280 Transactions of the Society. 



Methods of Etching. 



The chief advance made in metallography of recent date con- 

 sists in the method of developing the structures of metals. 



Dr. Sorby and Professor Arnold recommend the use of nitric 

 acid in different degrees of concentration, but, excepting in such 

 cases when vigorous action is required and for very pure steels free 

 from phosphorus and sulphur, nitric acid does not give good 

 results. 



M. Osmond devised a process of showing up the pearlitic and 

 other structures of steels, which he described as the " polissage 

 attack," which consisted of rubbing the polished metal section on 

 parchment, moistened with a solution of lirjuorice-root in water, or 

 a 2 p.c. solution of nitrate of ammonia in water. When the proper 

 pressure is applied, after a little practice, it is possible to get a very 

 perfect development of the structures of steels by this method. 



Tincture of iodine has been used by Osmond and others with 

 very satisfactory results, and for a long time I have found this a 

 most admirable reagent for differentiating between the appearance 

 of such portions in steels, which are relatively higher in phosphorus, 

 from those containing less of that element. Iron or steel con- 

 taining much phosphorus is less readily corroded or attacked by 

 iodine than similar material containing little of that element. By 

 taking advantage of this peculiarity, when a given piece of metal 

 contains more phosphorus in one part than in another, it is easy by 

 the action of dilute iodine to find out where the high phosphorus 

 portions are located. 



M. Ischewsky, in the laboratory of Professor H. Le Chatelier, 

 discovered that a 5 p.c. solution of picric acid in absolute alcohol 

 would give the same constant and perfect development of the 

 structure of pearlite in steel as was obtained by the "polish 

 attack " of Osmond, and this reagent is almost universally recog- 

 nised as one of the most valuable reagents for etching steels. 



On applying picric acid in the development of steels containing 

 high percentages of phosphorus, it has been found that by long 

 continued action it causes the portions highest in phosphorus to 

 become brown relatively more rapidly than the other parts. 



Brasses, containing varying proportions of copper, when placed 

 side by side in the picric solution, become coloured in different 

 degrees and at varying periods. 



It is possible that the same reagent may be used for other 

 alloys with advantage. 



Professor H. Le Chatelier has discovered that alkaline oxidising 

 reagents have the peculiarity of darkening carbide of iron when in 



