598 



COMPARATIVE ELECTRO-PHYSIOLOGY 



of light, is seen in the production of red phosphorus from the 

 yellow variety, and the insoluble from the soluble variety of 

 sulphur. 



It will thus be seen that the conductive aspect of a given 

 substance is not definite, but variable, the conductivity being 

 dependent on the particular molecular condition of the sub- 

 stance. This peculiarity may be represented in the accom- 

 panying model (fig. 366), if we give the cylinder representing 

 the sensitive molecule three main-conducting aspects ABC. 

 The non-conducting aspect is represented by C. With the 



sensitive substance in this 

 particular condition, inter- 

 posed in the electric circuit, 

 the current in the galvano- 

 meter would be zero. A is 

 the semi-conducting aspect 

 of the substance, under 

 which we may imagine the 

 corresponding deflection of 

 the galvanometer to be 50. 

 B is the highly conducting 

 aspect, the corresponding 

 galvanometer reading being 

 IOO. 



The model representing 

 the sensitive substance has 



its surface divided into six parts, the opposite sextants being 

 put in electric communication. The opposite sextants C C 

 are coated with shellac to represent the non-conducting 

 aspect ; the sextants A A are coated with graphite to repre- 

 sent the semi-conducting aspect ; and the highly conducting 

 aspect is represented by the sextants B B coated with tinfoil. 

 The three main aspects of the sensitive substance are thus 

 represented in the model ; it is to be understood that with 

 sensitive substances, under the action of stimulus, the transi- 

 tion from one aspect to the next is gradual, and not abrupt, 

 as represented. The sensitive substance is interposed between 



FIG. 366. Molecular Model 



