390 HOW CROPS GROW. 



sions of the flame are seen to diminish. It does not, 

 however, go out, but burns on for a time with continually 

 decreasing vigor. When the supply of liquid in the por- 

 ous body is insufficient to saturate the latter, there is 

 still the same tendency to equalization and equilibrium. 

 If, at last, when- the flame expires, because the combus- 

 tion of the oil falls below that rate which is needful to 

 generate heat sufficient to decompose it, the wick be 

 placed in contact at a single point, with another dry 

 wick of equal mass and porosity, the oil remaining ia 

 the first will enter again into motion, will pass into the 

 second wick, from pore to pore, until the oil has been 

 shared nearly equally between them. 



In case of water contained in the cavities of a porous 

 body, evaporation from the surface of the latter becomes 

 remotely the cause of a continual upward motion of the 

 liquid. 



The exhalation of water as vapor from the foliage of a 

 plant thus necessitates the entrance of water as liquid 

 at the roots, and maintains a flow of it in the sap-ducts, 

 or causes it to pass by absorption from cell to cell. 



Liquid Diffusion. — The movements that proceed in 

 plants, when exh-alation is out of the question, viz., such 

 as are manifested in the stump of a vine cemented into a 

 gauge (Pig. 43, p. 348), are not to be accounted for by 

 capillarity or mere absorptive force under the conditions 

 as yet noticed. To approach their elucidation we require 

 to attend to other considerations. 



The particles of many different kinds of liquids attract 

 each other. ' "Water and. alcohol may be mixed together 

 in all proportions in virtue of their adhesive attraction. 

 If we flll a vial with water to the rim and carefully lower 

 it to the bottom of a tall jar of alcohol, we shall find 

 after some hours that alcohol has penetrated the vial, 

 and water has passed out into the jar, notwithstanding 

 the latter liquid is considerably heavier than the former. 



