1916] THE GROWTH OF ETCH-FIGURES 259 
crystal, of such a nature that the form of any one could be predicted. 
This form is determined to a certain extent by the relative rates of 
solution in different crystallographic directions. Some pits are long 
and narrow on account of a marked difference in the rates of solution 
at right angles to one another, while others are square or even nearly 
round where there is no difference in the rates. This point has been 
worked out in some detail by Becke,”” who, by careful measurements of 
the diameter of spheres during the process of corrosion, deduced the 
rule that the velocity of solution varies with the crystal direction. 
From these velocities it would be possible to construct an ellipsoid of 
solution corresponding to those illustrating the phenomena of heat 
and light in the crystal. 
Another method of showing the relationship between the pits was 
suggested by the fact brought to light in the diopside experiments, 
described in an earlier section of this paper, that the pit on the base 
resembles that on the orthodome, while the latter resembles to an equal 
extent, the pit produced upon the orthopinacoid, the three differing 
mainly in their comparative lengths. In each pair there are some faces 
in common, and it would be possible to construct a negative crystal 
from the faces occurring in the pits of the orthodiagonal zone. The 
result would be substantially the same negative crystal which would 
be obtained by the union of two pits on opposite clinopinacoids. If it 
were possible for the corrosive to act within the mass of a crystal, just 
such a regular cavity would be the result, and, in passing, it might be 
pointed out that Professor Judd’s remarks as to the cause of schilleriza- 
tion, which are quoted on a subsequent page of this paper, would 
indicate that such an action is not so impossible an event as might at 
first appear. 
In order to discover experimentally whether we have in such a nega- 
tive crystal a method of correlating the pits of a crystal, a plaster model 
was made consisting of the following forms: (100), (110), (oro), (oor), 
and (o11). This is similar in shape to the cast obtained by filling the 
negative crystal formed by bringing together pits on adjacent clinopina- 
coids. The portion of the model cut off by any intersecting plane will 
then represent a cast of a similar portion of the original negative crystal. 
In order to obtain a cavity corresponding to that of the cut off part of 
the negative crystal, it is only necessary to imbed this in solidifying 
plaster in such a way as to keep the hypothetical plane of section in a 
horizontal position. This was done and the intaglio models so formed 
are illustrated in the accompanying plate. 
7 Min. u. petr. Mitth., N.F., 5: 457.1883. 
