850 HOW CROPS GROW. 



tiou, which have been alluded to. In many phenomena 

 of absorption, however, chemical affinity appears to super 

 vene with more or less vigor. 



Adhesive attraction is not manifested universally be 

 tween solids and liquids, as already hinted. Glass dipped 

 in mercury is not touched or wetted by it, and when a 

 capillary tube is plunged in this liquid, we see no rise, but 

 a depression within the bore. A greased glass tube de 

 ports itself similarly towards water. 



Adhesion may be a Cause of Continual Movement un 

 der certain circumstances. When a new cotton wick is 

 dipped into oil, the motion of the oil may be followed by 

 the eye, as it slowly ascends, until the pores are filled. 

 At this moment the adhesive attraction between cotton 

 and oil is satisfied, and motion ceases. Any cause which 

 removes oil from the pores at the apex of the wick will un- 

 satisfy their attraction and 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 molecule, and this process becomes per 

 manent when the wick is lighted. As the pores at the 

 base of the flame give up oil to the latter, they fill them 

 selves 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. 



In this process, the pores, if of the same material and 

 of equal size, exert everywhere an equal attraction for 

 the molecules of oil. The wick, above, contains indeed 

 less oil than below, for two reasons. In the first place, 

 gravitation, or the earth s attraction, acts most power 

 fully on the oil below, and secondly, time is required 

 for the particles of oil to pass upwards, and they cannot 

 reach the summit as rapidly as they might be consumed. 

 We get a further insight into the nature of this motion 

 when we consider what happens after the oil has all been 

 linked up into the wick. Shortly thereafter the dimon- 



