IV.] VARIOUS THEORIES, &c. 123 



able to water but not to air. Such a tube held 

 vertical lets no water flow out, because no air can 

 enter through the wet membrane above ; but if a drop 

 of water is placed on the upper membrane, it is at 

 once absorbed by the water inside, and a corre- 

 sponding drop appears on the outside of the lower 

 membrane. But the movement is not due to the 

 weight of the drop — it is caused by the weight of 

 the whole column in the tube ; the whole weight being 

 now greater than the difference of pressures at the 

 top and bottom of the system. 



If the tube is divided up by cross membranes, the 

 only difference is that the resistance of several 

 membranes has to be overcome, and a vertically 

 placed stick of Yew is just such a system, whence 

 Hartig's conclusion was wrong. 



But he was still more in error in regarding the 

 experiment as proving anything respecting the lifting 

 of the water — the filtration of the water downwards 

 is a very different matter from a lift upwards. Thus 

 supposing we place a piece of Yew branch, one meter 

 long, vertically into ninety cm. of water : if the pressure 

 of Hartig's drop sufficed to move the column, then 

 the pressure of ninety cm. of water ought to drive a 

 very fountain, which of course it does not. 



The real explanation of Theodore Hartig's ex- 



