72 
MR. S. W. J. SMITH ON THE THERMOMAGNETIC ANALYSIS OF 
methods of micrographic analyses have not yet been applied to the material at 
disposal. 
Of particular interest, however, in connection with the theory of solid solution is 
the San Cristobal meteorite. It contains about 26'5 per cent, of nickel (Cohen, 
‘ Sitzungsb. Akad. Wiss., Berlin,’ p. 608, 1898), and this, as will be seen from the 
previous diagram (fig. 23, p. 67), is approximately the percentage contained by the 
supposed eutectic corresponding to the point B of the diagram. The structure of this 
meteorite is described by Cohen ( loc. cit .), and also, more recently, by Brezina (Wien, 
‘Denkschr. Akad. d. Wiss., math.-naturw. Classe,’ vol. 78, p. 638, 1905)A In the 
photograph given by the latter (Plate II., fig. 12), representing a portion of the 
meteorite magnified 23 times, the left half of the field shows a remarkable resemblance 
in structure to the eutectic of iron-carbon alloys of the type known as lamellar pearlite. 
This resemblance is shown in the annexed figure, in which are placed side by side a 
portion of Brezina’, s figure and of one given by Osmond (‘ Microscopic Analysis of 
Metals,’ p. 85. Compare also ‘ Roberts-Austen Reports, Alloys Research Committee,’ 
1899 and 1904). 
Lamellar Pearlite. 
(Osmond.) 
Contains the eutectic 
percentage of carbon. 
x 23. x 1000. 
Fig. 24. 
§ 10. The present state of knowledge concerning the^general structure of nickel-iron 
alloys of natural origin (omitting those in which carbon, phosphorus, and other 
elements are present in considerable quantity) can be summarised as follows :— 
(1) Meteoric iron is, in general, homogeneous when it contains less than 7 per cent, 
of nickel. (2) It is heterogeneous when the percentage of nickel lies between 7 and 
14. (3) It becomes practically homogeneous again when the percentage of nickel 
exceeds some value, apparently between 15 and 35 per cent., which could possibly be 
fixed by further careful microscopic investigation. 
It is clear that these facts can be interpreted if we assume that the taenite (thin 
layers of nickeliferous material), which appears when the percentage of nickel exceeds 7, 
is a eutectic consisting of (1) “mixed crystals” containing about 7 per cent, of Ni, 
and (2) “mixed crystals” containing the considerably higher percentage of Ni 
corresponding to that at which the structure of the alloy again becomes homogeneous. 
* I am indebted to Mr. L. Fletcher, F.R.S., for drawing my attention to this paper. 
San Cristobal Meteorite. 
(Brezina.) 
(?) contains eutectic 
percentage of nickel. 
