wichita.edu
…being of the same nature as that which lies in the spermatic production of the egg, necessarily, though applied externally, performs the same function as innate warmth, whether inherent in a bird, a human, or elemental heat, as long as it has acquired a temperament suitable for producing the effect. For that vital internal heat, as the craftsman of its own…
https://www.wichita.edu/academics/fairmount_las/geology/wparcell/Kircher/IterII/IterExII_02_Dialog2.php