Septembek 8, 1 910] 



NATURE 



509 



microscope proved the presence of a few fir-tree crystallites 

 embedded in a ground mass of cementite and a eutectic 

 containing the two kinds of cementite, the No. i specimen 

 containing a much smaller proportion of the cementite 

 rich in silicon than No. 2. 



-As the metals had been somewhat rapidly cooled, the 

 alloy No. 2 was remelted, and was then allowed to cool 

 in the crucible, so as to obtain a more coarsely crystal- 

 lised eutectic. When cold, on polishing and " heat-tint- 

 ing," the eutectic was clearly seen. There were the re- 

 mains of large primary silico-austenite crystallites, plates 

 of the red-coloured cementite, and a well-developed eutectic 

 consisting of the (red) coloured and (white) cementites. 



The cooling having been slow, this compound con- 

 stituent had suffered partial decomposition in isolated 

 patches into graphite and silico-ferrite, whilst the cementite 

 coloured red remained intact. 



There can be little doubt that the residue left insoluble 

 in acid consisted of the two cementites, but in what pro- 

 portion it is impossible to tell, as a method for isolating 

 them has yet to be found. 



Had the* alloy contained a greater proportion of carbon, 

 the amount of cementite rich in silicon would have been 

 in much greater proportion. 



The trials, incomplete and necessarily imperfect as they 

 are, go far to prove, just as Gontermann premised, that 

 during the solidification of high silicon pig irons two 

 cementites fall out of solution together as a eutectic 

 mixture. 



They also have proved that the carbo-silicides are 

 exceedingly unstable, breaking up into silico-austenite 

 almost as soon as formed. It is the inslabilHy of these 

 silico-carbides which is mainly responsible for the graphitic 

 character of grey irons rich in silicon and low in sulphur. 



Summary and Conclusions. 



(i) The experimental results advanced show proof that 

 carbide of iron in presence of iron sulphide crystallises 

 with a minute quantity of sulphur not exceeding about 

 one-thousandth part of the weight of the carbide, but the 

 nature of the iron-carbon-sulphur compound has not yet 

 been determined. 



(2 1 It seems almost, if not absolutely, certain that it is 

 the sulphur crystallised with the carbide which makes the 

 latter stable. 



(3) The evidence appears to support the view, long held 

 by some, and more recently accepted by others, that 

 during the freezing of iron-carbon-hypo-eutectic alloys 

 after the crystallisation of the primary austenite, and in 

 the eutectic and hypereutectic alloys, it is the carbide, and 

 not graphite, which primarily forms, and that the carbide 

 afterwards decomposes into graphite and austenite. 



(4) It has been proved by chemical methods that when 

 the hypo-eutectic alloys, low in silicon, freeze, nearly all 

 the silicon crystallises out with the primary austenite ; and 

 it" follows that on gradually increasing the carbon so as to 

 reduce the quantity of primary austenite, the silicon re- 

 maining constant, the austenite which does form must be 

 as gradually enriched in silicon up to saturation point, and, 

 when that point is reached, the excess silicon crystallises 

 out with a portion of the carbide of iron to form carbo- 

 silicide of iron. Other elements remaining constant, the 

 same result must follow on gradually increasing the 

 silicon . 



(5) In the alloys of eutectic proportion and in the hyper- 

 eutectic alloys, as no primary austenite can form, the 

 silicon crvstallises primarily with the carbide. 



(6) In Cleveland pig iron containing about 1-5 per cent, 

 phosphorus, a ternary eutectic of ii-on-carbon-phosphorus 

 takes the place of the iron-iron-carbide eutectic. In white 

 irons containing 3 per cent, carbon and under 2 per cent, 

 silicon, after the primary austenite has fallen out of solu- 

 tion, carrving practically all the silicon, it is not iron- 

 iron-carbide which forms, but independent plates of 

 cementite, or carbide of iron, and after these have crystal- 

 lised and the residual mother liquor has arrived at the 

 composition of the ternary iron-carbon-phosphorus eutectic, 

 the latter solidifies at q45'' C 



(7) In Cleveland irons which become grey on cooling, 

 and in which there is no primary austenite, the same iron- 

 carbon-phosphorus eutectic is the only eutectic to form 



NO. 2132, VOL. 84] 



during cooling, and, instead of a ternary iron-carbon-silicon 

 eutectic, two independent cementites crystallise — one a 

 silico-carbide and the other carbide of iron, possibly con- 

 taining a little silicide in solid solution. The micro- 

 examination of the cold alloys, to which a little sulphur 

 had previously been added when the metals were melted, 

 led to the conclusion that it is the carbo-silico-cementite 

 which primarily crystallises. 



(8) There is evidence that the primary carbo-silicides are 

 exceedingly unstable, and are the first to decompose into 

 graphite and silico-austenite. 



(9) In the absence of any sensible quantity of phos- 

 phorus, two cementites form — one the silico-carbide 

 cementite, the other the carbide cementite— and these 

 crystallise together as a eutectic mixture. 



(10) The exact composition of the tw-o cementites has 

 not yet been determined, as no chemical method has been 

 found for their isolation. 



(11) It is evident that it is the exceedingly unstable 

 character of the silico-carbides which is responsible for 

 the greyness of commercial metals rich in silicon and low 

 in sulphur. 



(12) Silicide of iron when heated at 1000° C. with pure 

 white iron free from silicon effects the decomposition oT 

 the carbide of the white iron. Based on this observation, 

 the hypothesis seems justifiable, in cases where all the 

 silicon present in hypo-eutectic alloys crystallises out with 

 the primary austenite, that after the carbide has solidified 

 diffusion of the silicide follows, and this leads to the 

 decomposition of the carbide of iron into graphite of iron. 



(13) Many of the results arrived at by chemical analvsis 

 support the hypothetical conclusions of Gontermann, who 

 depended mainly on data obtained by thermal methods of 

 treatment. 



In conclusion, it will be clear from what T have stated 

 that there are many gaps yet to be filled. I hope that the 

 knowledge of this fact will lead others to follow up the 

 research, which, in its present stage, is far from complete. 



SUB-SECTION OF B. 



AGRICULTURAL SUB-SECTION. 



Opening .Address by .\. D. Hall, M.. A., F.R.S., 

 Chairman of the Sub-section. 



I BELIEVE it is customary for anyone who has the honour 

 of presiding over a section of the British Association to 

 provide in his presidential address either a review of the 

 current progress of his subject or an account of some 

 large piece of investigation by which he himself has 

 illuminated it. I wish I had anything of the latter kind 

 which I could consider worthy to occupy your attention 

 for the time at my disposal ; and as to a review of the 

 subject, I am not without hopes that the sectional meet- 

 ings themselves will provide all that is necessary in the 

 way of a general review of what is going forward in our 

 department of science. I have, therefore, chosen instead 

 to deal from an historic point of view with the opinions 

 which have prevailed about one central fact, and I pro- 

 pose to set before you this morning an account of the ebb 

 and flow of ideas as to the causes of the fertility of the 

 soil, a question which has naturally occupied the attention 

 of everyone who has exercised his reason upon matters 

 connected with agriculture. The fertility of the soil is 

 perhaps a vague title, but by it I intend to signify the 

 greater or less power which a piece of land possesses of 

 producing crops under cultivation, or, again, the causes 

 which make one piece of land yield large crops when 

 another piece alongside only yields small ones, differences 

 which are so real that a farmer will oay three or even 

 four pounds an acre rent for some land, whereas he will 

 regard other as dear at ten shillings an acre. 



If we go back to the seventeenth century, which we 

 mav take as the beginning of organised science, we shall 

 find that men were concerned with two aspects of the 

 question — how the plant itself gains its increase in size, 

 and, secondly, what the soil does towards supplying the 

 material constituting the plant. The first exoeriment we 

 h.ave recorded is that of Van Helmont, who placed 200 lb. 

 of dried earth in a tub, and planted therein a willow tree 

 weighing 5 lb. .After five years the willow tree weighed 



