-54 Scientific Proceedings, Royal Dublin Society. 



Indeed, such continuity would have been well-nigh impossible 

 to obtain. Hence, if the constraint, imposed by the surface- 

 tension forces around the gas in the main tube, enabled the water 

 there to get into a state of tension, discontinuities would have 

 immediately appeared in the water above the mercury and at the 

 point of contact of mercury and water. Thus, it would have 

 been impossible for the manometers to register anything but a 

 positive pressure, viz. the pressure of thejbubbles in the main tube 1 

 about the connexion of the manometer, or of bubbles in the 

 manometer itself. Other considerations, to be mentioned later, 

 render it extremely probable that tension did exist in the water in 

 the plaster; but the manometers, for reasons just given, could not 

 record it. 



2. The properties of plaster of Paris have often, rather unfor- 

 tunately, been likened to those of wood. At any rate, the large 

 resistance offered by plaster of Paris to the flow of water through 

 it, makes it, in this particular, to differ markedly from wood. 

 This resistance would prevent local differences of pressure being 

 quickly equalized. 



To give some idea of this resistance, I will quote one of several 

 similar experiments. 



A cylinder of plaster of Paris, 1*2 cms. in diameter and 5 cms. 

 long, transmitted, under a head of 35 cms. of water, 1*54 ccs. of 

 water in twenty-four hours. A piece of wood of Taxus baccata, 

 containing about \ of its volume of heart-wood, and having the 

 same dimensions and with the same head of water, transmitted 

 during the first hour at the rate of 20 - 3 ccs. in twenty-four hours. 

 The difference of permeability is, however, greater than this expe- 

 riment would indicate ; for, even in the first hour, there is a 

 considerable falling off in the rate of transmission through wood. 

 This falling off, due to clogging which takes place at the cut 

 surface of wood, does not occur in the case of plaster of Paris. 



The teaching of this experiment would appear to be, that the 

 resistance offered by plaster of Paris to the passage of water 



1 The actual pressure of the bubbles in the main tube will be very largely decided 

 by the dilatation they have undergone in expanding from their original to their final 

 volume. As this will be purely accidental — depending upon the fortuitous arrangement 

 ■ of plaster in the tube — we should expect that the pressure of these bubbles would be 

 quite different at different points in the tube. 



