COHESION OF LIQUIDS. 21 



and elasticity by hammering. Bright commercial brass wire is 

 not so soft as brass which has been thoroughly heated ; but it is 

 still somewhat flexible. The ring may be flattened by a good 

 smooth hammer upon an even hard surface, or best upon a small 

 anvil. A ring thus hammered equally all round opens slightly ; to 

 keep the two arms a close together, the hammer should be held in 

 a somewhat slanting direction, so as to make the external edge of 

 the ring thinner than the interior. 



On applying pressure at the ends d and e by thumb and fore- 

 finger, the bars a separate, and the rubber tube is inserted between 

 them ; if the pressure ceases, the bars approach each other and close 

 the tube. / 



If not carefully examined, liquid bodies manifest 

 neither the cohesion of rigi<J bodies, nor the expansive 

 force of gases. Poured from a vessel, a liquid breaks 

 up into single drops ; on immersing the hand in water 

 very little resistance is felt. Closer observation proves, 

 however, that cohesion exists between liquid particles, 

 although it is small. Without cohesion water poured 

 from a vessel would not form drops, but extremely fine 

 particles like dust. A small dry needle placed hori- 

 zontally upon water will not sink, because its small 

 weight is not sufficient to overcome the cohesion of the 

 liquid particles at the .surface, and to break through 

 them. But if the point is immersed, the needle imme- 

 diately sinks ; this proves that the needle was not 

 swimming upon the water, but was prevented from 

 sinking by the cohesion between the liquid particles. 



The cohesion of liquids is most obviously manifested 

 by the liquid pellicles or films which may be formed 

 from all liquid bodies, but most easily from a solution 

 of soap in water. These liquid films demonstrate not 

 only a comparatively strong cohesion, but also a distinct 

 tendency of the particles to contract and thus to occupy 



