30 PITYSTCOCIIKMTCAL BASIS OF PHYSIOLOGICAL PROCESSES 



equal and in opposite directions to each other. On the other hand, should 

 the concentration of the metallic ion in the solutions be unequal, the 

 electromotive force will flow from the one electrode to the other, and 

 the pressure with which it flows will be equal to the difference in con- 

 centration of the two solutions. This is the principle of a concentration 

 cell, and if .we know the concentration of one of the solutions composing 

 it, and then proceed to measure the electromotive force, we can obtain 

 the concentrations of the other solution by difference. To do this we 

 must employ a formula which takes into consideration the relation be- 

 tween the potential and the concentration of the solution. 



The potential of an unknown electrode composed of a metal in con- 

 tact with a solution of one of its salts may also be determined by making 

 it one pole of a battery of which the other pole is composed of a stand- 

 ard electrode of unchanging known potential. An electrode of the latter 



Fig. 7. Diagram to show type of electrodes used in studying electromotive force. The 

 metal in each electrode is connected (through a glass tube) with a platinum wire, to which 

 the apparatus for measurement of the voltage is connected. The metal dips into a solution 

 contained in the electrode vessel and filling the side tube. The latter dips into an inter-, 

 mediate vessel containing saturated KC1 solution. The currents flow through the circuit under 

 the following conditions: (1) dissimilar metals dipping into the same fluid; (2) similar metals 

 dipping into different fluids; (3) dissimilar metals dipping into different fluids. 



type can most readily be made by bringing pure mercury in contact 

 with a saturated solution of calomel (Hg 2 Cl 2 ) in normal potassium chlo- 

 ride solution. Under suitable conditions (i. e., when the circuit is com- 

 pleted), a potential of + 0.560 v. is developed in this so-called calomel 

 electrode* that is, positive ions of mercury are deposited on the mercury 

 from the calomel solution at this pressure. Suppose that we connect a 

 calomel .electrode, through the intermediation of some solution which 



*The calomel electrode consists of a suitably shaped glass vessel containing pure mercury, con- 

 nected by means of a platinum wire with a conductor, and filled with a saturated solution of pure 

 mercurous chloride in normal KC1 solution up to such a level that it also fills a side tube connected 

 with a vessel containing a saturated solution of potassium chloride. Into this vessel also runs a 

 similar side tube from the unknown electrode. By having an excess of undissolved calomel in the 

 solution in the calomel electrode its saturated condition is maintained during the chemical changes 

 which accompany the production of the electric current. 



