COLLOIDS 67 



on it depends the phenomenon known as adsorption, which in the case 

 of colloidal solutions may therefore be defined as the local concentra- 

 tion or condensation of dissolved substances at the interface between 

 the two phases. The amount of substance concentrated at the interface 

 can be calculated by a formula which takes into account the concentra- 

 tion of the dissolved substance, the temperature, and the surface tension 

 at the interface (the Gibbs formula). After adsorption has occurred, vari- 

 ous reactions of a chemical, electrical or purely physical nature (e. g., dif- 

 fusion) may follow at a rate which depends on the amount of the 

 condensation. 



Reactions Which Depend on Adsorption 



1. Decolorization of liquids by charcoal. That no chemical reaction occurs in such 

 a case is readily shown by the ease with which the pigment can be extracted from 

 the charcoal. 



2. Adsorption of gases by such solids as charcoal and spongy platinum. In these 

 cases there must be great condensation, even a liquefaction of the gas, during which 

 heat must be evolved. By adsorbing oxygen and hydrogen, spongy platinum causes 

 these gases to combine and form water. The hemoglobin of blood may take up 

 oxygen by a similar process. 



3. Formation of solid surface films on solutions of protein, saponin, etc. The con- 

 densation may lead to coagulation, which explains why, if the froth produced by beat- 

 ing the white of an egg is allowed to stand, it can not be again beaten into a froth, 

 the albumin having gone out of solution by surface coagulation. 



An interesting phenomenon depending on the surface tension occurs 

 when the protoplasmic contents of a ciliated infusorian is pressed out in 

 water. A new membrane forms on the protoplasm because of surface con- 

 centration of all constituents which lower surface energy. By application 

 of the principle of Willard Gibbs, A. B. Macallum 18 concludes that not only 

 adsorption, as exhibited in a colloidal solution, but also the local accumula- 

 tions of material often seen in cells, are associated with changes in sur- 

 face energy. His conclusions are based largely on microscopic studies 

 of various forms of cell exhibiting different degrees and types of activity, 

 and ingeniously stained for potassium by cobalt hexanitrite. By such 

 a means the potassium stains intense black. In vegetable cells, local 

 accumulations of potassium occur either near the interface between the 

 clear and the chlorophyl-containing parts of the cell (spirogyra) or 

 under a portion of the cell wall from which later a protrusion grows out 

 to form the first stage in conjugation. The outgrowth from the cell, 

 as well as the accumulation of potassium, may be the result of a low 

 surface tension. In unicellular animal organisms, such as Vorticella, 

 much less potassium is present, being confined to the base of the cilia, 

 which Macallum believes indicates that the structures are produced as 

 an outcome of low surface tension. 



