Fig. 7- 



FURTHER NOTES ON TIDESWELL DALE QUARRIES. 13 



similar change in its material, as we have above noted. Now, it 

 is obvious that when once a film is formed, this re-agent can only 

 reach the surface of the unaltered part by passing through it ; and 

 it is equally obvious that in so doing the re-agent will tend to 

 reach this surface by the shortest " cuts." For instance, the part 

 or area of the surface of the unaltered part, a, in Fig. 7 (which 



represents a section of a part of our 

 ,! W cube) will draw its supply of re-agent 

 from the outer surface at b, 

 immediately superjacent to it j and 

 these two corresponding areas, a 

 and b, of the two surfaces, when 

 situated in the central region of 

 any of the sides of the cube, will 

 be equal in size, or approximately 

 so- that is, a square inch at a will draw its supply of re-agent 

 from the square inch of outer surface at b, immediately above it. 

 But the case is otherwise, in this latter respect, in the regions of 

 the angles of the cube, as d in the section ; for it is manifest that 

 instead of & drawing from an outer area equal to itself, it will 

 draw from a strip in breadth equal to its own diameter, and 

 extending from c to A, and from A to d-* surface immensely 

 greater. But the contrast becomes still greater if we suppose 

 d to be immediately under one of the " corners " of the cube ; 

 in that case, instead of supplies of a breadth equal to its own 

 diameter converging upon it with an angle of nearly 90 , they will 

 pour on to it from all directions within that angle. What will be 

 the result ? The greater the supply of re-agent, the greater the 

 amount of chemical change, and the more rapid the recession of 

 the surface of the nucleus acted upon. Hence a rounded nucleus 

 must at length be the result. But when this is accomplished, the 

 broader surface which the nucleus presents to the outer angles, 

 combined with the thicker " shell," will tend to neutralise these 

 differences, the surface of the former receiving a more equitable 

 supply of re-agent throughout. 



