COLLOIDS 67 



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. 



In the cells of higher animals, deposits of potassium are also localized ; 

 in striated muscle, for example, they occur in a zone at each end of the 

 doubly refractive band and immediately adjacent to the singly refrac- 

 tive band. Changes in surface tension, associated with changes in the 

 distribution of potassium, are believed by many to be responsible for 

 muscular contraction. In nerves and nerve cells, potassium is concen- 

 trated at the axon and at the surfaces of the cells. Interesting sugges- 

 tions are offered to explain the relationship among changes in surface 

 tension at the terminations of axons (synapses, terminations in gland and 

 muscle cells) brought about by the nerve impulse acting as a change in 

 electric potential. Surface condensation of potassium has also been 

 observed at the lumen border of gland cells (pancreas), and on the lu- 

 men surface of the cells of the renal tubules. Such observations indicate 

 in what way surface tension may be called into play to control cellular 

 activities. The field is new and almost unexplored, but there is already 

 much to indicate that surface energy plays a most important role in the 

 performance of many cellular activities. 



Conditions That Influence or Are Influenced by Adsorption 



Electric Changes. Besides mere concentration, other forces come 

 into play to assist or retard adsorption. One of the most important of 

 these is electrical. Most solids when present as particles in a fluid carry 

 a negative charge of electricity, some a positive one. In conformity with 

 the "Willard Gibbs law, a constant tendency will exist for this free energy 

 to be diminished by the neutralization of the electric charge. This can 

 occur by deposition on the interface of other particles carrying an 

 electric charge of opposite sign or by the action of that present on ions. 

 Charcoal in suspension in water, for instance, has a negative charge. 

 If colloidal iron, which has a positive charge, is added to the solution, it 

 will become deposited on the charcoal, as will also the cations of an 

 inorganic salt. On account of electric adsorption, dyestuffs and bile 



