86 



ANIMAL ELECTRICITY. LECTURE IV. 



currents, which underlie the electrotonic alterations of 

 excitability touched upon in the previous paragraph, 

 and which are undoubtedly the effects of electrolytic 

 polarisation. 



Preliminary Experiment. — The two platinum 

 electrodes of a battery of two or three volts dip into 

 a rectangular glass vessel containing a mixed solution 



Fig. 37. 



of dextrine and potassium iodide. The molecule of 

 the potassium iodide is composed of a basic moiety 

 potassium, and an acidic moiety iodine, and the latter 

 as soon as it is set free by the electrolytic disruption 

 of the molecule, will signify its presence by striking 

 a red colour with dextrine. [I have taken dextrine 

 in preference to starch because iodine strikes blue 

 with starch, and to most of us there is an association 

 of thought between redness and acid, blueness and 

 base.] The circuit is now completed, and at once 

 you see that the anode is becoming surrounded by a 

 red halo, indicative of the presence of free iodine, the 



