100 BIOLOGY OF PLANTS. 



the point of junction. Cohesion gives to fluids a globular 

 form as in the case of drops of water. A modification of this 

 power, called capillary attraction, has considerable influence 

 in the ascent of the sap through the common vessels, and in 

 the absorption of moisture by the leaves and roots of plants. 

 The mode, in which this is done, may be shown by placing 

 straws in a basin of water ; the water in the straws will rise 

 much higher than that in the basin. The principle is exem- 

 plified in the wick of a lamp, the oil being drawn up by this 

 power ; also in the sponge, in sugar, and almost any body con- 

 taining small tubes or pores. The force of cohesion, however, 

 does not fully account for the ascent of the sap, although it 

 may aid other forces in promoting its circulation. 



III. Chemical Affinity. This power differs from gravita- 

 tion and cohesion in the circumstance, that its force is always 

 exerted between different kinds of matter.* A bar of iron, 

 for example, is held together by cohesion, and is attracted to 

 the earth by gravitation; but when the iron is moistened, the 

 oxygen of the water, a very different substance from iron, 

 unites with it and forms iron rust. This is effected by chem- 

 ical affinity. So in the common soda powders, the tartaric 

 acid and the soda, two different kinds of substances, combine 

 by the force of chemical affinity. Various kinds of matter 

 possess this attraction with different degrees of force ; thus, 

 in the soda powders, the soda is combined by affinity to car- 

 bonic acid, but the tartaric acid has a stronger attraction for 

 the soda than the carbonic acid has, and displaces it; the 

 liberated gas passes up through the water, and gives rise to 

 the foam and eflfervescence which is so desirable and grate- 

 ful, when such waters are used as a drink. This is some- 

 times called elective affinity, and gives rise to all the decom- 

 positions of matter. It will be readily perceived, that this 

 agent must exert great influence in the phenomena of vege- 

 tation. The soil itself is composed of substances united by 



* See Introduction. 



