" Nickel grains" in Fuming Nitric Acid. 257 



Nickel Company consisted of three thin-walled nickel tubes about 

 9 mm. in diameter, 12 or 13 cm. in length and each weighing 

 between 3 and 4 gm. Although an absolute purity was not 

 claimed for it, the metal was stated to be practically pure and 

 free from copper and cobalt. The tubes had a brilliant metallic 

 sheen over their convex surface, and a dull frosted appearance 

 within. A careful examination showed that whilst there were 

 many minute indentations externally, the small blister-shaped 

 prominences were entirely confined to the inner aspect of the 

 tube. When split lengthwise and flattened out a tube was readily 

 converted into a thin plate of nickel. The plate (a) so formed, 

 although less ductile and tenacious, was practically of the same 

 thickness (i.e. 01 mm. approximately) as the sample of thin sheet 

 nickel (b) obtained from Johnson, Matthey and Company. The 

 two served well for comparison. When subject to the action of 

 nitric acid in the manner already described they were both found 

 to be very resistant, especially (6). After prolonged exposure the 

 metal plates were reduced to grilles, but the openwork tracery of 

 grille (a) was very unlike that of (6). 



Final observations. To the present time I have dealt almost 

 exclusively with descriptions more or less imperfectly worded of 

 the structural changes which take place in different samples of 

 commercial nickel when they are exposed to the action of fuming 

 nitric acid. I wish before concluding to draw attention to a few 

 phenomena observed during this investigation which may assist 

 in elucidating the physical processes whereby these changes are 

 occasioned. 



Grain-nickel is sometimes met with in the form of a flattened 

 plate. If one of these is subjected to the prolonged action of 

 nitric acid, the plate under suitable conditions, when reduced to 

 a thickness of 0'75 mm. approximately, becomes so friable that it 

 can be crushed to powder between the thumb and finger. The 

 powder so formed and that obtained from nickel-grains under the 

 conditions mentioned in my former note seem to me to be alike in 

 every respect. Apparently the disintegrating agent, whatever it 

 may be, although doubtless most active over the surface of the 

 metal can penetrate its substance to the depth of at least 

 0375 mm. In the case of the grain-nickel plate I am now con- 

 sidering, this great change in cohesiveness was effected without 

 any visible alteration in its contour or aspect beyond the general 

 shrinkage due to persistent acid reaction. 



If we examine the grilles of Mond nickel and of sheet nickel 

 respectively under a low power objective we cannot fail to observe 

 their great dissimilarity. Whilst the Mond nickel is perforated 

 with, it may be many, circular or nearly circular holes, the sheet 

 nickel is riven by narrow fissures which cross each other at various 



