174 ON THE ORIGIN OF SEVERAL MECHANICAL INVENTIONS. 



It appears, then, that Mr. Perkinses invention was not 

 barren to the outer world, since the use of tubular boilers 

 by him led to their extended and beneficial employment in 

 railway engines and steam-boats, and were, I believe, first 

 employed by Stephenson, a few years after the steam-gun 

 experiments had been put hors-de-combat ; and it would be 

 safe to foretell, that this description of boiler will ulti- 

 mately be extended to all high-pressure steam-engines. 



Note. — Although it is needless to describe the process of 

 case-hardening, so generally known, it may be well to ex- 

 plain that of decarbonating the steel plates for engraving ; 

 this process is as follows : — The prepared steel plates are 

 placed in a cast-iron box, and covered about an inch deep 

 with an oxide of iron, prepared by subjecting iron-filings 

 to alternate wetting and drying until they are mostly con- 

 verted into red oxide. Over this covering a clay luting is 

 placed, so as to exclude the air, and the box is then placed 

 in a furnace and kept at a red heat for about sixty 

 hours, when the oxide in contact with the steel will have 

 taken up the carbon from its surface to the depth of about 

 a sixteenth of an inch, and thus convert the surface into 

 pure iron, as mentioned in the text. 



Any other oxide in the form of powder, in which the 

 affinities are weaker than those of the oxide with carbon, 

 might be employed in lieu of iron-filings. It is not im- 

 probable that this fact of bringing oxygen in contact with 

 cast iron in a melted state may have suggested the Bessemer 

 process of purifying cast iron. 



