18 



PHYSICOCHKMICAL BASTS OF PHYSIOLOGICAL PROCESSES 



ally made in a cylindrical vessel of hard glass (from soft glass enough 

 alkali might be dissolved to affect the results), the electrodes being circu- 

 lar plates of platinum firmly cemented at a known distance from each 

 other (Fig. 5).* This conductivity cell, as it is called, is connected 

 with a suitable electric apparatus for measuring the resistance offered 



Fig. 5. Diagram of conductivity cells. The platinum discs are represented by the thick 

 black lines. They are held in position by thick-walled glass tubes, through which they are 

 connected with the terminals by platinum wires. (From Spencer.) 



by the solution to the passage of an electric current (Wheatstone Bridge) 

 (see Fig. 6). The resistance is of course inversely proportional to the 

 conductivity. 



As a saline solution is progressively diluted, its specific conductivity 

 naturally decreases (since there are now fewer molecules between the 



Fig. 6. Wheatstone Bridge for the measurement of electric resistance: a-b, bridge wire; c, 



the movable contact. 



two opposite faces of the centimeter cube, and the space between ions or 

 molecules is increased). This result will not, however, tell us whether 

 the salt itself is undergoing any alteration in conducting power as a con- 

 sequence, for example, of greater dissociation. To ascertain this we must 



*This distance is determined not by direct measurement but by calculation from results obtained 

 by testing the actual resistance of a solution whose specific resistance is accurately known. 



