ANIMAL AND VEGETABLE 57 



of 2000 mm. positive and B of 40 mm. negative upon the 

 scale of the galvanometer I have mentioned. After about 

 an hour, or less (according to the size of the room), the air 

 of the laboratory would become charged by reason of the 

 neuro-electricity emanating from the persons of A and B, 

 and as 200 positive minus 400 negative = 200 negative, 

 the air must become negatively charged, increasing in 

 tension or pressure with time or varying with any alteration 

 in the insulation. 



In this we have one of the sources of error. The tension 

 and sign of the atmosphere in the testing-room have 

 always been unknown quantities. 



PERSONAL CAPACITY 



I have not of recent years taken any actual measure- 

 ments, but the mean of a former series of tests gave 

 nearly four microfarads as the average capacity of 

 the body. Now if B (= 400 mm. negative) touched A 

 ( = 200 mm. positive), A would become 200 mm. negative 

 so long as he remained shut up with B, or, failing direct 

 contact between the two, the air of the room would 

 charge A as certainly as water would find its level. In- 

 ductive capacity introduces another and equally per- 

 plexing source of confusion, as a flash of lightning, a 

 powerful earth-current, wireless telegraphy, or the proxi- 

 mity of a charging station or of an electric railway or tube 

 would not only affect the persons experimenting, but also 

 the subject of experiment, although a galvanometer of 

 the d'Arsonval type might not be perceptibly influenced. 



CAPACITY or LIQUIDS AND MOIST SUBSTANCES 



But that is not all. Physiologists, overlooking conductive 

 and inductive capacity, have invariably used what they call 

 non-polarisable electrodes, or contacts to which the objects 

 under examination are connected, for the purpose of 



