Physics Defined. -- Physics is often defined as the science of matter and energy.
Section 223. The Ether.—Exhaust the air as far as possible from a glass bell jar. Place a candle on the far side of the jar; it will be seen as clearly before the air has been let into the bell jar as after. It is obvious that the medium conveying light is not the air and it must be something that exists even in a vacuum. Physicists have agreed to call this medium the ether. It exists everywhere, even penetrating between the molecules of ordinary matter.