432 Prof. Challis on a Theory of Galvanic Force. 



internal atomic actions are equal and opposite at any point of 

 the interior if the density of the body be uniform. Hence, 

 besides the rapid gradations of density near the boundary, which 

 give rise to the superficial attractions and repulsions, there must 

 be a much slower gradation of density throughout the interior 

 of the substance ; and as the atomic and molecular repulsions 

 are much more affected by difference of density than the mole- 

 cular attractions, the force of gravity is counteracted by repul- 

 sions, and the density consequently decreases from the lower to 

 the upper part of the body. Now it is clear that in this example 

 the deviations of the superficial atoms, and those of the interior 

 atoms, from their undisturbed state, are mutually dependent, and 

 that all are connected with the molecular action at the point of 

 attachment; for if the attachment were suddenly cut off, the 

 atoms would immediately return to the undisturbed state, and 

 would all equally obey the force of gravity. While in this 

 instance the atoms are all in disturbed positions, because all are 

 acted upon by the force of gravity, the counteracting atomic 

 and molecular forces consequent upon this state of disturbance, 

 must evidently have a single resultant at the point of attach- 

 ment. It is therefore conceivable that conversely a superficial 

 disturbance at a single point, or at several points, may extend 

 its influence over all the atoms of a body. This, as a matter of 

 experience, appears to be the case when two dissimilar substances, 

 conductors of electricity, are in contact. 



2. The definition of physical contact is, that the superficial 

 atoms of each body are acted upon at the points of contact by 

 the molecular forces of the other ; not that the atoms them- 

 selves come into contact, this being prevented by the atomic 

 repulsion, which is the most energetic of all the physical forces. 

 When two dissimilar substances are in contact, the superficial 

 atoms of each at and near the points of contact must, in general, 

 on account of the dissimilarity, be put by the molecular forces 

 into positions of disturbance ; that is, according to the theory of 

 electricity which I have proposed, the two substances are in an 

 electric state. If they are good conductors of electricity, the 

 local disturbance extends throughout their superficial atoms, 

 and the substances are in a state of induced electricity, which 

 would immediately disappear upon separating them. Under 

 these circumstances, from what is argued above, there would be 

 a gradation of density throughout the interior of each, and thus 

 the conditions for generating secondary currents, stated in art. 

 18 of the "Theory of Electricity," would be fulfilled. The in- 

 tensity of these currents will depend on the amount of disturb- 

 ance at the points of contact ; and this again must depend on 

 the constitutions and qualities of the two substances. It might 



