434 Prof, Challis on a Theory of Galvanic Force, 



and parallel to each other, they are at the same time free to 

 move. 



4. The question as to whether the galvanic currents owe 

 their origin to the contact of dissimilar substances, or to che- 

 mical action, has been much discussed on experimental grounds. 

 Looking at it from the point of view of the hydrodynamical 

 theory, the conclusions that will be come to are, that the cur- 

 rents are due to the mutual molecular actions of dissimilar 

 substances in contact; that chemical affinity between the sub- 

 stances augments the molecular actions, and consequently the 

 strength of the currents ; and that chemical combination, or 

 analysis, is not necessary for the generation of the currents, but 

 may be a consequence of them. I consider the theory to be in 

 complete accordance with the two following inferences, which 

 occur in an exposition of the experimental evidence bearing on 

 this point by Mr. Gassiot (Philosophical Transactions, 1844, 

 p. 39). " Elective affinity is greatly concerned in the antecedent 

 action, of which chemical combinations, when the circuit is 

 closed, are the consequence" (§ (26)). " The higher the che- 

 mical affinities used, the greater was the evidence of the deve- 

 lopment of tension " (Note at the end of the Paper). 



5. The explanation which the hydrodynamical theory gives of 

 the chemical action consequent upon the generation of the cur- 

 rents, will be best stated after a more particular consideration 

 of the nature of the currents, considered as instances of steady 

 iiuid motion. The origin of the movement being at the battery, 

 and the current flowing from the right hand through the battery 

 to the left hand, the fluid will be impelled on the left hand, and 

 will be drawn on the right hand. It will enter the conductor 

 at one pole, and issue out at the other ; and by this circum- 

 stance the two poles will be permanently distinguished. The 

 direction of the stream depends on the order of the elements, 

 the stream resulting from a difference of action of the elements ; 

 and if the order be not the same throughout the battery, op- 

 posing streams will be generated. Now it does not appear 

 possible for steady currents to give rise to forces adequate to 

 produce chemical analysis, except by considerable changes of 

 velocity within small spaces. By hydrodynamics, the variation 

 of pressure dp in an elementary portion ds of a line s drawn 

 in the direction in which the stream flows, is proportional to 

 — WV, d\ being the corresponding variation of the velocity V 

 of the stream. Hence an increment of velocity in the direction 

 of the stream is accompanied by a decrement of pressure, and 

 consequently an accelerative force, in the same direction ; and, 

 on the contrary, a decrement of the velocity in the direction of 

 the stream produces an accelerative force in the opposite direc- 



