1916] THE GROWTH OF ETCH-FIGURES 255 
in which a current of somewhat concentrated corrosive was directed 
against a crystal of colemanite, the well-defined pits which were pro- 
duced apparently instantaneously, differed only in size from those 
produced slowly by a more dilute solution. . 
The inference is that by the use of a strong corrosive the pits are 
produced more rapidly than with a weaker one. Many authorities have 
gone a step farther by the claim that such pits are also more perfect. 
Miers summarizes this as follows: ‘‘Experiments with hydrochloric 
acid on cylinders of calcite indicate that with a more concentrated 
solvent the etched faces are produced more readily and in greater per- 
fection than with a dilute solvent”.2!. The results of the foregoing 
experiments, however, show that approximate perfection in the figure- 
faces depends, not upon the strength of the solvent, but upon the com- 
pleteness of the processes involved. With a weak solvent this will take 
place slowly, but in time the result will be the same. 
That there is a development of some kind in the formation of the 
pit has long been known. It is referred to by Baumhauer,” who points 
out that the pits become more numerous and larger until the surface is 
practically covered. He also observed that after a time they become 
rounded, but the stages in this process have not hitherto been definitely 
laid down. It was also made note of by Daly, who remarks, ‘‘As the 
process of etching continues the pit usually increases in size, often 
(depending partly on the symmetry of the etched plane) changes in 
shape of outline. ... The first stage of development in a pit may be 
called its ‘initial’ form. The development ends where the outline 
begins to be seriously impaired by the solution of the surrounding part 
of the etched surface. Just preceding this point in the history, the pit 
may be called ‘mature’, and the process intervening between the initial 
and mature stages is that of maturing. Von Ebner’s ‘instantaneous’ 
and ‘retarded’ types are connected by transitions, but are not easily 
to be compared to ‘initial’ and ‘mature’ figures since his types refer 
simply to the length of time required to develop the pits and are not 
restricted to the use of one solvent’’.*8 
By the application of the method of interrupted etching, the exist- 
ence of this development was proved and the various stages were isolated 
and described. They may be stated as follows: 
(1) The first stage in pit development is ultra-microscopic, and con- 
sists of the attack of the corrosive upon the weak point in the crystal. 
From this, the rate of attack depends upon the strength of the corrosive. 
31 “‘Mineralogy”’, Lond. 1902, page I1§. 
2 op. cit., page 3. 
% op. cit., page 376. 
