THE CONDUCTIVITY BALANCE 481 



agents on an identical specimen. Again, in a series of 

 chemical compounds which differ but slightly in effect from 

 one another, an arrangement has to be devised by which 

 the most minute excitatory variations will be conspicuously 

 displayed. The same delicacy of experimental adjustment 

 also becomes necessary when we wish to investigate the 

 varying effects of time and quantity in the application. 



Similar considerations are involved when we attempt to 

 observe the effects of various agents on conductivity and 

 receptivity ; and still more complicated are the difficulties to 

 be overcome when we have to study the property of con- 

 ductivity versus responsivity or receptivity, or of receptivity 

 versus responsivity, under the action of the same external 

 agent. The methods hitherto available are neither perfect 

 nor delicate enough for a complete and satisfactory determina- 

 tion by their means of the various problems which arise in 

 this connection. I: shall now, however, describe a very 

 perfect and delicate method carried out by an experimental 

 arrangement which I have devised, and shall designate as the 

 Conductivity-Balance, by which the variation of an affected 

 region may be continuously compared with a normal area 

 as regards each of the three different aspects of the ex- 

 citatory reaction, namely, receptivity, conductivity, and 

 responsivity. In this method, moreover, the result is un- 

 affected by any variation of resistance in the circuit that may 

 be induced by changed conditions. It also enables us to 

 solve the various difficulties encountered in comparing the 

 relative changes induced in conductivity with those induced 

 in receptivity or responsivity, or in the two last in respect to 

 each other, under the influence of a given reagent. 



In fig. 289 is given a diagrammatic representation of the 

 principal parts of this Conductivity- Balance. The thermal 

 stimulator produces stimulation of the enclosed area of the 

 specimen. The excitatory wave travels along both arms of 

 the balance, through the conducting region c and c', and 

 induces excitatory electromotive effects at the two responsive 

 points E and E'. The excitatory electrical effects at E and E r 



II 



