CHAPTER XIII. 



PROPERTIES CONFERRED BY THE COLLOIDAL CON- 

 STITUENTS: STRUCTURE AND CONSISTENCY. 



THE EMULSION-STRUCTURE OF PROTOPLASM. 



One of the most important aspects of the relationship of the Lipoids 

 to the properties and behavior of protoplasm is that arising out of the 

 marked effect upon the tension of protoplasmic surfaces which the 

 lipoids and their decomposition-products are capable of bringing about. 

 The Surface-tension of the interface between water and gas, or an 

 immiscible fluid or solid, is very markedly reduced by Oils, Fatty 

 Acids or Soaps, and this fact contributes in the first place to the deter- 

 mination of the distribution of these substances in the cell, and in the 

 second place to the stability of the emulsified substances in living cells, 

 which, despite their immiscibility in water, remain suspended in the 

 form of stable emulsions within the material composing the protoplasm. 



The distribution of soluble fatty materials in the cell, such as the 

 Lecithins must be considerably influenced by the extent and variety 

 of the surfaces which are presented by the sponge- or foam-like structure 

 of protoplasm. The reason for this is that all those substances which 

 reduce superficial tensions also tend, if possible, to become concen- 

 trated upon any surfaces presented to them. This is very strikingly 

 shown, for example, in a classical experiment adduced by J. J. Thomson. 

 If a deeply colored solution of Potassium Permanganate be passed 

 through a long column of well washed and finely ground quartz-sand, 

 the first few drops of fluid which percolate through the column will 

 be found to be colorless, the whole of the permanganate in this first 

 quantum having been abstracted from the solution by the surfaces 

 over which it has passed, not because of any chemical interaction 

 between the sand and the reagent, but in consequence of the reduction 

 of the tension of the water-sand interface by the permanganate. 

 Similarly, if aqueous solutions of Saponins or of Bile-salts be shaken 

 up with petroleum-oils, the dissolved substance will be found to have 

 become concentrated at the surface of the oil-drops, and in the foam 

 which forms when saponin solutions are shaken in air, the saponin is 

 more concentrated than it is in the body of the liquid. 



The mechanism of this retention of dissolved substances by sur- 

 faces is as follows: In the accompanying diagram (Fig. 14) of a 

 spherical droplet partially enclosed by a layer of molecules which coat 

 it and separate it excepting at the gap A from direct contact with 

 the surrounding medium, if the enveloping molecules reduce the 



