398 cox. 



a definite temperature and finally to divide the weight in air by the 

 difference and correct for the difference in the specific gravity of water 

 at the given temperature. It is realized that complete saturation is 

 difficult to attain, but I think the method used is satisfactory. A cube 

 of Guadalupe tuff was slowly (several hours were required) immersed 

 in water and finally allowed to remain over night completely covered 

 with water. The results have been computed in two ways, first, on the 

 basis of the sample air-dried in my laboratory for eight months (July to 

 March), and second, on the basis of the sample absolutely freed from 

 interstitial water by heating in an air-bath at 110° for eight hours. 

 The sample retained only 0.7 per cent interstitial water after being air- 

 dried for eight months. The specific gravities (water at 1:°^1) are 

 as follows : 



I. II. 



1.895 1.907 



Porosity. — To obtain this factor,'' the weights of the dry and saturated 

 stone were used. The sample was saturated as described abo^•e, the 

 surface qtiicldy dried by pressing in filter paper and the weight taken. 



The difference in the weights was multiplied by the specific gravity 

 1.907 and the product added to the dry weight. The difference of the 

 dry and saturated weights multiplied by the specific gravity divided by 

 the above sum gives the actual pore space, compared with the volume of 

 the sample tested, as 28 per cent. 



Temperature changes. — The coefficient of expansion is a most important 

 factor in the disintegration of rocks in many places. Alternate freezing 

 and thawing and extreme heat in case of conflagration are the commonest 

 causes of disintegration in northern countries. Eocks often contain 

 grains of widely different coefficients of expansion and with change of 

 temperature internal strain is produced which aids in the destioiction of 

 the rock. A porous rock is somewhat able to accommodate itself to such 

 a strain. This volcanic t\iff , as is -triie of all very porous stones, can not 

 be used in a cold country because the absorbed water freezes and, breaking 

 the bonds, causes the stone to cramble, but the uniform temperature of 

 the Philippines favors the long life of a soft, porous stone. 



Water upon freezing at 0° and under one atmosphere pressure expands about 

 one-ninth of its volume. If the water is not allowed to expand it must remain 



' Buckley, Joe. cit.. 69. 



