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. 



Hydrochloric Acid + Absolute Alcohol. — To ioo 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 + 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 -f 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 A cid + A Icohol. -- 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 ringer tip in washing will usually clear away the 

 obliterating film. 



Silver Nitrate. — Dissolve 5 grams in 100 cubic centimeters 

 of water. After washing, rub the surface very lightly with a 



