56 COLLOIDS IN BIOLOGY AND MEDICINE 



ability of true albumins to diffuse into gelatin jellies. This fact was 

 demonstrated by means of the precipitin reaction. If goat serum 

 is mixed with rabbit serum nothing noteworthy occurs. If the 

 rabbit has previously been injected with goat serum and the serum 

 of the previously treated rabbit (called " goat-rabbit serum") is 

 mixed with goat serum there occurs a precipitation of an albumi- 

 nous substance, called " precipitin." BECHHOLD mixed a 1 per cent 

 gelatin solution containing 0.85 per cent NaCl with an equal vol- 

 ume of goat-rabbit serum. The jelly was solidified in the ice box 

 and goat serum was layered over it. At the end of 24 hours a cloudy 

 precipitate formed in the gelatin which in the course of 120 hours 

 penetrated as far as 5 mm. The same phenomena occurred when 

 the gelatin was mixed with goat serum and goat-rabbit serum was 

 layered over it. Thus in both cases actual constituents of the serum 

 had diffused into the gelatin. 



Similarly, Sv. ARRHENIUS* and TH. MADSEN showed that not 

 only diphtheria toxin and tetanolysin, but the highly colloidal diph- 

 theria antitoxin and antitetanolysin could diffuse into 5 per cent 

 gelatin jellies. 



Such a diffusion of colloids into a jelly naturally may be expected 

 if the meshes are quite wide, i.e., if the jelly is quite dilute. 



Membranes. 



It will be appropriate to introduce the concept of "membrane" in 

 the following way: if we make the colloid medium, the jelly, thicker 

 and thicker, i.e., poorer in water, diffusion must be increasingly hin- 

 dered. We very soon reach a point where no colloids are able to 

 diffuse into it and we have reached a special case in our previous 

 exposition, the membrane. We may describe membranes as irrever- 

 sible gels, whose surface is very great in relation to their thickness. 

 They play an important role in the organism, but- we shall here 

 discuss their general properties only, as their biological functions 

 will be considered in Part III. 



An excellent general resume with a very complete bibliography 

 has been published by H. ZANGGER (" Membranes and the Functions 

 of Membrane"). 



On account of the great physical and chemical differences of the 

 membranes of the organisms the employment of artificial membranes 

 is preferable for the study of their chief properties. 



If a very dilute solution of potassium ferrocyanid is carefully 

 layered over a concentrated solution of copper sulphate, there is 

 formed at the layer of contact, by chemical interchange, a very thin 

 brown film of copper ferrocyanid. Naturally this film is very deli- 



