Read APPENDIX A:  The Fire-walk in Spain of Modern Mythology, free online book, by Andrew Lang., on ReadCentral.com.

One study occasionally illustrates another.  In examining the history of the Earl Marischal, who was exiled after the rising of 1715, I found, in a letter of a correspondent of d’Alembert, that the Earl met a form of the fire-walk in Spain.  There then existed in the Peninsula a hereditary class of men who, by dint of ‘charms’ permitted by the Inquisition, could enter fire unharmed.  The Earl Marischal said that he would believe in their powers if he were allowed first to light the fire, and then to look on.  But the fire-walkers would not gratify him, as not knowing what kind of fire a heretic might kindle.