104 BITLLETIN 18 4, UNITED STATES NATIONAL MUSEUM 



At its best, and with certain octahedrites, the result is a strikingly- 

 beautiful selective coloration when viewed under low or moderate 

 magnification. The success of this method apparently depends 

 upon the degree of variation between the nickel content of the taenite 

 and that of the adjoining kamacite. If the conditions of transfor- 

 mation produced taenite with 25 or 30 percent nickel, it is colored 

 strongly as compared with the nickel-poor kamacite; if the taenite 

 contains only 15 percent nickel, there is only a slight contrast in the 

 deposition of the red precipitate. 



After tedious experimenting, by the author and later by his assist- 

 ant, with many materials and manipulations, the best results were 

 obtained with a 4 percent solution of gelatine filtered through bone 

 black and hardened with formalin; a 1 percent solution of nitric acid 

 in alcohol; and a saturated aqueous solution of dimethylglyoxime 

 with a few drops of formalin, used at about 40° C. 



The method cannot be recommended for any ordinary purposes 

 of study. 



Polarized light. — The usefulness of polarized light is limited by 

 the fact that most of the components of meteoric irons are isometric 

 in crystallization and therefore isotropic. Exceptions are troilite, 

 which is hexagonal, and schreibersite, which is tetragonal; therefore 

 minute inclusions of those substances should show extinctions, which 

 might help to identify them when other methods are not satisfac- 

 tory. In practice, however, the author has rarely made use of 

 polarized light. 



Spectroscopic analysis. — The spectroscope reveals extremely small 

 percentages of component substances, and though its use generally 

 is unnecessary it has been found by the author to be of value for the 

 identification of cohenite. If the spectroscope reveals no phosphorus 

 in an inclusion, it is not phosphide. 



Grains or small inclusions of chromite, otherwise not easily recog- 

 nized, might also be identified by the spectroscope. It has been 

 used to reveal small percentages of the rare metals and other minor 

 constituents; this, however, is a matter of chemistry or mineralogy, 

 because such constituents have no effect upon the structure of 

 meteoric irons. 



X-ray analysis. — The use of X-ray diffraction analysis in metal- 

 lography is a development of recent years. By revealing the posi- 

 tion of atoms in a crystalline structure, this method has made it 

 possible to establish the space lattice for different transformation 

 phases, and for various metals, and to determine compositions and 

 crystalline planes. 



Even very recently some metallographers have doubted its practi- 

 cal value, although that now seems to have been abundantly proved 



