Among the widely agreed facts of physics in the late nineteenth century was the existence of luminiferous ether: the medium through which light was thought to travel. Theorised to be a highly rarefied substance, the ether accounted for the movement of light, gravity and even heat across a vacuum. It also had great implications for spiritualism. Where thought was not proven to be a result of chemistry in the brain, the presence of ether allowed for the idea that cognition and emotion might exist independently of a physical body. First published in 1925, this monograph by the eminent physicist and ether advocate Sir Oliver Lodge (1851–1940) was written for the non-scientific reader. With a focus on straightforward explanations rather than mathematical theory, his book still represents a fascinating introduction to the topic today.