1MPOR TANT DEFINITIONS. 9 



recompositions, as in the case of liquids, it is termed electro- 

 lytic conductivity. Finally, when it requires for its existence 

 a preventive action of condensation, such as takes place in 

 the badly conducting bodies called insulators, it takes the 

 name of electro-tonic conductivity. It is this conductivity 

 which produces the effects of electro-static induction in sub- 

 marine cables. 



The majority of non-metallic' substances, such as minerals, 

 woods, the human body, tissues, &c., are conductors only 

 by reason of their electrolytic conductivity ; and their resist- 

 ance depends, therefore, on their greater or less hygrometric 

 power. Most metallic minerals, however, unite to this kind 

 of conductivity a very marked conductivity proper.* 



Finally, to finish these definitions, we shall state that by 

 the word electrodes is understood the metallic plates which 

 are plunged into an imperfectly conducting medium in order 

 to electrify it intimately, and over a surface sufficiently large 

 for it to acquire a certain amount of conductivity. The zinc 

 and copper plates of a battery constitute electrodes, and the 

 carbon pencils of an electric lamp are likewise the electrodes 

 of the luminous arc between them. 



Electrodes may also be used not only to communicate 

 electricity to a badly conducting medium, but also to collect 

 from it its polarity when it has by any circumstances become 

 charged. In all cases there results from this system of 

 electric communication a particular reaction, which is called 

 polarization, and which, being produced in a direction oppo- 

 site to that of the original electric action, disturbs the ope- 

 ration of the currents. This action is tolerably simple with 

 liquids, but it is very complicated with minerals, and gives 

 rise to peculiar and curious effects, which I have studied in 

 detail,* but as these have nothing to do with electric light- 

 ing they must here be passed over in silence. 



Most frequently polarization effects arise from electrolytic 



* See the author's RechercJies sur la conductibilitt lectriq^le des corps 

 mediocrement condztcteurs. 



