"What is the object in doing that?"
"If cooled too suddenly the plates, will crack, but by heating the slates and then cooling them down gradually, we anneal the glass, in a measure. You remember how we annealed the steel by gradually cooling it down? Glass, however, cannot be annealed so that it will not fracture, although attempts have been made for years to find a means for doing it. The man who can discover a process that will enable it to bend without breaking, can command any price for the discovery."
CHAPTER XV
MYSTERIOUS HAPPENINGS ON THE ISLAND
The Professor supervised every part of the operation with the utmost care. "Before the plates are heated you must put a raised margin around each slate square, so the molten material will not run off."
"How high shall we make the margins?"
"About a quarter inch above the surface of the slate."
"As we are now ready to heat up the crucible, how shall the materials be mixed?"