"Nickel-grains" in Fuming Nitric Acid. 259 



angles. Direct perforations are few and more or less oval in 

 shape. In other words the lines of least resistance in the two 

 plates lie presumably in the one case in planes at right angles to 

 the surface and in the other in a plane parallel to the surface. 

 This conjecture is however only partly true, for if we examine the 

 etched surface of Mond tube-nickel before perforation has taken 

 place (and this can be conveniently done in the accompanying 

 photomicrographs), we shall find that in the Mond nickel lines of 

 least resistance, mapped out by furrows, also lie in planes parallel 

 to the surface. These intricate figures (Figs. 1 and 2), so highly 

 suggestive of wave and vortex motion, are mainly met with in 

 my experience on the outer or convex surface of a Mond tube. 

 They appear to indicate that in this highly purified metal the 

 perforations take place as a rule in one direction, namely from 

 without inwards. 



I am indebted to Mr Caush, an amateur photographer of 

 Brighton, for the accompanying photomicrographs. 



