184 WRITINGS OF JOSEPH HENRY. [1855- 



its lower end placed in water, the liquid will rise perhaps to 

 an inch above the general level of the liquid in the vessel, 

 and here it will remain. The cause of this ascent is the at- 

 traction of the glass for the liquid and the liquid for itself, 

 and is familiarly known under the name of capillarity. A 

 perpetual flow of water can never be produced by this action 

 since if we cut off the tube before-mentioned, leaving but 

 three-fourths of an inch above the water, the attraction of the 

 glass will draw the liquid up to the very top, but will not 

 permit it to run over, because the same attraction which sus- 

 pends it will prevent it from overflowing. The atom of 

 water at the top of the tube will be attracted as much down- 

 ward by the glass as the next one below will be attracted 

 upwards; hence an equilibrium will ensue. 



If however we apply heat to the upper surface, which will 

 evaporate the water, a new portion will be drawn up to re- 

 store the equilibrium ; and if this process be continued, a con- 

 stant current will be maintained, and a definite amount of 

 mechanical work will be performed. If the liquid contain 

 different substances in solution, these will be retained, it may 

 be in a solid form, and in this way a solid substance may 

 be brought up and deposited at the end of the tube. If across 

 the lower end of the tube a porous membrane be stretched, 

 and if the liquids above this, and that in the vessel below, 

 be of a different quality, which would necessarily result on 

 account of the evaporation mentioned, then the ascensional 

 power would be very much increased by the process called 

 endosmose. Without considering at present this action very 

 minutely, we may apply the principles we have here given 

 to the means by which heat becomes a motive power in build- 

 ing up a plant. The stem of a tree is an arrangement anal- 

 ogous to an assemblage of minute tubes, such as we have 

 described, terminating in leaves above, from the surface of 

 which constant evaporation is going on, and a current of 

 liquid ascending called crude sap, which consists of water 

 containing in solution the various substances imbibed by the 

 roots, and elaborated by the leaves. The tubes are not con- 

 tinuous, but are elongated cells analogous to a glass tube, the 

 ends of which are closed with porous membrane. 



