CH. IV] FILTRATION EXPERIMENT. 81 



rubber tube to the short arm of a U tube. By pouring 

 mercury into the long arm it will be found possible to 

 force air through ; to make this obvious paint the upper 

 surface of the wood with olive oil in which the escaping 

 bubbles are visible. 



(ii) A similar cylinder which has been thoroughly 

 soaked in water is fitted into the U tube : the wet cell- 

 membranes will be found to be extremely, though not 

 absolutely, impermeable to air. 



(iii) If the U tube is now filled with water it will be 

 found that a very slight pressure of water forces water 

 through. 



(102) Oozing of water from the lower end of wood. 



The experience gained in experiment 101 enables us 

 to understand the following experiment'. A yew-branch 

 (two inches in length) is placed in water until thoroughly 

 soaked. When removed from the water and held with its 

 axis vertical no water escapes from the lower surface 

 (although water is contained in the tracheids) because if 

 water is to escape air must enter, and air does not easily 

 pass wet membranes. If however a drop of water is added 

 (e.g. with a wet paint brush) to the upper surface, water 

 immediately oozes out below. 



(103) Permeability of splint wood. 



A modification of the experiment may be used to 

 illustrate the fact that the water travels in the splint-' 



' See Sachs in his Arbeiten, ii. p. 296. Also Godlewski, Pringsheim's 

 Jahrbilcher, xv. 



D. A. 6 



