1916] THE GROWTH OF ETCH-FIGURES 249 
De Kalb, N.Y., reached its greatest clearness in about fifty minutes. 
These experiments indicate that a maximum of perfection, in which 
the figure-faces approach to the condition of possible crystal faces in 
their evenness and position, is attained only after etching has been 
continued for a considerable time. Further exposure results in 
gradually decreasing clearness, until the pits have quite lost their original 
character. 
It is interesting to note in passing that in these experiments, as in 
others in this research, one or two of the pits attained a size many times 
greater than that of the rest on the same face, persisting even after 
others had flattened out of all resemblance to their original form. This 
fact is of significance in developing the theory of the origin of etch-figures 
as will be seen by the discussion in the second part of this paper. 
(d) Puts on the orthodome. 
The faces around the ends of the vertical axis of diopside are almost 
invariably extremely rough and corroded and quite unsuited for crystallo- 
graphic examination. In order to overcome this difficulty some crystals 
were obtained which had the forms (1o1r) and (oor) artificially cut and 
polished upon them. Before being etched they were examined and the 
artificial faces were found to be correctly cut. 
After an exposure of 20 minutes to very dilute hydrofluoric acid, 
quite definite but minute pits were obtained on the domes. In general 
form they were somewhat lozenge-shaped. They were each bounded 
by four figure-faces, two pairs of triangles, one of which was consider- 
ably longer than the other. In some cases, a fifth face, small and four 
sided, formed the bottom (Plate XXVI, fig. 30). It will be noted that this 
arrangement 1esults in a figure, one of whose ends is deeper than the 
other. At this stage, the pits were too small to yield any definite re- 
flections of any kind. 
On further exposure to the same corrosive the pits gradually increased 
in size and number and to a certain degree changed in outline. A few 
were observed which were enclosed by two curved figure-faces. This 
stage of their development is shown in Plate XXVI, fig. 31. 
After thirty minutes’ exposure it was found that the pits were still 
too shallow to admit of measurements, and so a stronger corrosive, 
commercial hydrofluoric acid diluted with an equal volume of water, 
was used. This, after an exposure of 20 minutes at 100° gave pits 
which, while they retained their original outlines, were much larger 
and deeper. While the perfecting of the faces took place apparently 
in the same way as in the case of the pits on other faces, the action was 
not continued long enough to give any clear reflections, 
