14 CAPILLABT ATTRACTION. 



hair. In such phenomena the attraction of adhesion may be inferior 

 in force to the attraction of cohesion, or equivalent thereto, but it is 

 so great as to overcome the attraction of gravitation and to elevate 

 the contained column of water above the level of the mass to which 

 the attraction of gravitation would lower it but for the conteracting 

 influence thus exercised upon it. The quantity of water thus raised 

 is proportional to the area of the surface of adhesion, and to this the 

 attraction is confined ; and thus it comes to pass that capillary 

 attraction cannot raise water till it overflow, as it does in the experi- 

 ment on endosmosic action which has been spoken of, and for which 

 the diiferent action by which endosmose is efl^cted enables us to 

 account. That action has already been explained. 



If a worsted thread six inches long have two inches at the one end 

 immersed in water filling a basin to within half an inch of the top, 

 and the other end be allowed to hang down outside the vessel, the 

 water may rise in it and after a time begin to drop from the pendant 

 extremity, which it will do more readily if it be thoroughly 

 moistened immediately before ; and this may seem at variance with 

 what I have just said. If, however, we distinguish things which 

 difier, we may see that it is not so. The water may be raised in the 

 capillary interstices of the worsted thread in opposition to the attrac- 

 tion of gravitation to the level of the basin, and it may be onward 

 beyond it ; it may have been attracted a little way on its descent out- 

 side by the same force, but here there came into operation along with 

 this the attractive force of gravitation; as the water was drawn 

 onwards and downwards by capillary attraction, the co-operating force 

 of gravitation would be increased ; and at length the worsted thread 

 without ceasing to act by capillary attraction would become a verit- 

 able syphon, in which the gravity of the water in the longer limb, 

 and the pressure of the atmosphere on the surface of the water in 

 the basin would occasion a continuous flow. 



I have not said all that might be said in regard to this ; but I have 

 said enough to show that this is different from the overflow at its 

 upper extremity of a tube filled by endosmose from below, and con- 

 versely that this is different from that, leaving it free to indicate that 

 endosmosic action may, as has been said, supply an explanation of 

 the rise of sap in lofty trees, though capillary attraction fails to do so. 



I have referred to an opinion that the rise of the sap may be 

 attributable to a vis a tergo, and some of my readers may think that 

 in endosmose he has found this vis a tergo, or, at least, a vis a tergo 

 which will explain all ; and to this I would nest attend, ,' 



