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. 
AwMOfiiuin Hydfoxide -j- Hydfogcn PeToxidc.^ 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 drop wise. 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. 
Afntnofiiufn PeYSulphdte. — 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 + 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. 
