2o8 LECTURE XIII. 



one another ; or rather they become forcibly driven apart by the penetrating water. 

 This process of swelling is thus something quite other than the entrance of water 

 into a porous unorganised body, e.g. dry gypsum ^ or into a heap of sand, etc. In 



^ Although the view of the structure of organised bodies and those capable of swelling cited 

 in the previous note had been established and accepted by Naegeli, the view was nevertheless 

 held tenaciously for a long time, that the imbibition of water by such bodies could be referred 

 to the laws of capillarity in narrow, hollow tubules. That the capillary theory is in no way 

 capable of explaining particularly the movements of fluids in wood follows from Nsegeli and 

 Schwendener's considerations {'Das Alikroskop^ II Aufl. § 371), and indeed the more forcibly since 

 these investigators proceeded from the view that it was self-evident that capillarity was concerned in 

 the matter. I have in my work, ' Ucbcr die Porosität des Holzes^ (' Arbeiten des bot. Inst, in Würz- 

 burg,' II. Bd., p. 305, 1879), and jireviously in a preliminary notice, expressed myself on the matter 

 in question as follows: 'This view, that imbibition is only a special case of capillarity, was first 

 expressed by De Luc, and in fact because hygroscopic bodies after being completely saturated with 

 water on being brought into alcohol apparently maintain their condition of imbibition. The fact is 

 however incorrectly apprehended. If bodies capable of swelling and free from water, such as animal 

 glue, coagulated dry proteid, dry stems of Laiiiiuaria, &c., are placed in alcohol almost free from 

 ■water (98 "/o) they never swell up in it at all, and never increase in weight, or only very slightly. If 

 they are jDlaced dry in water they absorb very much of it, as is shown by weighing, and their volume 

 increases to nearly the volume of the absorbed water. This increase in volume proves that the 

 water does not penetrate into pre-formed cavities (capillaries), but that it drives asunder the mole- 

 cules of the substance, and that to the extent of its own volume. If such a saturated body is again 

 allowed to dry up it again assumes the previous volume, and the cavities which the water had 

 produced and filled disappear, the molecules again applying themselves to one another. Alcohol 

 and thick glycerine are not able to diive asu«der the molecules of dry bodies capable of swelling, 

 and therefore do not penetrate into them. Since, therefore, cavities in which water or glycerine or 

 alcohof could penetrate forthwith do not exist in dry bodies of this kind, a comparison of these 

 processes with the capillary entrance of fluids in large bodies can of course scarcely be spoken of. 



' When water, alcohol, or other fluids penetrate into bodies such as cast gypsum, chalk, or burnt 

 clay, which in the dry state possess actual capillary cavities, they drive before them the air 

 contained in the cavities, and this can be exhausted and measured ; when water, on the other hand, 

 penetrates into a dry body capable of swelling, no air is driven out, simply because it penetrates 

 into spaces which it first opens itself.' 



If dry bodies capable of swelling, which do not absorb alcohol or glycerine, are first placed in 

 water until they are completely swollen, and then brought into very strong alcohol or glycerine, the 

 effect may be very different according to the nature of the body. Glue contracts energetically, since 

 the imbibed water is withdrawn from it without an equal volume of alcohol or glycerine passing in. 

 Laminaria behaves quite otherwise : it contracts but little in 98 % alcohol, and, as is shown by 

 weighing and determining the volume, alcohol passes into the spaces left by the water. Here, 

 however, the internal condition of the Laminaria is altered : in the watery state it was flexible and 

 soft, in the alcohol it becomes hard and brittle. Even if the alcohol which has replaced the 

 water is driven out by heat, the Laminaria no longer contracts to its previous volume in the dry 

 state: it now contains evident capillary cavities filled with air, since it swims on water, while the 

 dry Laminaria sinks at once. The alcohol is thus unable to drive asunder the molecules 

 of the cell- walls when dry ; if the water however has driven these apart, the alcohol penetrates 

 into the cavities occupied by the water, because as it advances it makes the molecules of the cell- 

 walls immovable and prevents contraction. These experiments explain also why alcohol is so 

 extremely serviceable as a medium for preserving and maintaining the form of plants : it takes the 

 place of the water of the cell-walls, while it prevents the contraction of the molecules of the latter. 

 If plants are laid in alcohol quite fresh they preserve their fresh appearance, if drooping portions 

 are placed in it they maintain their drooping aspect. The protoplasm lying within the cell-walls 

 so stiffened contracts on the other hand, since it becomes rigid in alcohol. 



The imbibition of the cell-wall may be better compared with the process of solution of a salt, 

 than with the capillarity of porous bodies. Just as the water seizes upon the molecules of a crystal, 

 and takes them between its own molecules in solution; so the dry body capable of imbibition seizes 

 upon the water molecules, and forces them between its own. Both processes require time. \\ hen 



