, alone, indicates the value of a 
stone for building purposes. It was pointed out that the size of 
the pores is by far the more important consideration. This can 
be estimated roughly, when the size and shape of the grains and 
percentage of pore space are known. If only the. ratio of 
