ETCHING LIQUIDS 441 



to the acid to just dissolve the surface film and yet not attack 

 the polished surface. 



The following are a few of the most generally useful of etching 

 reagents. For the development of certain specific structures 

 the student must consult the literature dealing with these prob- 

 lems. 



Ammonium Hydroxide + Hydrogen Peroxide. 1 — Immerse the 

 alloy in ammonium hydroxide diluted to such a strength (1:4) 

 that the alloy is not rapidly etched. Add hydrogen peroxide 

 from a pipette dropwise. This method gives better results 

 than mixing the reagents before the specimen is immersed. 

 Great care must be observed to avoid too rapid an attack and 

 too deep etching. Excellent for alloys high in copper. 



Ammonium Persulphate. — Dissolve 5 grams in 100 c.c. strong 

 ammonium hydroxide. Rub the specimen with cotton dipped 

 in dilute sodium hydroxide, wash at once and dip into the per- 

 sulphate solution. After a few seconds, remove wash and 

 examine. If not sufficiently etched, dip again. Repeat until 

 the structure has been sufficiently developed. Etches /3-Brass 

 more readily than a-Brass. Useful with most copper alloys. 



Ferric Chloride. — Prepare a hot, almost saturated solution 

 of ferric chloride; filter, and add an equal volume of concen- 

 trated hydrochloric acid. For use, dilute one part of this stock 

 solution with twenty parts of alcohol. If upon trial the etching 

 is too energetic, dilute still more; if not energetic enough, add 

 more stock solution. 



Useful in studying bronzes of high tin content, in etching 

 a-Brass and copper alloys in general. 



Ferric Chloride -f- Alcohol. — Robin 2 prepares this reagent as 

 follows : 



Per cent. 



Ferric chloride 5 



Water 5 



Hydrochloric acid 3° 



Iso-amyl alcohol 3° 



Ethyl alcohol 3° 



1 Ramsay, Chem. N., 87 (1903), 291. 



2 Traite de Metallographie. 



