396 PROCEEDINGS OF THE AMERICAN ACADEMY. 



a hypothecated process of "secondary solution" was incidentally re- 

 ferred to in connection with the actinolite figures ; its effects can 

 often be seen where straight-edged outlines of a figure are replaced 

 by curved edges as the figure is undergoing destruction by prolonged 

 solution. 



Plate I. Fig. 7 furnishes a noteworthy variation on the normal and 

 simple process of pit development. The figure is compound, and consists 

 of three pits, formed respectively at the bottom of the next oldest pit. 

 Each must represent an abrupt stage in the solution of this part of the 

 surface. The side AD remains sensibly parallel to itself in all three 

 steps, but the angle BAH grows larger from the first to the third num- 

 bered downwards; at the same time, the edges of the successive figure- 

 faces against (110) are seen to curve more in the first than in the second 

 and in the second than in the third. These facts accord with those 

 observed in the case of the pits that grew continuously, not intermit- 

 tently, to maturity from the initial and less advanced stages of previous 

 exposures to the acid. The stepjied figure seems, then, to show that 

 the formation of pits may (though not always) take place spasmodically, 

 so to speak (•' sprungweise," in the German phrasing), the attack the 

 affair of a moment and preceded and followed by longer periods of 

 almost perfect quiescence as regards other than "secondary" solution. 

 Klocke believed, similarly, that the formation of figures on the alums 

 is an instantaneous thing.* The stepped form is jiresumably not due 

 to a zonal structure in the hornblende, because such a hypothesis would 

 imply very considerable variations in the attackability in passing from 

 the exterior to the inner zones of the crystal, — variations improbable 



* Zeit. fiir Kryst., 1877-78, Bd. II. p. 131. The rapidity of the reverse process, 

 that of healing over an etched surface, was early commented upon by Sir David 

 Brewster in connection with his studies on the instructive group of crjstalline 

 substances, the alums, especially' with reference to his now familiar " light-figures." 

 On immersing an etched crystal of an alum in a concentrated solution of its own 

 substance, he observed that, "in an instant," the pits of corrosion began to fill. 

 He says, "The singular fact in tliis experiment is the inconceivable rapidity with 

 which the particles in the solution fly into their proper places upon the disinte- 

 grated surface and become a permanent portion of the solid crystal." Phil. Mag., 

 1853, Vol. V. p. 27. The intermittent character of the process of etching was 

 noticed by Rinne (Neues Jahrbuch, 1885, Bd. II. p. 15), who etched milarite with 

 dilute HF. At first, the base became covered with regular hexagonal pits placed 

 symmetrically on (0001). On further attack, these are suddenly modified by the 

 appearance in each case of a second hexagon in the bottom of the first, but now 

 turned through an angle of 30°. 



