THE METALLOGKIAPHY OF METEQiRIC IRON 99 



irons may not appear consistent with that of similar but much larger 

 inclusions of the same nature in meteoric irons etched in the same 

 manner. 



Etching for phosphide. — Boiling neutral sodium picrate is the 

 most reliable method for identifying schreibersite and all phosphide- 

 bearing areas. Alkaline sodium picrate may or may not color 

 phosphide. Used boiling it usually does not color it unless long 

 applied. Used electrolytically it is apt to do so unless the immersion 

 is brief. Etching with a boiling neutral solution is the only 

 perfectly dependable method if picrate is employed. 



Because of its dangerous nature, neutral sodium picrate should 

 not be prepared or used without competent instruction or advice. 

 This caution applies especially to the preparation of the picrate in 

 crystalline form, which is highly dangerous because the crystals are 

 a powerful explosive which may go off easily from the effects of 

 warmth or manipulation. 



It can be prepared and used, however, with safety in the wet way 

 without crystallizing out and drying the picrate. When the solution 

 is to be used boiling, in the ordinary manner, it is prepared in the 

 proportions of 2 grams of picric acid crystals and 25 grams of sodium 

 hydroxide dissolved in water, the acid being added to the hydroxide 

 solution. 



It is not easy to determine the neutrality of the solution by the 

 use of an indicator, as it quickly changes in color to orange, red, and 

 brown, but litmus paper can be used satisfactorily. The solution 

 must be neutral; for if it is acid it is inefi'ective, and if it is alkaline 

 it will color carbide as well as phosphide. 



The necessary length of boiling is variable, depending on the 

 changes in the solution as well as on the character of the phosphide 

 areas. Usually a few minutes is enough to darken all phosphide. 

 The solution must be used immediately, as it rapidly loses its potency. 



To bring out eutectic structures in phosphide inclusions the sample 

 should be immersed in the boOing solution repeatedly for brief 

 periods (5 to 10 seconds) as the action is often rapid, examining the 

 sample after each immersion. In such cases the process is delicate, 

 the structure appearing with just the right length of application and 

 disappearing in a few more seconds in a uniform black. 



Etching for carbide. — Boiling alkaline sodium picrate darkens 

 carbide (cohenite). The solution, which must be freshly made, is 

 prepared by dissolving 25 grams of sodium hydroxide in 100 cc. of 

 water and adding 4 grams of picric acid. A larger quantity may 

 be prepared in the same proportions. The use of the solution in- 

 volves no danger. It must be used immediately, and care must be 

 taken to make sure that it is at all times alkaline, as it tends to become 

 neutral and in that case it would not darken carbide. 



