Lecture XXV. 



225 



which is closed at the end of the long arm, is partly filled with 

 water. By inclining the tube the water may be made to occupy 

 the long arm and the bend. Now if the tube be brought into 

 an upright position the water will remain in the long arm. It 

 is held there by the atmospheric pressure pressing on the surface 

 in the short arm. This is easily proved by attaching an air- 

 pump to the short arm, and removing the atmospheric pressure 



B. 



FIG. 79. J-shaped glass tube ; A, in vertical position, contained water lying 

 in the bend; B, inclined to bring the water into the long limb; C, in 

 vertical position, the water being retained in the long limb by its 

 cohesion and by its adhesion to the glass. 



Immediately the water falls down out of the long arm and 

 occupies the bend rising to the same level in each arm. In this 

 second similar tube special precautions have been taken to free 

 the inner surface of the tube from grease, so that the water 

 will wet it and adhere to it. Further, by prolonged boiling I 

 have made certain that all dust particles in the water itself are 

 also wetted. While the water in the tube was still boiling the 



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