PROFESSOR IN BERLIN 317 



minutes or so there was a similar detonation, and we could get 

 within 100 paces of it, as the glowing slag fell back regularly 

 upon the surface of the new cone. The most recent streams of 

 lava exhibited a very slow and hardly perceptible motion. One 

 could feel it warm to the soles of one's feet, and on pouring 

 water into a crack it hissed out again at once. Some cracks in 

 the newest lava still glowed red. The alpenstocks caught fire, 

 and the guide picked out some of the viscous glowing mass. 

 The old walls of the crater were steaming with a penetrating 

 vapour, and brightly coloured with yellow sulphur, white salt, 

 and green copper. It was in the highest degree interesting and 

 impressive, but also rather fatiguing and expensive.' 



From Naples he went by Rome and Trient to see Lenbach 

 in Munich, and thence back to Berlin. 



As soon as Helmholtz was free of his Rectorial duties he 

 returned to the electrical work that had been interrupted for 

 a year, starting off with a study of the contact theory of electri- 

 city, which appeared in Wiedemanris Annalen with the title 

 ' Studies in Electrical Contact-layers '. If, in the theory of the 

 distribution of electricity in conducting bodies, the forces of this 

 agent, as known by their action at a distance, are alone taken 

 into consideration, it is found that in a condition of equilibrium 

 the electricity leaves the interior of the body, and forms an 

 infinitely thin layer upon its surface alone. But if there should 

 be a sudden change in the value of the potential function at the 

 limit of two different bodies, as e.g. at the contact of two con- 

 ductors under the influence of a galvanic force acting between 

 them, then in this case a double electrical layer will be formed 

 along the surface: Helmholtz denotes the product of the 

 density of the positive electricity into the distance between the 

 two layers as the electric momentum of the layer, where the 

 distance is to be regarded as small, but not infinitely small, 

 since otherwise the work employed in producing the layers 

 would have to be infinitely great. 



The supposition of the formation of a double layer, as 

 previously made for bodies electrified by contact, was now 

 extended by Helmholtz to the case of contact between any 

 two bodies. The expressions for the potential difference led 

 him in the first plac$ to explain the production of electricity 

 by friction, and he succeeded in deriving the relations of the 



