REPOUT ON CAPILLAHY ATTRACTION. !?5& 



founded on them, deserve to be brought as much as possible 

 into notice. 



Capillary Attraction. — ^The theory of capillary phsenomena 

 will be best exhibited by tracing historically the principal steps- 

 by which it has arrived at its present state. 



Dr. Hooke is among the earliest speculators on the cause of 

 capillary attraction. He attributed the risfe of the fluid to a di- 

 minution of the pressure of the atmosphere within the tube, by 

 reason of friction against its interior surface. This opinion was 

 shown to be erroneous when the fluid was found to rise as high 

 under the receiver of an air-pump as in the open air. 



Hauksbee*, whose experiments on the capillary action of 

 tubes and glass plates have not even yet lost their value, made 

 the beginning of a true theory of the phsenomena, by ascribing 

 them to the attraction of the tube or plate. Having ascertained 

 by experiment that the thickness of the matter of the tube made 

 no difference as to the height to which the fluid ascended, he 

 saw that the attractive force must emanate entirely from the 

 particles of the tube situated at its interior surface. He does 

 not, however, pronounce a decided opinion whether the sphere 

 of their attraction extends mediately or immediately to the par- 

 ticles of the fluid situated about the axis of the tube ; and he is 

 in error in supposing that this attractive force, by pressing the 

 fluid particles perpendicularly against the capillary surface at all 

 the points with which the fluid is in contact, diminishes the 

 weight of the suspended column. 



In this last particular the explanation of Hauksbee was 

 shown to be untrue by Dr. Jui-inf, who found by experiment, 

 that the height to which water would rise in a tube, of which 

 the portion occupied by the fluid consisted of two cylinders of 

 dififerent diameters, depended only on the diameter of the upper 

 cylinder. If the lower cylinder was the larger of the two, as 

 soon as it was completely filled, the water rose in the upper cy- 

 linder to the same height above the external level as it would 

 have done in a tube uniformly of the same bore as this latter. 

 Hence he was led to ascribe the phsenomena to "^ the attrac- 

 tion of the periphery or section of the surface of the tube to 

 which the upper surface of the water is contiguous and coheres." 

 On this hypothesis he easily showed that the heights of ascent 

 in tubes of the same matter are inversely as their radii. For the 



• Physico-Mechanical ETperiments. London, 1709. pp. 139—169. Also va- 

 rious papers in the Philosophical Transactions for the years 1711 and 1712. 



+ Philosophical Transactions 1718, No. 355, p. 739. An additional paper 

 on the same subject, Phil. Trans. 1719, No. 363, p. 1083. 



