216 Nature's 



off, leaving a shell of graphitic substance 

 formed from the coke. 



After the current is shut off and the fur- 

 nace has cooled down, a cross-section through 

 the whole mass becomes a very interesting 

 study. The core itself, owing to the intense 

 heat it has been subjected to, has had the im- 

 purities driven out of the coke, leaving a sub- 

 stance like black lead, that will make a mark 

 like a lead-pencil, and is really the same sub- 

 stance, known as plumbago, in one of it?, 

 forms. It is the carbon left after the impuri- 

 ties have been driven out of the coke. Sur- 

 rounding the core for a distance of ten or 

 twelve inches, radiating in every direction, 

 beautifully colored crystals of carborundum 

 are found, so that a single furnace will yield 

 over 4000 pounds of this material. Beyond 

 this point the heat has not been great enough 

 to cause the union between the carbon and 

 silica, which leaves a stratum of partly-formed 

 carborundum; outside of that the mixture ib 

 found to be unchanged. 



These carborundum crystals are next crushed 

 under rollers of enormous weight, after which 

 the crushed material is separated into various 

 grades for use in making grinding-wheels of 

 different degrees of fineness. This crushed ma- 

 terial is now mixed with certain kinds of clay, 

 to hold it together, and then pressed into 

 wheels of various sizes in a hydraulic press, 



