CONSTITUTION AND TEMPEEATllRE ON MAGNETIC SUSCEPTIBILITY. 
2(55 
The apparent discontinuities of the susceptibility temperatui'e curve are suggestive 
in this connexion (fig. 6). The upper branch of the curve AB for the so-called /?-lron 
is practically continuous with the branch CD for the ^-range above 1400° C. Between 
B and C there is a break, the branch BE representing ;d-iron over a somewhat narrow 
range just below the critical temperature and the branch EF representing 
paramagnetic y-iron. The locus BEFC corresponds to a crystalline modification of 
iron which is more stable over this temperature interval than the crystalline grouping 
or groupings over the ranges AB and CD. 
If this view is correct, the molecular field is operative in iron over a temperature 
interval from 1400° C. upwards as well as below the critical temperature. The 
existence of this forcive above 1400° C. implies a crystalline symmetry involving an 
appreciable mutual action between the molecules consistent with the enhanced 
susceptibility found by CuRiE'^ and by Weiss and FoExt over this range. In the 
intermediate range, between 900° C. and 1400° C., iron shows’a paramagnetic quality 
only. Perhaps we may regard the molecular state in this range as more allied to a 
gel, consisting of very small interlocked grains, each with relatively few molecules,;}; 
rather than to a coarse grain crystalline arrangement of the molecules. The orientations 
of the molecular axes as we pass from one small grain to another will be different, so 
that each grain is, as it were, surrounded by a surface of vitreous material. As stated 
on p.' 251 the molecular field would then l)e non-effective in so far as the production of 
* ‘ Annales de Chimie et de Physique,’ ser. iv., vol. v., p. 289, 1895. 
t ‘Archives des Sciences, Geneve,’ ser. 4, vol. xxxi., p. 88 , 1911. 
I This smallness of grain structure above A 3 in wrought iron or mild steel is consistent with the 
experiments of J. E. Stead, ‘Iron and Steel Institute,’ 1898, No. 1 , p. 145. The reverse effect, that of a 
coarse structure on cooling from above A3, observed by Stead and Carpenter (‘ Iron and Steel Institute,’ 
No. 11 , p. 119, 1913) in the case of thin strips of electrolytic iron, may possibly be attributed to surface 
forces. See also a paper “ On the Part Played by the Amorphous Phase in the Hardening of Steels,” b}' 
J. C. W. Humfrey, ‘Trans. Faraday Soo.,’ May, 1915. 
2 O 2 
