436 



NATURE 



[July 31, 1919 



THE METALLOGRAPHY OF IRON AND 

 IRON-CARBON ALLOYS. 



AT the May meeting of the Iron and Steel Institute 

 two papers of decided scientific importance were 

 presented. In one of these Prof. G. Cesaro, of Li^ge 

 University, a distinguished Belgian man of science, 

 has endeavoured by careful mathematical analysis to 

 ascertain the course of the curve joining the points at 

 which molten iron-carbon alloys commence to solidify, 

 if the abscissae are taken either as x, the number of 

 atoms of carbon contained in a unit of the alloy, com- 

 posed of a hundred atoms, or as y, the number of 

 molecules of cementite FejC contained in a unit of the 

 alloy composed of a hundred molecules, and assuming 

 an iron molecule to contain two atoms. The author 

 has used for his data the experimental results obtained 

 by Carpenter and Keeling fifteen years ago, which are 

 generally accepted as valid for the liquidus of the 

 series. He comes to the conclusion that whether 

 Raoult's law of the depression of the freezing point 

 or the more general law expressed by the Le Chatelier- 

 Schroeder formula 



2VT0 t/ 



where T is the absolute temperature and z is the 

 number of molecules in the solvent contained in a unit 

 of the alloy forming a single molecule, be adopted, the 

 calculated figures agree decidedly better with the ex- 

 perimental results on the iron-cementite than the iron- 

 carbon hypothesis. Further, the results which agree 

 best are obtained on the assumption 'of a rectilinear 

 variation afforded by the Fe3C-Fe2 hypothesis. 



In the second paper Prof. Honda, of the Tohoku 

 Imperial University, Sendai, Japan, returns to a con- 

 sideration of the allotropic forms of iron. It is now 

 generally agreed that pure iron undergoes two trans- 

 formations between the freezing point and the ordinary 

 temperature, which are allotropic. The first of these 

 . is the A4 transformation, and takes place at 1394° C, 

 about 130° below the freezing point. It is completed in 

 a few minutes. The second is the A3 transformation, 

 and occurs at about 900° C. This transformation, 

 although not so rapid as the previous one, can be com- 

 pleted in a very narrow temperature interval, provided 

 the heating and cooling are sufficiently slow. In a 

 recent experiment by Ishiwara, where the complete 

 transformation required about three hours, the differ- 

 ence between the Ac^ and Ar3 points did not exceed 

 5° C. 



The Ao change is of a different nature. It does not 

 take place at a definite temperature or within a small 

 range, but begins at the ordinary temperature, its rate 

 becoming greater as the temperature is increased, 

 until it is completed at 785° C. The various physical 

 properties, such as heat absorption or evolution, inten- 

 sity of magnetisation, electrical resistance, thermal 

 conductivitv, etc., varv similarly with one another, the 

 values changing slowly at lower temperatures, and the 

 change becoming faster as the temperatures approach 

 785° C. When thermal equilibrium is established the 

 change in any one of the properties does not increase 

 by a prolonged heating. The properties are definite 

 functions of the temperatures. It is this distinction, 

 according to the author, which constitutes the essential 

 difference between allotropic and non-allotropic 

 changes. According to him, therefore, an allotropic 

 change is the transformation of a substance from one 

 phase to another which proceeds at a definite tem- 

 perature if sufficient time be allowed for the trans- 

 formation. 



The A. critical point, as determined thermally, is 

 usually taken as 768° C. This is the temperature at 

 which the rate of heat evolution or absorption is at 



NO. 2596, VOL. 103] 



the maximum on cooling and heating respectivelv. 

 This figure is somewhat lower than 785° C, which 

 is the value proposed by Honda, and is the tempera- 

 ture at which the Aj transformation begins on cooling 

 and terminates on heating. 



In the case of carbon steels, in addition to the 

 above, there are two other transformations. A, and 

 Ao. The former is a change of phase, while the 

 latter is a change in cementite of a similar nature to 

 A2- Accordingly, whereas A^, A3, and Ai are phase 

 changes, the A^ and A^ transformations extend from 

 the critical to the lowest temperature. Every stage 

 of these changes is a definite function of the tem- 

 perature, and, from the point of view of the mole- 

 cular theory of magnetism, they may be regarded as 

 processes in which the molecules acquire rotational 

 energy about their magnetic axes. 



H. C. II. Carpenter. 



SEX, REPRODUCTION, AND HEREDITY 

 IN PIGEONS AND FOWLS. 



p\R. OSCAR RIDDLE has previously brought for- 

 *--' ward evidence to show that male pigeons arise 

 from eggs (yolks) of less storage metabolism, which 

 implies small size and higher (oxidising) metabolism, 

 and that females arise from eggs (yolks) of greater 

 storage metabolism, which implies large size and 

 lower (oxidising) metabolism. -He has now {Journ. 

 Exper. Zoology, vol. xxvi., 1918, pp. 227-54) studied 

 two cases of female "identical twins," and seeks to 

 show that the ova (yolks) which produced both of 

 them were extraordinarily and abnormally large. 



Of course, the yolk of an egg cannot be directly 

 weighed on a balance and then put back to see what 

 it will develop into; Dr. Riddle's evidence is neces- 

 sarily indirect. The eggs when laid were very large 

 compared with all the other eggs produced by the 

 particular parents (totals of 116 and 134 eggs). 

 Double-yolked eggs in doves are practically restricted 

 in their production to hybrids from wider crosses, or 

 to birds showing striking reproductive abnormalities, 

 or to both of these, and would not be expected to 

 appear in the series in which the two cases of "iden- 

 tical twins" were found. 



It is suggested that the blastoderm-borders will be 

 abnormally raised in extraordinarily large eggs, and 

 abnormally lowered in extraordinarily small ones, and 

 that this might lead, for physical reasons, to the 

 establishment of two independent foci of development. 

 If male "identical twins" were found developing 

 from a verv small egg. it would be an interesting 

 corroboration of the author's theory. Meanwhile, he 

 thinks that the available data point to the conclusion 

 that each pair of female " identical twins " arose from 

 a single ovum of high storage metabolism. 



In healthy doves and pigeons the right testis is 

 larger than the left in a very high percentage of cases, 

 vet in the female it is the left ovary that persists. 

 The left testis more nearly approaches the ovary than 

 does the right. In disease, particularly in tuberculosis, 

 the testes undergo extreme atrophy, but more in the 

 right than in the left ; the ovarv does not seem to 

 suffer reduction in size. The right testis of the very 

 young birds (from embryos to squabs a few weeks 

 old) is normally lonjjer than the left. The single (per- 

 sistent) left ovary of voung female squabs is twice, or 

 more than twice, as long as is either testis in males 

 of similar age (three to seven weeks). Now Dr. 

 Riddle finds (Anat. Record, vol. xlv., 1918, pp. 283- 

 334) that In hybrids the normal size relations of the 

 two testes are often disturbed, sometimes reversed, 

 approximating to the female condition. The number 



