.ABSORPTION OF MATERIALS IN GENERAL III 



Briicke (1843) advanced a theory of diffusion through septa, based upon 

 the observation that if two liquids are separated by a membrane, the one that 

 wets the membrane more thoroughly (i.e., in which the latter swells 

 more rapidly) penetrates more rapidly. For example, if a membrane of rubber 

 or collodion be employed, then alcohol passes through more rapidly than water, 

 but with a membrane of animal bladder the opposite is true. Rubber and 

 collodion membranes imbibe alcohol more rapidly than water and they also 

 swell more in alcohol. Thus alcohol passes through such septa more rapidly. 

 But animal bladder swells in water and shrinks in alcohol, and water passes 

 through such a membrane more quickly than does alcohol. Animal bladder 

 swells more in pure water than in salt solution, and the former passes through 

 such a septum more rapidly than does the latter. These facts indicate that the 

 water is more forcibly attracted by the membrane substance than are the salts, 

 so that the concentration of the imbibed solution in the pores of the membrane 

 increases as the distance from the pore walls becomes greater. Ludwig 1 has 

 shown further that if dry pieces of animal bladder are placed in a solution of 

 sodium sulphate or sodium chloride, the solution that is imbibed is less concen- 

 trated than what remains. By means of a hand press he pressed some of the 

 liquid out of such impregnated pieces of bladder, and found that the expressed 

 solution possessed a concentration higher than the average concentration of 

 the solution originally within the pores of the bladder." 



Osmotic pressure is studied by various kinds of osmometers. BaranetskiiV 2 

 osmometer consists of two chambers separated by a membrane, one containing 

 a salt solution, which is to increase in volume, while the other contains water 

 introduced through a funnel that is attached by a rubber tube. As the solution 

 increases in volume a rubber-tube outlet from the solution chamber allows 

 the overflow to be caught in a graduated flask. The surface of the water in 

 the funnel must be kept at the same height as that of the solution in the exit 

 tube. The movement of liquids through the membrane continues until the 

 concentration of the two solutions is the same on both sides. 



Experiments upon diffusion of dissolved substances through membranes 

 have shown that all water-soluble substances may be classified into two groups 

 according to their relation to the membrane, those which can pass through the 

 membrane (crystalloids) and those which cannot (colloids). Upon these dif- 

 ferent properties of colloids and crystalloids depends the method of dialysis, 

 by which colloid material may be separated from crystalloids. Many plant 

 substances are colloids and they cannot, therefore, diffuse out of the cells.* 



1 Ludwig, C, Ueber die endosmotischen Aequivalente und die endosmotische Theorie. Poggendorff's 

 Ann. Phys. u. Chem. 154 ("der ganzen Folge") : 307-326. 1849. 



2 Baranetskii, I. Investigations on diosmosis as related to plants. [Russian.] Inaug. Dissertation. 

 St. Petersburg, 1870. Barenetzky, J., Diosmotische Untersuchungen, Poggendorff's Ann. Phys. u. Chem. 

 223 ("der ganzen Folge") : 195-245. 1872. 



These considerations give the reason why the membrane is more permeable to one sub- 

 stance than to the other, or, they merely state this fact in other terms. — Ed. 



h But the matter is not so simple as this. Many water-soluble crystalloids fail to pass cer- 

 tain membranes that are permeable to water, and some colloids do pass them. Colloids and 

 crystalloids are difficult to distinguish accurately, these terms referring to the state rather than 



