BY END0SM08E, 13 



accounted for by what was called a, vis a tergo, a power or pressure 

 from behind ; but this, like the older aphorism that Nature abhors 

 a vacuum, will not stand examination ; and the elasticity and tension 

 of the wood accounts for all that was seen in the experiment which 

 suggested the phrase vis a tergo, or pressure from behind, though not 

 for all the phenomena connected with the rising of the sap. But in 

 endosmose there may be found a satisfactory mode of accounting for 

 all that is known to take place in connection with the flow of the sap. 



I do not know a more simple illustration of exosmose and endosmose 

 than that which I have given, but to illustrate more fully the pheno- 

 mena, with a view to the explanation of the rising of the sap, I may 

 give the following : Let a piece of bladder be stretched over the 

 mouth of a funnel and tightly fastened there, and let a tube, if one 

 of glass SQ much the better, be attached to the orifice of the funnel ; 

 let this funnel, half-filled with a solution of salt, be immersed in water 

 with the tube projecting upwards. The water in the vessel wUl 

 become slightly saline through the escape, by exosmose through the 

 organic tissue, of a small portion of the solution of salt, but this will 

 be replaced by a much greater quantity of water entering by endos- 

 mose, and so increasing the quantity of water within the funnel 

 and tube, that this will be seen to rise in the tube, until — through 

 the dilution of this, and the loss, by exosmose, of a portion of the 

 substance held iu solution — an equilibrium be established between 

 the strength of the solution on the one side and on the other of the 

 separating membrane. But shoidd the tube be insufficient to contain 

 such a quantity as this would render necessary, the surface of the 

 solution will rise till the solution overflows, and it wiU continue to 

 flow until the equilibrium spoken of be established. 



This experiment, with its results, makes manifest the difierence 

 between endosmose and capillary attraction, depending, as they do, on 

 difiPerent actions; and it seems to indicate that the process of endosmose 

 may supply a satisfactory explanation of the rise of the sap in lofty 

 trees, though capillary attraction fails to do so. 

 ^ The phrase capiUary attraction is employed to account for the rise 

 of liquids to a considerable height in tubes of small diameter, and 

 through interstices of similar size in solid bodies. If several tubes 

 of different diameters of bore have their lower extremities immersed 

 in water and be held erect, the water will rise in them to different 

 heights and be sustained so elevated. The smaller the bore is the 

 greater will be the height to which the water will rise and at which 

 it will be sustained, hence the derivation of a name from gapilla, a 



