FRUITS FROM THE TROPICS 299 



The full power of the molecular forces that, 

 acting jointly, carry the water to the tree tops 

 will best be understood when it is recalled that if 

 a rubber tube is put tightly about the end of an 

 amputated twig, water in this tube will be forced 

 upward by the pressure of water in the cells of 

 the twig. This experiment, first made by Hales 

 in 1727, in itself shows how r utterly different are 

 the conditions of water in the tree from the mere 

 mechanical condition of pressure that governs the 

 water in a closed tube, or otherwise standing in 

 a single receptacle. 



TITANIC MOLECULAR FORCES 



Many boys have made the experiment of burst- 

 ing a barrel by the pressure of water in a small 

 iron pipe projecting upward from the barrel. 



Whoever has seen the experiment will not 

 doubt that the physical laws governing the water 

 in the trunk of the tree are quite different 

 from those that govern the water in the iron 

 tube. And the difference is due, the physicists 

 assure us, to the interposition of the molecular 

 forces. 



Whether or not the laws of osmosis, above out- 

 lined, as discovered by Vant Hoff, give full ex- 

 planation is matter for the physicists to decide. 

 As yet they are not quite sure about it. But that 



