Nov. 14, 1889] 



NATURE 



33 



seconds to fall through an interval of temperature which 

 hitherto and subsequently only occupies about 6 seconds. 

 Turn to the diagram, and see what actually happens when 

 the iron contains carbon in the proportion required to 

 constitute it mild steel (shown by thin continuous line, 

 Fig' 7) ; there is not one, but there are two such breaks in 

 the cooling, and both breaks occur at a different tempera- 

 ture from that at which the break in pure iron occurred. 



As the proportion of carbon increases in steel, the first 

 break in cooling travels more and more to the right,'']and 

 gradually becomes confounded with the second break, 

 which, in steel containing much carbon, is of long dura- 

 tion, lasting as much as 76 seconds in the case of steel 

 containing V2^ per cent, of carbon (thick i_continuous 

 line, (Fig. 7). 



[In the experiments shown to the audience the'spot of 



Fig. 7. — The curves in this diagram show ho* the rate of inovcmeiit of the spot of light varies with different samples of steel. The stoppage of 

 the movement of the spot of light of course indicates the evolution of heat from the coaling mass of steel, f (Fig. 5). 



light moved slowly and uniformly along a screen ten feet 

 in length. It halted for a few seconds as the temperature 

 of the cooling mass of steel fell to about 850' C, and 

 when the metal was at dull redness, the spot of light 

 remained stationary for 68 seconds, and then resumed its 

 course.] 



Now, it may be urged, evidently the presence of carbon 

 has an influence on the cooling of steel when left to itself : 

 may it not affect molecular behaviour during the rapid cool- 

 ing which is essential to the operation of hardening? We 

 know that the carbon, during rapid cooling, passes from 

 the state in which it is combined with the iron into a state 

 in which it is dissolved in the iron ; we also know that, 

 during slow cooling, this dissolved carbon can re-enter 

 into combination with the iron so as to assume the form 

 in which it occurs in soft steel. Osmond claims that this 

 second arrestation in the fall of the thermometer corre- 

 sponds to the recalescence of Barrett, and is caused by 

 the re-heating of the wire by the heat evolved when 

 carbon leaves its state of solution and truly combines with 

 the iron. 



If it is hoped to harden steel, it must be rapidly cooled 

 before the temperature has fallen to a definite point, not 

 lower than 650^, or the presence of carbon will be un- 

 availing. But what does the first break in the curves 

 mean? You will see that a break occurs in electro- 

 type iron which is free from carbon (thin dotted 

 line, Fig. 7) ; it must then indicate some molecular 

 change in iron itself, accompanied with evolution of 

 heat — a change with which carbon has nothing what- 

 ever to do, for no carbon is present ; and Osmond 

 argues thus : — There are two kinds oiuon, the atoms of 

 which are respectively arranged in the molecules so as to 

 constitute hard and soft iron, quite apart from the 

 presence or absence of carbon. In red-hot iron the mass 

 may be soft but the molecules are hard— let us call this 



/3 iron ; cool such red-hot pure iron, whether quickly or 

 slowly, and it becomes soft ; it passes to the a soft modi- 

 fication — there is nothing to prevent its doing so. It 

 appears, however, that if carbon is present, and the metal 

 be rapidly cooled, the following result is obtained : a 

 certain proportion of the molecules are retained in the 

 form in which they existed at a high temperature—the 

 hard form, the 3 modification — and hard j/^(?/ is the result. 



a. OR SOFT 

 IRON 



IRON. 



WHEN/^IRON COOLS 

 DOWN FROM BRIGHT 

 REDNESS TO 855° C. 

 IT CHANCES TO<X IRON 

 < 



/S: OR HARD 

 IRON 



PURE IRON AT TEMPERATURES 

 BELOW 855°C, AND IRON 

 CONTAINING CERTAIN OTHER 

 ELEMENTS IF COOLED SLOWLY. 



Esmond) 



IRON AT HIGH TEMPERATURES 



OR, IF CERTAIN OTHER 



ELEMENTS BE PRESENT, 



AFTER BEING RAPIDLY COOLED. 



(pSMONO) 



Fig. 8. 



The main facts of the case may, perhaps, be made clearer 

 by the aid of this diagram (Fig. 8) which shows the relation 

 between a and /3 iron. This molecular change from ^ 

 iron to a iron during the slow cooling of a mass of iron or 

 steel is, according to Osmond's theory, indicated by the 

 first break in the curve, representing the slow cooling of 

 iron, as is proved by the fact that it occurs alone in electro- 

 iron, A second break, usually one of much longer dura- 

 tion, marks the point at which carbon itself changes from 



