68 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES 



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 



Electrical 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 

 \vhen present as particles in a fluid carry a negative charge of electricity, some a posi- 

 tive 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 suspen- 

 sion in water, for instance, has a negative charge. If colloidal iron, which has a pos- 

 itive 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 salts are adsorbed much more freely than they would be if the process depended 

 solely on surface condensation; that is, if the Gibbs formula is used to calculate the 

 adsorption, it will give values that are much below those actually found. 



If the dissolved substance and the particles both have the same electric sign, ad- 

 sorption will not occur. Filter paper, for example, has a negative charge and can not 

 therefore adsorb a negative dye such as congo red (as shown by the depth to which 

 it becomes stained) ; whereas it readily adsorbs night blue, which is positively charged. 

 If the negative charge of the paper is lowered, it becomes capable of adsorbing some 

 of the negative congo red. This can be effected either by placing the paper in al- 

 cohol or by adding inorganic salts (NaCl) to the water with which it is in contact. 

 The positive-charged ions of Na, produced by dissociation, neutralize some of the nega- 

 tive charge on the paper, and allow a certain amount of adsorption of the negative- 

 charged congo red to occur. As would be expected, acids and alkalies are capable of 

 greatly altering the electric charges by the II and OH ions which they contribute. 



Chemical Forces. If the nature of the phase at the surface of which adsorption 

 occurs is such that it can enter into chemical combination with the substance ad- 

 sorbed, reactions will occur that do not obey the laws of mass action. By adsorption, 



