4 COMPARATIVE ELECTRO-PHYSIOLOGY 



of response does not depend on the electro-positivity or 

 negativity of the substance is seen in the fact that while 

 highly electro-positive potassium gives positive response, 

 the equally electro-negative dioxide of lead gives a response 

 of the same sign. Substances like magnesium, aluminium, 

 and iron give negative response. 



It is found, again, that the same substance, under different 

 molecular conditions, will give responses of opposite signs. 

 For example, a particular molecular variety of silver, Ag', 

 gives positive (fig. 5), whereas ordinary silver gives nega- 

 tive response. Again, while 

 Ag' normally gives positive, 

 yet the sign of this response 

 is gradually reversed to nega- 

 tive, under the long-continued 

 action of very strong stimulus. 

 By the employment of the 

 same _ method of resistivity 

 variation, I have been able to 



obtain excitatory response records from living tissues also. 

 Details of these will be given in a subsequent chapter. 



The electric response, however, employed to obtain 

 the excitatory reaction of living tissues, depends upon the 

 electro-motive variation of the substance under stimulation. 

 This electric reaction has been regarded as vitalistic in 

 contradistinction to physical. But I have shown that 

 similar responses are given by inorganic substances also. 

 That is to say, the molecular excitability on which the 

 phenomenon of response depends is not distinctive of animal 

 tissues alone, but is common to all matter, both organic and 

 inorganic. If, then, we desire to understand those funda- 

 mental reactions which underlie the response of living tissues, 

 it will be well to observe its occurrence in the much simpler 

 case of the inorganic body. 1 



If we take an inorganic substance, say a piece of metal 



1 For a detailed account cf. Bose, Response in the Living and the Non- 

 Living, 



