Nov. 7, 1889] 



NA TURE 



15 



to indicate the appearance when soft, hardened, and 

 itempered steel are respectively treated with a solvent 

 ■which acts gently on the mass. 



IRON AND CARBON. 



(02 TO 1-5 PER CEaT OF CARBON.) 



Heated to bright redness 



I -• n 



SLOWLY COOLED 

 . I 



SOFT" 



QUICKLY COOLED 



"HARD" 



CONTAINS 



CAR8IDE-CAR60N 



fe jC ^ 



MEOHADICALiy MIXED 



APPEARANCE OF METAL WHEN 

 ETCHED WITH A SOLVENT 



CHANGE OF CARBON 



TAKES PLACE 



RAPIDIY. 



> 



CONTAms 

 tURDENING-CARBON' 



TEMPERATURES VARYING / ^S" y 

 FROM 200 'C. TO 400 "C. / «J<„ X-^ 



TEMPERED" ,//// 



m ANNFALEO STEEL THE 



CAhdiOt IS IN GP.ey, 



SCALES 



TEMPERED AT 40P C. 

 THECAitBlOE IS FINELV OlVIDEO. 



Fig. 4. 



A Study of the above diagram and of the admir- 

 able work of Ledebur"^ will show how complex the 

 relations of carbon and iron really are, but, for the 

 purposes of the present inquiry it may fairly be asked, 

 Does a change in the " mode of existence" of carbon in 

 •iron sufficiently explain the main facts of hardening and 

 tempering? It does not. It is possible to obtain by rapid 

 -cooling from a certain temperature steel which is per- 

 fectly soft, although analysis proves that the carbon is 

 present in the form which we have recognized as 

 ^' hardening carbon." No doubt in the hardening of steel 

 the carbon changes its mode of existence, but we must 

 seek some other theory to explain all the facts, and in 

 order to do this we will turn to the behaviour of the iron 

 itself. 



In approaching this portion of the subject a few elemen- 

 tary facts relative to the constitution of matter must be 

 recalled, and in doing so I must again appeal briefly to 

 history. It is universally accepted that metals, like all 

 elements, are composed of atoms of definite weights 

 and volumes grouped in molecules. In order actually 

 to transmute one metal into another it would be 

 necessary to discover a method of attacking not the 

 molecule but the atom, and of changing it, and this, so 

 far as is known, has not yet been done ; but it is possible, 

 by influences which often appeir to be very slight, to 

 change the relations of the molecules to each other, and 

 to alter the arrangements or distribution of the atoms 

 within the molecules, and by varying in this sense the 

 molecular arrangement of certain elements, they may be 

 made to pass into forms which are very different from 

 those in which we ordinarily know them. Carbon, for 

 instance, when free, or when associated with iron, m.\y 

 readily be changed from the diamond to the grapiitic 



' Stahl nnd Eisen, vol. viii,, 1888, p. 742)1 



form, though the converse change has not as yet been 

 effected. 



Sulphur, again, with which you are familiar as a hard, 

 brittle, yellow solid, may be prepared and maintained for 

 a little time in the form of this brown viscous mass, but 

 this latter form of sulphur soon passes spontaneously and 

 slowly at the ordinary temperature, and instantaneously 

 at 100", to the solid octahedral yellow modification with 

 evolution of heat. The viscous form of sulphur is an 

 allotropic modification of that element. A few cases of 

 allotropy in metals have already been established, and 

 when they do occur they give rise to problems of vast 

 industrial importance. Such molecular changes in metals 

 are usually produced by the addition of a smill quantity 

 of foreign matter, and I have elsewhere tried to show that 

 the molecular change produced by the action of traces 

 upon masses is a wide-spread principle of nature, and one 

 which was recognized at the dawn of the science of 

 chemistry, even in the seventh century, although distorted 

 explanations were given of well-known facts, and gave rise 

 to entirely false hopes. But it is the same story now as 

 in mediaeval times : the single grain of powder which 

 Raymond Lully said would transmute millions of its 

 weight of lead into gold— the single grain of stone that 

 Solomon Trismosin thought would secure perpetual 

 youth — had their analogues in the small amount of 

 plumbago which, to Bergman's astonishment in the 

 eighteenth century, converted iron into steel. By his 

 time it was recognized that the right use of alchemy con- 

 sisted in the application of its methods to industry, and . 

 we still wonder at the minuteness of the quantity of \ 

 certain elements which can profoundly affect the proper- 

 ties of metals. The statements are true, and are not 

 derived from poetical literature, early or late. Even in 

 the moral world the significance of the action of traces 

 upon masses has been recognized, and the method of 

 the alchemist survives in the administration of the small 

 quantity of powder which, in the imagination of Robert 

 Louis Stevenson, will produce the malevolent Hyde 

 modification of the benevolent Dr. Jekyll. In thus 

 borrowing an illustration from one of the, most refined 

 and subtle writers of our time, I do not fear the taunt of 

 Francis Bacon,^ that " sottishly do the chymics appro- 

 priate the fancies and delights of poets in the transforma- 

 tion of bodies to the experiments of their furnaces ; " for, 

 although it may not be possible to transmute metals, it is 

 easy so to transform them, by very slight influences, that 

 as regards special service required from them they may 

 behave either usefully or entirely prejudicially. 



In attempting to illustrate this part of the subject I 

 cannot take the most striking cases, as it is difficult to 

 demonstrate them in the time at my disposal. The 

 following experiment, which does not, however, depend 

 upon the action of a trace upon a mass, will enable me 

 to lead up to the point I wish to insist upon. It consists 

 in the release of goll from its alloy with potassium. 

 When the alloy is treated with water, the gold comes 

 down in a finely divided, dark brown, chemically active 

 state. [Experiment shown on the screen.] 



I have chosen this experiment because it was a similar 

 one that first roused suspicion that pure iron could exist 

 in more than one form. 



The question at once suggests itself, Can iron behave 

 in a similar manner : is an allotropic form of iron known 1 

 Joule afforded experimental evidence for an affirmative 

 answer to this question nearly forty years ago by 

 communicating to the British Association in 1850 a 

 paper on some amalgams. The result of his experiments, 

 published in detail later,- in a paper which has been 

 sadly neglected, showed that iron released from its 

 amalgam with mercury is chemically active, as it com- 



« Preface to th; " Wisdom of the Ancients." . , ?' , 



= "0.1 sons .\n%l3ani," M;m..Li.t. P.iil. bo:. M n;h;st;r, v jL lu [3I 



p. 115. 



