62 TIMBER AND SOME OF ITS DISEASES, [chap. 



thicker and thinner, the chapelct de Javnn is even 

 more effective. 



Jamin has shown, further, that porous bodies such 

 as gypsum, absorb water with a force equal to a 

 pressure of several atmospheres : when such bodies arc 

 saturated, they are practically impervious to air, though 

 easily permeable by water. Hence a block of gypsum 

 may be fixed to each end of a glass tube, the apparatus 

 saturated with water and the tube filled, and if the 

 lower block of gypsum is placed in wet sand and the 

 upper exposed to the air, the evaporation at the ex- 

 posed end is compensated by a flow from below. 



Jamin thought this explained the ascent of water 

 in plants, and that the lumina of the vessels, &c., corre- 

 sponded to the capillaries of his system. Hofmeistcr 

 on the contrary thought the experiment confirmed 

 Meyen's view that the water passes up as imbibed 

 water — supposing the wood-walls to correspond to 

 the porous body. We shall see how Sachs has 

 extended this idea ; but it should be clearly appre- 

 hended that Sachs's idea of imbibition is a very 

 different one from the old notion of its dependence 

 on capillarity. In a capillary system there are pores, 

 and air may be driven out : the water of imbibition 

 is inter-molecular (or at most inter-micellar) water. 



Such were some of the views which led gradually 



