IMBIBITION. 209 



these cases the water penetrates into cavities — into small visible and invisible pores, 

 previously filled with air which now becomes forcibly expelled by the water 

 passing in : no pushing asunder of the solid parts occurs here, as is obvious from 

 the fact that the volume of the porous body is not perceptibly enlarged by the 

 penetrating water. 



The chief point in connection with the swelling of an organised body, and upon 

 which stress is to be especially laid, is, as follows from the preceding remarks, that 

 the water of imbibition by no means passes in through pre-formed cavities or pores ; 

 but that it penetrates by forcing asunder the minute particles {niicellcB) of the 

 swelling body, as is evident at once by the increase in volume. Organised bodies 

 are therefore not porous in the ordinary sense of the word; and the penetration 

 of water into them does not occur by capillarity, so-called. A second point of 

 the greatest importance in the phenomena of the swelling of organised bodies, 

 is with respect to the extraordinary force with which the water penetrates 

 and drives asunder the solid particles.- This may be recognised, for instance, in 

 that dry wedges of wood driven into blocks of granite, and then moistened, expand 

 with a force so great that the stone may be split. The impossibility of completely 

 squeezing the water out of swollen vegetable cell-walls by pressure, however great, 

 also shows with what immense force the molecules of water are held between those 

 of the cell-wall, and gives a measure of the force with which they have penetrated into 

 the dry membrane. We shall make use of these facts in the theory of the move- 

 ment of water, and they will remove for us all those difficulties which have previously 

 been felt. The further fact that bodies capable of imbibing, such as dry cell-walls and 

 starch-grains, condense the aqueous vapour from the surrounding air to such an 

 extent that they become entirely or almost entirely saturated with water, or swollen, 

 may also be taken as evidence of the magnitude of the attractive forces between 

 molecules of water and these organised bodies. These however are only incidental 



however the water molecules are at length distributed equally between those of the swelling body, 

 they are there held as fast as the salt molecules distributed in the water of the solution. 



The water-molecules contained in a cell-wall in a state of imbibition evidently exert mutual 

 pressure just as little as the salt-molecuIe5 in a solution : the imbibed water-molecules form a 

 coherent mass of fluid just as little as the dissolved salt-molecules of a crystal do. Of course the 

 case is otherwise in a porous capillary body. In such a body provided with pre-formed capillaries 

 the height of the capillary column depends upon the weight of the continuous column of water, 

 and this exerts a pressure on the walls according to its height. In an imbibing body the weight 

 of the water does not come into consideration. It is therefore immaterial whether the imbibed 

 water in the cell-walls of a tree is 20 or 100 metres high. 



The comparison of the imbibed water in a cell-wall or any other body capable of swelling and 

 imbibing with the condition of the water of crystallisation is perhaps still more apparent : no one 

 would assume that this is contained in capillary cavities of the crystal. The water of crj-stallisation 

 is also present between the molecules of the salt in a form in which it can no longer be designated 

 a fluid : it exists in a form which prevents the water molecules from exerting pressure on one 

 another and obeying the laws of hydrostatics applicable to a capillary water column, however fine. 

 Like the water of imbibition, the water of crystallisation also may be driven off by heat, at least in 

 some cases; but of course the crystalline form is then destroyed. P'undamentally, however, some- 

 thing of the same kind seems to take place in the conducting cell-walls of the wood ; for their 

 desiccation even at lower temperatures produces essential alterations in the imbibing properties, since 

 air-dry wood loses the specific property of rapidly conducting the water of imbibition ; it may 

 therefore be assumed that a permanent alteration in the molecular structure of the cell-walls is 

 effected by a certain degree of desiccation. 



[3j. 



