1898.] PRESTON — ETCITIXG IRON METEORITES. 265 



The principle in etching so as to bring out the Widmanstatten 

 figures, or crystalline structure of the iron, is to use a solvent that will 

 more readily attack one portion of the mass than another. As the 

 kamacite contains a much larger percentage of iron it is more readily 

 attacked or dissolved by the acid than the other alloys, thus allowing 

 the bands of kamacite to become depressed or intaglio, while the 

 taenite and plessite stand out in relief. For this purpose various 

 methods have been used, bichloride of mercury, nitric or hydrochloric 

 acid, or a combination of the two of various degrees of strength, 

 depending upon the readiness or backwardness with which the iron 

 will etch, etc., etc. In some irons it is only necessary for them to 

 come in contact with the acid for three or four seconds in order to 

 bring out distinctly the crystalline structure, while with others fifteen 

 or twenty minutes or more are required. 



There are several reasons why the use of acids has been unsatis- 

 factory for the work. In the first place they act upon the taenite and 

 plessite almost as readily as upon the kamacite, so we do not get the 

 detail required from the lack of relief in the more nickeliferous alloys. 

 In the second place most or all iron meteorites are filled to a greater 

 or less extent with fissures passing between the kamacite and taenite 

 bands. The acids percolate through these fissures to considerable 

 depths, so that it is almost impossible to treat the newly etched slices 

 so as to thoroughly neutralize the effect of the acid that has pene- 

 trated them ; the result being that in a short time, even though the 

 surface be well oiled or coated with paraphine, we observe a discolora- 

 tion extending in lines over the surface. Many times these are the 

 first indications we have of a fissure or crack, as they were not per- 

 ceptible to the eye on the newly etched face. After a time this line 

 commences to spread and finally a thick layer of rust is formed, which 

 not only mars the beauty of the surface, but in many cases is impos- 

 sible to remove without repolishing the slice. For these reasons it 

 has been my desire to substitute a solvent for the acid and thus over- 

 come these annoyances. 



Bichloride of mercury (corrosive sublimate) has long been used 

 to take the place of acids, and will overcome many of the difficulties 

 caused by the acids, but there are objections to it. In the first place 

 it is a very dangerous solution to have around ; again we frequently 

 get a coating of mercury on the iron, which at times causes some 

 trouble to dispose of, and as far as my experience has gone with it I 



