402 cox. 



The irregularities in the curves can be directly accounted for by the 

 variations in the humidity and the temperature of the air from day 

 to day. The curves show this sample of tuff to be in equililn-ium with 

 the atmosphere when the latter contains about 1.75 per cent of loosely 

 held water. All of this may be removed by drying at a temperature 

 just above the boiling point of water, but the tuff quickly regains most 

 of it when' again exposed to atmospheric conditions. Upon resaturation 

 the cubes absorbed enough water to return to the original saturation 

 value. 



If there were no other elements influencing the loss of water, the 

 rate of evaporation from l)locks should be directly proportional to the 

 surface exjwsed, i. e., the cttrve expressing it would be a linear function 

 of the latter; however, several other factors enter. The exposed surface 

 of tlie cubes used bear to each other the relation 1, 4, 16, and 64. It will 

 be noticed (fig. 1) that the cubes reach equilibrium with the atmosphere 

 not in the above time ratio, but in approximately 36, 108, 334, and 972 

 hours, respectively, or a ratio of 1, 3, 9, and 37. The chief factor causing 

 the differences in these ratios is capillarity and it is constant. Hence 

 the following curve can be plotted, from which the time required for 

 a cube of anv size to drv may be directlv read: 



