Hooke was aiming the sparks at a blank sheet of paper. 'Mark where they strike,' he said to Daniel. Daniel hovered with a pen, and whenever an especially large spark hit the paper, he drew a tight circle around it. They examined the paper under the Microscope, and found, in the center of each circle, a remnant: a more or less complete hollow sphere of what was obviously steel. 'You see that the Alchemists' conception of heat is ludicrous,' Hooke said. 'There is no Element of Fire. Heat is really nothing more than a brisk agitation of the parts of a body – hit a piece of steel with a rock hard enough, and a bit of steel is torn away – '