CAUSES OP THE MOTIOX OP JUICES. 351 



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

 ever go out, but burns on for a time with continually de- 

 creasing vigor. When the supply of liquid in the porous 

 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 combustion of 

 the oil falls below that rate which is needful to generate 

 heat sufficient to decompose it, the wick be placed in con- 

 tact at a single point, with another dry wick of equal 

 mass and porosity, the oil remaining in the first will enter 

 again into motion, will pass into the second wick, from 

 pore to pore, until equilibrium is again restored and the 

 oil has been shared 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 DiifilSionf — The movements that proceed in 

 plants, when exhalation is out of the question, viz., such 

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

 guage, (fig. 43, p. 248,) 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 difierent kinds of liquids attract 



eacli other. Water and alcohol may be mixed together 



in all proportions in virtue of their adhesive attraction. 



If we fill 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. If the wa-^ 

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