Condition of Iron by Heating to Redness and Cooling. 479 



Before describing these experiments, I wish to mention a few- 

 experiments made with other kinds of iron. 



It is well known that no sharp distinction, depending only 

 on percentage of carbon, can be drawn between wrought 

 iron and steel. For the property of hardness characteristic 

 of steel is dependent not only upon the quantity of carbon, 

 but also upon the presence of traces of other metals ; so that 

 iron containing only about 0*5 per cent, of carbon, if it con- 

 tains also other metals as impurities, may be hardened, and 

 thus behaves like steel. Only when it contains still smaller 

 quantities of carbon (less than 0*5 per cent.) can iron be indis- 

 criminately characterized as soft iron, i. e. it acquires only 

 slightly increased hardness by being suddenly cooled in cold 

 water; and this increase is the less the smaller the percentage 

 of carbon and other foreign substances. This is the usual view, 

 and Karsten expresses himself to the same effect in his Hand- 

 buck der Eisenhuttenhunde, when he says, " It is characteristic 

 of all kinds of bar-iron that their hardness is not perceptibly 

 increased by being suddenly quenched in cold water." 



My experiments entirely agree with this result : the varie- 

 ties of iron wire (b) and (c) were found to be distinctly, but 

 not considerably, harder after rapid cooling than when slowly 

 cooled, since they offered more resistance to flexure. The 

 hardness has therefore no connexion ivith the simultaneous 

 change in density; since, whilst the density of (b) decreased, 

 that of (c) increased. I was not able to obtain any definite 

 result with the iron wire (a) on account of its greater thickness. 

 The electrolytic iron was certainly not made harder by sudden 

 cooling. Lastly, I have tested the cast iron, as well as the 

 brittle white pig-iron, with the file, but without any certain 

 result. 



Lastly must be mentioned that platinum and copper do not 

 appear to become harder by sudden cooling than by slow 

 cooling. It appears, then, that the property possessed by 

 metals of being hardened is associated with the commence- 

 ment of a chemical change, In the case of iron it is chiefly 

 carbon, which, when it is present in certain quantities, neither 

 too great nor too small, undergoes chemical change upon 

 sudden cooling, and is thus the cause of the hardening which 

 takes place. 



Five pieces were cut from a cylindrical rod of steel of 0*55 

 centim. diameter. Four of these of the same length (about 

 3 centim.) were heated and cooled quickly, (a) and (b) in 

 water of 45°, (c) and (d) in water of 15°. The fifth piece (e) 

 was only 2*3 centim. long. It was made red hot and allowed 

 to cool gradually to the temperature of the room. After deter- 



