apart act upon each other with unit force, then each of them is a unit of active mass.
The same definition of unit mass must also fit if applied to gravitational attraction, but there is a difference. We know from experimental evidence (T. Erismann, Arch. d. Science, Jan. 1911, pp. 36-45) that the attractive force of gravity is not in the least influenced by the medium which fills the intervening space. Two bodies in air attract each other with exactly the same force as in water; nor would the force be altered if we placed a wall between them. There would of course be an additional attraction between each body and the wall, but no additional force of the attraction between the bodies themselves.
With magnetic and electric forces it is different. If the force acting between two electrically charged bodies be measured, first in air and then when immersed in oil or separated by a wall of glass, we should find a decrease of force in the latter cases. In these cases the whole or part of the medium which at first was air has been replaced by some other substance with the result of an alteration in the force. We thus find that not only the magnitude of the charges and their distance, but also the
