THE SURFACE TENSION OF LIQUIDS. 



595 



homogeneous medium, he takes the envelope itself — a drop void of 

 water, or rather full of air — represented for convenience of ma- 

 nipulation by a soap-bubble, and consisting of two films separated 

 by an extremely thin mass of water. The pressure is the same in 

 every part, and the curvature uniform, and that which gives the 

 least possible surface — a sphere. The pressure is strong enough 

 to drive tobacco-smoke back through a pipe-stem or to blow out a 

 candle. The curved film may be deformed by passing it through 

 rigid frames, but it will always preserve a geometrical shape, for 

 it can not continue to exist except upon the condition of exercis- 

 ing an equal pressure throughout upon the air imprisoned within 

 it ; but some of the shapes it will assume within this rule are very 

 curious. 



If a drop of water is poured upon another liquid, it is still im- 

 prisoned in its contractile sac, but in one having two walls of 

 unequal elasticity ; the upper wall resting against the air, and the 

 lower one against the liquid. The line of suture of these two 



Pig. 3.— A Paper Box closing upon itself when Water is poured into it. 



walls floats in three different media — air, water, and the subjacent 

 liquid ; or, to use M. Gossart's figure, it is like a cord drawn by 

 three different forces, which are represented in this case by the 

 upper and lower walls of the sac and the uncovered membrane 

 of the inferior liquid, pulling against one another, as when three 

 ropes are pulled by three men of unequal strength. Suppose, as 

 the extreme case, that the attraction of the membrane exterior to 

 the drop so prevails over the tension of the two walls of the sac 

 that they can not rest in equilibrium. Then the sac will be drawn 



