350 HOW CROPS GROW. 
tion, 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 
-sucked up into the wick. Shortly thereafter the dimen- 
