Page:Elementary Text-book of Physics (Anthony, 1897).djvu/316

302 Faraday's theory that the electrification of a conductor is due to an action in the dielectric surrounding it, may be performed with a jar so constructed that both coatings can be removed from it. If the jar be charged, the coatings removed by insulating handles without discharging the jar, and examined, they will be found to be almost without charge. If they be replaced, the jar will be found to be charged as before. The jar will also be found to be charged if new coatings similar to those removed be put in their place. This result shows that the true seat of the charge is in the dielectric. The experiment is due to Franklin.

That the arrangement in the dielectric is of the nature of a strain is rendered probable by the fact, first noticed by Volta, that the volume occupied by a Leyden jar increases slightly when the jar is charged. Similar changes of volume were observed by Quincke in fluid dielectrics as well as in different solids.

Another proof of the strained condition of dielectrics is found in their optical relations. It was discovered by Kerr that dielectrics previously homogeneous become doubly refracting when subjected to a powerful electrical stress. Maxwell has shown, from the assumptions of his electromagnetic theory of light, that the index of refraction of a transparent dielectric should be proportional to the square root of its specific inductive capacity. Numerous experiments, among which those of Boltzmann on the index of refraction of light in gases and those of Hertz and others on the index of refraction of electromagnetic waves in solids and liquids are the most striking, show that this predicted relation is very close to the truth.

It has further been shown that the specific inductive capacity of sulphur has difEerent values along its three crystallographic axes. This is probably true also for other crystals.

Some crystals, while being warmed, exhibit on their faces positive and negative electrifications, which are reversed as the crystals are cooling. This fact, while as yet unexplained, is probably due to temporary modifications of molecular arrangement by heat.

If a jar be discharged and allowed to stand for a while, a second