1858, as a proposed word for "electricity considered as a material substance possessing weight," from pyro- + -gen. Meaning "fever-producer, substance which, introduced into the blood, induces fever" is from 1896. Related: Pyrogenic. Greek pyrogenes meant "born in fire, wrought by fire" (compare pyrogenesis).
"a foretaste," 1880, from preview (v.); specifically "a showing of a book, film, etc. before public release" by 1920.