CAUSES OF THE MOTION OF JUICES. 389 



Capillary Attraction. If two squares of glass be 

 set up together upon a plate, so that they shall be 

 in contact at their vertical edges on one side, and one- 

 eighth of an inch apart on the other, it will be seen, on 

 pouring a little water upon the plate, that this liquid 

 rises in the space between them to a hight of several 

 inches where they are in very near proximity, and curves 

 downwards to their base where the interval is large. 



Capillary attraction, which thus causes liquids to rise 

 in narrow channels or fine tubes, involves indeed the 

 adhesion of the liquid to the walls of the tube, but also 

 depends on a tension of the surface of the liquid, due to 

 the fact that the molecules at the surface only attract 

 and are only attracted by underlying molecules, so that 

 they exert a pressure on the mass of liquid beneath them. 

 Where the liquid adheres to the sides of a containing 

 tube or cavity, this pressure is diminished and there the 

 liquid rises. 



Adhesion may be a Cause of Continual Move- 

 ment under certain circumstances. When a new cotton 

 wick is dipped into oil, the motion of the oil may be fol- 

 lowed by the eye, as it slowly ascends, until the pores 

 are filled and motion ceases. Any cause which removes 

 oil from the pores at the apex of the wick will disturb 

 the equilibrium which had been established between the 

 solid and the liquid. A burning match held to the 

 wick, by its heat destroys the oil, molecule after mole- 

 cule, and this process becomes permanent when the wick 

 is lighted. As the pores at the base of the flame give up 

 oil to the latter, they fill themselves again from the 

 pores beneath, and the motion thus set up propagates 

 itself to the oil in the vessel below and continues as long 

 as the flame burns or the oil holds out. 



We get a further insight into the nature of this motion 

 when we consider what happens after the oil has all been 

 sucked up into the wick. Shortly thereafter the dimen- 



