It is clear that one body cannot act upon another at a distance without some means of communication. Dr. Faraday has proved that the intervening non-conducting substance or dielectric has a great influence upon induction. Thus the inductive force is greater when sulphur is interposed between the two bodies than when shellac is the dielectric, and greater when shellac is the dielectric than glass, &c. Professor Matteucci has proved by the following experiment that the intervening substance is itself polarized by induction. A number of plates of mica in contact were placed between two plates of metal, one of which was electrified, so that the whole was charged like a Leyden jar. On separating the plates with insulating handles, each plate of mica was electrified; one side of it was positive and the other negative, showing decidedly a polarization by induction throughout the whole intervening non-conducting substance; and thus, although the interposed substance or dielectric is incapable of conducting the electrical force from one body to the other, it becomes by induction capable of transmitting it. In the atmosphere induction is transmitted by that of the intervening strata of air. It is true that induction takes place through the most perfect vacuum we can make, but there always remains some highly elastic air; and even if air could be altogether excluded, the ethereal medium cannot, and it must be capable of induction, since, however attenuated, it must consist of material atoms, otherwise it would be a nonentity.