INTERNAL COHESION OF LIQUIDS. 9 



again, the mercury remained up to the end of the tube, 

 except about one eightli of an inch, which was filled with 

 sulphuric acid. The unbalanced column of mercury was 

 therefore 31 inches long. At first the full significance of 

 this phenomenon was not recognized ; but in order to as- 

 certain that the tube was cleared of air, it was moved gently 

 up and down to see if the mercury clicked, as it usually 

 does when the tube is free from air, but the mercuiy did 

 not move in the tube. The rapidity of the oscillation was 

 thereupon increased until it became a violent shake, and, 

 as the mercury still remained firm, it was clear that some 

 very powerful force was holding it in its place. The tube 

 being in a vertical position, was then left in order that the 

 barometer might be consulted. This was standing at 29 

 inches. After a few seconds, when the gauge was again 

 examined, the column no longer reached the end of the 

 tube, but stood at 29 inches. As it was singular that the 

 mercury should have quietly settled down after having re- 

 sisted such violent shaking, the tube was again inclined 

 until the mercury and acid came, apparently, up to the end 

 of the tube ; but this time on the erection of the tube the 

 mercury at once settled down. That is to say, it settled 

 down gradually as tlie tube was erected. At first what 

 appeared to be a very small bubble opened in the sulphuric 

 acid; and this enlarged as the top of the tube was raised. 

 On again inclining the tube until it was horizontal, and 

 examining it closely, a minute bubble could be seen in the 

 acid, and it was this bubble which expanded as the tube was 

 erected, and so allowed the mercury to descend. To get 

 rid of this bubble, the tube was turned down so as to allow 

 the bubble to pass along the tube ; but, owing to its small 

 size, it did not pass many inches along the tube before 

 it became fixed between the mercury and the glass. When 

 the bubble came to a standstill at about six inches from the 



