PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 861 



tain analogous facts, well known as resulting from treating wrought 

 and cast iron under similar conditions, where intense heat is pre- 

 sent, for instance. 



In the manufacture of cast steel, by common process, the best 

 wrought iron is chosen, and hence the softest. This, cut into small 

 pieces, is placed in a muffled crucible, Avith wood charcoal, and the 

 whole exposed for several hours, subject to intense heat in a fur- 

 nace, until, by the absorption of carbon, as carbonic oxide, or 

 some unknown condition, the iron is carburetted and poured into 

 ingots of cast steel. 



For the manufacture malleable iron, the hardest cast iron is 

 employed. In this process the objects to l^e treated are packed in 

 iron cases with pulverized charcoal, and then subjected to high 

 heat, in a furnace, for a protracted period, and when withdrawn, 

 is found not only decarburetted, but more ductile than the best 

 quality of wrought iron. 



It is reasonable to believe, therefore, that also, under moderate 

 temperature, in the absence of free oxygen, wrought iron, exposed 

 to the gases of carbon, will absorb that carburettiug property, in 

 a measure due to the intensitj;- of the heat and its own purity or 

 standard of affinity. 



And, conversely, that under like treatment cast iron has the pro- 

 perty of parting with a portion of its carbon, proportionate to 

 the measure of the calorific force to expel it. Indeed, so little is 

 really known of the rationale of the phenomena attending those 

 combinations that, both in science and practice, we are compelled 

 to reason rather from coincident results than by known positive 

 causes. 



ExTEENAL Deposit. 



The spaces between the tubes of wrought iron boilers are not 

 only subject to deposit from the water, and generally inaccessible, 

 but their interior or heating surfaces are always greatly impover- 

 ished from an accumulation of those non-conductors, ashes and 

 soot, which are usually removable only when the tires have been 

 drawn. Besides, under the most favorable circumstances, boiler 

 tubes serve as heat conductors but a short distance from the fur- 

 nace end, beyond which point they serve simply as conduits of the 

 non-heatino; jjases and smoke. 



The first products of combustion, the uudecomposed gases, are 

 carried into the tub.es from the furnace, beyond the reach of oxygen, 

 and are swept ofi" by the draught to waste through the chimney, 



