THE METALDOGKIAPHY OF METEORIC IRON 103 



metallography. An instructive article on the subject by the late 

 Prof. H. M. Boylston is reprinted as Appendix II of Sauveur's text- 

 book on the metallography of iron and steel (1938). 



Other methods of study. — A number of other methods of research, 

 in addition to visual examination and photography, are at times 

 useful. These relate mostly to the qualitative determination of 

 various components. 



Heat tinting. — This method, whereby Widmanstatten discovered 

 the structure named for him, is sometimes used to reveal variations 

 in the phosphorus content of artificial irons. The specimen is 

 gently heated on a hotplate, over a Bunsen burner, or preferably 

 floated on molten soft solder, and as the temperature rises there is 

 selective coloration from the varying oxidation of the components. 

 It is not practically useful with meteoric irons, as both the structure 

 and the phosphide are revealed more satisfactorily by etching. 



Sulphur 'printing .—Inchisions of troilite are quickly revealed by a 

 sulphur print. A sheet of contact printing paper is soaked in 2 per- 

 cent sulphuric acid and laid on a sheet of glass, and the polished 

 specimen (preferably before etching) is pressed gently upon it for 

 10 or 15 seconds. The paper is then treated with hyposulphite, 

 washed, and dried. Brown spots indicate the sulphide inclusions. 

 This test is convenient to ascertain quickly the number and location 

 of such inclusions, and sometimes to distinguish readily inclusions 

 of graphite, which often are associated with troilite and which do 

 not show on a sulphur print. 



Hardness tests. — Hardness tests are rarely important but are 

 convenient for a quick identification of schreibersite or cohenite, 

 which sometimes might be mistaken for taenite. There is also an 

 observable difference in hardness between troilite and daubreelite. 

 The use of a fine needle is usually sufficient. For a more exact test 

 a Bierbaum hardness tester is convenient, the diamond point carrying 

 a 2-gram weight. 



Quantitative nickel coloration. — To show the varying proportions 

 of nickel in areas of taenite and plessite, the author after much 

 difficulty developed a process of quantitative coloration by di- 

 methylglyoxime. Briefly, the surface is coated with gelatine, im- 

 mersed in very dilute nitric acid, then in an alkaline solution of 

 dimethylglyoxime in water. The effect is to develop nickel nitrate 

 on the surface beneath the gelatine, in quantities proportional to 

 the nickel content at each point. The dimethylglyoxime then 

 penetrates the gelatine and produces the characteristic red precipi- 

 tate, faintly or strongly as the nickel content varies. 



The technique is difficult and the results are uncertain, sometimes 

 yielding success and again failm-e with apparently identical methods. 



