THE MOLECULAR RESPONSIVENESS OF MATTER 



resistance decreased, under the action of light. In fig. 3 is 

 shown a number of responses to light, given on a series of 

 separate exposures, each of one second's duration, the inter- 

 vening periods allowed for recovery being of one minute each. 



FIG. 3. Response of Selenium to the Stimulus of Light 

 (Resistivity variation method) 



These responses were obtained by recording the increased 



deflection due to decreased resistance under the impact of 



light, and the subsequent recovery. Such responses, by means 



of decreased resistance, we may arbitrarily distinguish as 



negative. Similar responses are 



also given by a mass of metallic 



particles when acted upon by 



electric radiation. In fig. 4 are 



seen several of these negative 



responses given by galena under 



the action of this stimulus. 



There are, on the other 

 hand, some substances which 

 give positive responses ; that is to 

 say, their resistance is increased, 

 or conductivity decreased, under 



the action of stimulus. The deflection of the galvanometer 

 under a constant electro-motive force now undergoes diminu- 

 tion during the impact of stimulus. Such positive responses 

 are obtained with potassium and sodium. "That the sign 



B 2 



FIG. 4. Negative Response of Galena 

 to Hertzian Radiation 



(Resistivity variation method) 



