82 MR. W. K. BALDWIN-WISEMAN OX [Feb. 1907, 



percolating fluid, and the pressure to which that fluid is subjected : 

 it is also still further complicated, as I shall show later, by variations 

 in the physical structure of the rock and by the cushioning of the air 

 in the innermost interstices. 



Thus, for instance, the porosity of marble is very small if it be 

 determined by immersion in water under ordinary conditions of 

 temperature and pressure, but if it be determined by immersion in 

 oil the result is somewhat higher ; and, with water under conditions 

 of high temperature and great pressure, much larger values would,, 

 in all probability, be obtained than with water under normal 

 conditions of temperature and pressure. 



During the course of my experiments upon flow and resoakage — 

 recorded, with other data, in a paper published in the Minutes of 

 Proceedings of the Institution of Civil Engineers (24) — I found that 

 when water was forced into the rock under pressures of 40 or 60 lbs. 

 per square inch, the water-capacity after the removal of the pressure 

 was much higher than that for ordinary tank-soakage, or for soakage 

 under reduced pressure, as when the soaking-basin was placed under 

 the receiver of an air-pump. Thus, a piece of Upper Chalk from 

 Micheldever (Hampshire) had a storage-capacity, after subjection to 

 a pressure of 40 lbs. per square inch, of 0*4679 of its total volume, 

 or a storage-capacity of 2-92 gallons per cubic foot. Upon re- 

 charging, it took up a volume of water equal to 28*1 per cent, of its 

 total porosity in the first second, which had increased to 57 '2 per 

 cent, in 4 seconds and to 74*7 per cent, in 9 seconds : the volume of 

 the several quantities of absorbed water varying, roughly, in the 

 ratios 1, 2, and 3, and the time-interval in the ratios of 1, 4, and 9, 

 or as the squares of the first ratios. But thereafter the rate of 

 absorption continually decreased, and thus only 772 per cent, of the 

 total storage-capacity was charged in the first half-hour ; while the 

 rate increased still more slowly to 78 per cent, in 1 hour, 79 per cent, 

 in 4| hours, and 83*1 per cent, in 47 hours. 



Under the receiver of an air-pump, the absorption during the first 

 half-hour was somewhat greater than that under ordinary conditions 

 of soakage, the quantity absorbed being about equal to that of 

 4| hours of ordinary soakage ; but thereafter the rate of absorption 

 did not appear to be accelerated by further operation of the air- 

 pump, nor were any observable air-bubbles given off from the 

 stone. 



Similar characteristics in the rate of absorption to that of the 

 \Ticheldever Chalk are shown for other stones in the appended 

 Table (I, p. 83). 



It is interesting to note, as Prof. Boyd Dawkins pointed out in his 

 James-Forrest Lecture before the Institution of Civil Engineers (1), 

 that the quantity of water taken up by a rock upon recharging is 

 somewhat less than its original water-content ; and the explanation 

 seems to me to be that, as the water penetrates to the inner- 

 most pores, it expels the air from the pores into which it enters, 

 part of the air escaping externally, while the remainder passes into 

 the innermost and finer interstices. With further additions of 



