38 Henry H. Dixon. 



taken as an indication that the gas-pressure within the tube is very 

 slight. Care must indeed be taken not to let the concussion become 

 too violent, as in that way the tube may be easily shattered. 



If now, by carefully inclining- the tube, the long limb is com- 

 pletely filled with water (Fig. 3 B) and all the bubbles are chased 

 out of that limb by holding the bent end uppermost, so that no 

 breaks, even the most minute, remain, we will find, on inverting the 

 tube and bringing the bent end under, that the water remains in the 

 long limb and does not .under the force of gravity take up the lowest 

 possible level in both limbs (Fig. 3 C). From the level in the two 

 limbs it is evident that the hydrostatic pressure of the shorter column 

 cannot possibly balance the pressure of the column in the longer 

 limb. The one is about 85 cms higher than the other. The water 

 in this case, like the sulphuric acid in Donny's experiment, hangs 

 in the tube. The liquid in the long limb is in contact with the glass 

 all over, and, since it wets it perfectly, it adheres to it. To the film 

 of water adhering to the glass the rest of the water coheres, and 

 this cohesion is much more than able to sustain the weight of the 

 column of water which is counterbalanced by no other upholding force. 

 In this way the lower part of the water in the longer limb of the 

 tube transmits through the upper part a stress to the glass equivalent 

 to its gravitational pull. 



The reality of this pull becomes all the more striking when, by 

 destroj^ing the cohesion at one spot, a rupture is started. This rupture, 

 which may be at first invisibly small, rapidly spreads across the whole 

 column. The rupture may usually be started by a sharp knock 

 administered to the side of the longer limb; but, when the cohesion 

 is very perfect, to produce a rupture may require a shock so violent 

 as to be liable to shatter the tube. When the rupture is started, 

 the lower part tears suddenly away from the upper part of the 

 column and falls into the bend of the tube. The upper part follows 

 it more slowly, trickling down the inside of the tube, and all the 

 water comes to occupy a position in the lower part of the tube 

 (Fig. 3 A). 



It is instructive to note how the cohesion of the water in these 

 experiments is overcome. The rupture starts as an extremely small 

 space or discontinuity in the water. Immediately surface tension 

 forces develop at the surface of this bubble. At its inception, being 

 extremely small these forces are very great, but if the bubble 

 enlarges, the surface tension forces tending to close it rapidly diminish. 

 In our experiments the forces tending to open it are (1) the momentum 

 of the water conferred on it by the shock, and (2) the gravitational pull 

 giving rise to the tension in the liquid. We may neglect the vapour 

 pressure of the bubble, as it is balanced by the vapour in the other 



