The derivation of the word seems to indicate some connexion in men's minds with the idea of Fire: the other three "elements," Earth, Water, Air, representing the solid, liquid, and gaseous conditions of ordinary matter respectively. The name AEther suggests a far more subtle or penetrating and ultra-material kind of substance. Newton employs the term for the medium which fills space-not only space which appears to be empty, but space also which appears to be full; for the luminiferous ether must undoubtedly penetrate between the atoms-must exist in the pores so to speak-of every transparent substance, else light could not travel through it. The following is an extract from Newton's surmises concerning this medium: - "Qu. 18.