Dixon — The Cohesion Theory of the Ascent of Sap. 51 



^air, was erected vertically. The tube was composed of short 

 lengths, connected end to end by rubber-joints. These short 

 lengths were filled with the plaster separately, and, as the plaster 

 set, were boiled and cooled, and then kept immersed in boiling 

 water for several hours. At the beginning of the experiment the 

 whole tube terminated above in a funnel, also filled with plaster 

 of Paris, which supported a chemically-produced semi-permeable 

 membrane. The lower end terminated in a (J -tube containing 

 mercury and water. At a height of 8*4 m., by means of a T-joint, 

 it was possible to connect a capillary tube, the lower end of which 

 dipped under the surface of mercury. 



At the beginning of the experiment, while evaporation was 

 going on from the funnel, and before the lateral capillary was 

 connected, mercury rose in the lower U-tube to the height of 

 150 mm. in four days, the rate of rise apparently slightly falling 

 ■off. How much of this rise is due to evaporation above, and how 

 much to other causes, is uncertain. There are no contemporary 

 records of temperature and air-pressure given. 



After four days the funnel was removed and the upper end of 

 the tube closed. During the next few days the lateral capillary 

 was put into connexion with the main tube ; and later on the wide 

 U-tube below was replaced by a tube of similar bore to the upper 

 capillary. With these arrangements mercury rose in both capil- 

 laries. According to the final records, the mercury stood in the 

 lower capillary at 428 mm., and in the upper one at 550 mm. 

 above the external mercurial level. 



Copeland seems to believe that his results indicate a continuous 

 rise of water in the tube, due to the difference of pressure at the 

 top and bottom of the tube indicated by the mercurial level in 

 the two capillaries ; and that this difference was established by 

 evaporation from the funnel, and maintained by a kind of a lag. 

 But, he says, " an analysis showing the elementary factors by 

 which the water is raised has baffled me, even in my own appara- 

 tus." And he comes to the astonishing conclusion, "that a 

 difference in pressure of less than one atmosphere between top and 

 bottom will lift water much more than 10 m. under the peculiar 

 •conditions here present." 1 And again : — " The positive result of 



1 Loc. cit., p. 165. 



