34 LECTURE III. 



Let us now see how imbibition can be explained from 

 this point of view. Strasburger points out that there is 

 reason to believe that the feeble diffusibility of all colloids 

 is due to the connexion of their molecules in the manner 

 described above. Some colloids, such as the acrylcolloid 

 mentioned above, gum, gelatine and others, are capable, 

 whereas others, such as gelatinous silicic acid, are incapable 

 of swelling up. The conclusion from this is that the molecu- 

 lar network is in some cases extensible, in others inextensible, 

 that is, that the molecules may or may not be mobile about 

 their position of equilibrium. Inasmuch as the proportion 

 of water in a colloidal substance depends upon the size of 

 the intermolecular meshes, swelling-up, that is, the absorption 

 of water, must be due to the increase in size of the meshes 

 of an extensible molecular network. The limit of swelling-up 

 is reached when the capillary attraction is equal to the chemical 

 affinity ; when the former exceeds the latter the molecules 

 become dissociated and the organised structure is destroyed. 



But certain facts yet remain to be explained. How are 

 we to account, from this point of view, for the fact that 

 different parts of an organised body contain different pro- 

 portions of water, and for the fact that the swelling-up of 

 such a body is unequal in different directions ? With regard 

 to the first of these, Strasburger points out that the amount 

 of water which a colloid which does not swell up, such as 

 silicic acid, can absorb, is just that which it contained at its . 

 first formation ; in other words, that the size of the meshes 

 of the molecular reticulum is determined by the amount 

 of water present in them when the reticulum is formed. 

 Thus the unequal distribution of water in an organised body 

 may be ascribed to the unequal size of the intermolecular 

 meshes in different parts. This affords also some expla- 

 nation of the unequal swelling-up of organised bodies, but 

 not a complete one. The important feature in the swelling- 

 up of organised bodies to which Strasburger draws attention, 

 a feature which distinguishes these bodies which have been 

 formed by living protoplasm from the unorganised colloids 

 which are capable of swelling, is that it bears a definite 

 relation to their anatomical structure. This relation is that 



