SIR ROBERT HADFIELD, BART. 25 



Preparation of Specimens and Etching.— In the preparation 

 of specimens for micro-examination great skill and ingenuity have been 

 displayed by numerous investigators from the time of Sorby onwards. 



When it is considered that a maximum magnification has now been 

 reached of about 8,000, the difficulties to be overcome will be readily 

 recognised. Supposing the surface of one side of a cube, say one 

 twenty-fifth of an inch square, to be under examination, this ^ has 

 meant that the area under observation has been multiplied or magnified 

 to a surface of say 30 ft. square, or about 900 square feet. It will be 

 * seen how the slightest scratch or groove, imperfect polishing, bad 

 etching, or other defect will at once interfere with the desired results 

 being obtained. 



In this connection I should like also to call attention to an 

 interesting Paper read by Sir G. T. Beilby, F.R.S., before the Royal 

 Society in February, 1914, entitled " Transparence or Translucence 

 of the Surface Film produced in Polishing Metals." Some beautiful 

 Photomicrographs are there shown, photographed by a 3 mm. Oil immer- 

 sion Lens of 1.4 N.A. The thickness of the films covering the slight Pits 

 on a Copper surface was stated by Sir George to be probably of the 

 order of 10 to 20 micro-millimetres i ^^^^looo ^^ 1.200000 i^^^)- 



Although if it was possible to get raw surfaces free from all grooves, 

 scratches, and other blemishes, some structure would be developed, 

 it must be remembered that not even the finest polishing will display 

 structure, therefore etching must be employed. 



The etching accomplishes two things : it removes the amorphous 

 layer, and then attacks the various constituents differently. The 

 products of the etching attack usually differ in appearance more than 

 the original constituents. 



For high power the etching must be very light, that is, the time 

 of etching must be short. A 5 per cent, solution of picric acid in 

 alcohol gives the best results. The perfect flatness of the polished 

 surface must be retained, and only the lightest possible etching is 

 given. In low power w^ork the etching is fairly strong in order to- 

 obtain contrast between the light and dark portions. 



As regards the effect of different kinds of etching, I invite attention 

 to Photomicrographs, Figs. 9, 10 and 11. These are from a Gun 

 Tube Steel containing .42 per cent. Carbon, .74 per cent. Manganese, 

 and representing material as forged, that is without further 

 treatment. 



Fig. 9 was etched with 5 per cent. Picric Acid in Alcohol. 

 Fig. 10 was etched with 5 per cent. Nitric Acid in Alcohol. 

 Fig. 11 was etched with 5 per cent. Solution Meta-Nitro-Benzol- 

 Sulphonic Acid. 



The structure shows grains of Ferrite on a ground mass of Pearlite 

 and the Photomicrographs prove that the Structure developed is in Ic- 

 pendent of the particular etching reagent used. The number of etching 

 reagents might be extended on this work with practically the same 

 results in each case. Most Alloy Steels, for example. Manganese 

 Steels, quenched and tempered. Nickel Chromium and other Steels,. 



