166 SECONDARY ELECTROMOTIYE PHENOMENA IN 



an organic nature, and in the case of the latter either organised or 

 not organised. In the latter case, again, they might be either 

 dead like wood, or living like fresh tissue of plants 1 . It may 

 be stated generally that every porous body which is not too 

 bad a conductor and is soaked with an electrolyte which, in com- 

 parison with the porous body, does not conduct too well, is sus- 

 ceptible of negative internal polarisation ; i. e. after the passage of 

 the current each transverse section lying between two isoelectric 

 surfaces has an electromotive action in a direction opposite to that 

 of the polarising current. This can be explained by the suppo- 

 sition that the current divides itself between the soaking fluid and 

 the porous framework, and that the latter becomes polarised just 

 like a metallic septum by the ions set free on its surface. Each 

 one of the innumerable minute septa has an electromotive action 

 in a direction opposite to that through which the current passed. 

 The partial currents thus generated take their way through two 

 paths, one presented by the internally polarised body itself, the other 

 by the galvanometer-circuit applied to the body. Their strength 

 in the latter depends on the dimensions of the polarisation- 

 object, on the position of the little septa in its interior, on the 

 nature of the material composing the porous framework, and on 

 the soaking fluid. The total current in the galvanometer-circuit 

 results from the superposition of all the partial currents, whose 

 strength is naturally an unknown and complicated function of the 

 strength and duration of the polarising current, and the length 

 of time since its opening. All that can be said is that it must 

 increase with the increase of the two first variables up to a certain 

 point, and must diminish with that of the third. 



We have a clear example of internal polarisation in a cylinder 

 of thoroughly burned wood-charcoal soaked with weak sulphuric 

 acid, and through which a current is passed axially. As a material 

 it conducts sufficiently well, so that a portion of the current finds 

 its way through it notwithstanding the good derivation which the 

 dilute acid affords. Each tract of the charcoal cylinder of equal 

 length has an equally strong secondary electromotive action. The 

 action of the whole cylinder equals the sum of all the individual 

 tracts. Wood is also capable of strong internal polarisation. It 

 is necessary, however, as the substance of the wood conducts badly, 



1 Untersuchungen fiber thierische Elektricitat, vol. i. part i. p. 380. Gesammelte 

 Abhandlungen, loc. cit. 13-28: * Uber innere Polarisation poroser, mit Elektrolyten 

 getrankter Halbleiter' (August 4, 1856). 



