CAUSES OF THE MOTION OF 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 porcus 
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 Diffusion. — 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 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 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. Ifthe wa 
