174 INTRODUCTION TO GENERAL PHYSIOLOGY 



Take first normal hydrochloric acid, which can be bought. 

 Adjust the slider until on closing the key a deflection is obtained 

 which does not exceed the limits of the scale of the galvanometer. 

 Note the maximum reading. This rapidly diminishes if the circuit 

 is kept closed, on account of polarisation of the electrodes. In 

 accurate work an alternating current is used to avoid this effect, as 

 described below, and the electrodes are coated with platinum black 

 (see Findlay's " Practical PhysicalChemistry," p. 1 50). 



Repeat the experiment, using acetic acid in the same mole- 

 cular concentration. It is made by taking about 54 c.c. of glacial 



FIG. ii. 



A is a resistance of 1000 ohms. 



B is the cell containing the solution to be measured. 



C and D are the slide wire of one metre length, graduated. 



E is a small induction coil, such as used for medical purposes, but provided with a 



light steel spring as vibrating contact to give a high pitched note. The secondary 



winding is connected to the circuit. 

 F is a telephone. 



acetic acid and diluting to a litre with distilled water. It is then 

 titrated with standard sodium hydroxide, using phenol-phthalein as 

 indicator. It is best to make it rather too strong and then diluting 

 to the proper volume. The standard alkali can be bought, but 

 should be tested against the normal hydrochloric acid. The 

 deflection obtained with the acetic acid will be less than that with 

 hydrochloric acid. Hence there are more conducting constituents 

 in the latter than in the acetic acid. In other words, the strong 

 acid is more highly dissociated electrolytically than the weaker 

 acid. 



To make more accurate determinations, the use of alternating 

 currents is necessary. The following arrangement of the Wheat- 



