442 ELEMENTARY CHEMICAL MICROSCOPY 
The etching is rapid and needs careful attention to prevent 
over treatment, one to three minutes being the average exposure 
required. 
Valuable in studying aluminum bronzes and brasses. 
HydfochloTtc Add -f- Absolute Alcohol. — To loo cubic centi¬ 
meters of absolute alcohol add i cubic centimeter of concentrated 
hydrochloric acid. This is the general etching reagent of 
Martens and Heyn for all iron-carbon alloys. Applicable 
to all specimens but must be used with care. With extra 
hard steels and certain alloy steels this reagent does not work 
well. In these cases Martens suggests the nitric-alcohol reagent. 
Neither reagent is permanent, but must be freshly prepared for 
use. 
Hydrochloric -f Nitric Acid. — Mix three parts of dilute hydro¬ 
chloric acid with one part of dilute nitric acid, add 2 or 3 drops 
of platinum chloride per 100 cubic centimeters of mixture. 
A valuable etching liquid for copper-nickel alloys. 
Nitric Acid -j— Absolute Alcohol. To 100 cubic centimeters 
of absolute alcohol add 4 cubic centimeters of concentrated 
nitric acid. Prepare just before using. Useful in the case of 
very hard steels and with certain alloy steels. Especially valu¬ 
able in developing Troostite. Used also on nonferrous alloys. 
One of the most generally useful etching reagents. 
Picric AcidAlcohol. —Employ a 5 per cent solution of 
picric acid in absolute alcohol. 
Useful for all iron-carbon alloys, especially those high in 
carbon. Pure iron (ferrite) is not appreciably attacked save 
after long'exposure. 
With low-carbon steels a higher concentration than 5 per cent 
is advisable. 
This reagent not only etches but stains the specimen. Often 
a surface film, especially with high phosphorous irons and steels, 
is formed of such a character as to mask the structure. Gentle 
rubbing with a finger tip in washing will usually clear away the 
obliterating film. 
Silver Af/m/c. — Dissolve 5 grams in 100 cubic centimeters 
of water. After washing, rub the surface very lightly with a 
