1916] ‘Tue GrowTH or Etcu-FIGuRES 237 
This milkiness was found to be due to the presence of numerous 
small inclusions. There would thus appear to be some connection 
between the number of pits and the number of inclusions, and it seemed 
quite possible that the uncovered top of an inclusion might form a ready 
point of attack of the corrosive. But, while in a given experiment the 
pits are of approximately the same size, the inclusions on the other 
hand vary, some being many times the size of the largest pit. Further, 
it was found that the places where most pits were produced were not 
the areas of the greatest distribution of the inclusions, and finally, in 
no case was an inclusion seen to be the starting point of a pit. We 
may safely conclude then that the relation between inclusions and pits 
is not that of cause and effect, but rather that of a common origin. 
The ability of a growing crystal to take up impurities from the sur- 
rounding solution, and the frequency of the occurrence of possible 
points of attack for the corrosive on the faces of a mature crystal, alike 
depend upon abnormalities in its cohesion. 
An observation of Baumhauer’s in his paper on the apparently 
anomalous pits of colemanite, when more fully worked out, gave a clue 
to the elucidation of this problem. He found that if the two cleaved 
halves of a crystal which have been subjected to the action of a cor- 
rosive, be placed together in their original position, pits on the two 
halves will be found to coincide and thus form a negative crystal, which 
is divided, sometimes in the centre, but more frequently unequally, by 
the plane of cleavage. The bearing of this phenomenon upon the 
problem of the origin and cause of pitting, was not discussed or suggested 
by this investigator.’ 
Following this experiment, a piece of the mineral was cleaved into 
halves, which were then etched separately, but under similar conditions. 
It was found that the size, number and distribution of the pits on ad- 
jacent sides of the plane of cleavage were quite similar, and even in 
some cases complementary pits could be identified. But the distribu- 
tion of pits on opposite sides of the same fragment was always different, 
and sometimes a difference was also observed in their size and develop- 
ment and even in their shape. 
A suggestive fact which came to light in this experiment was that 
the negative crystal which would be formed by bringing the two pits 
on adjacent faces together, was not divided symmetrically by the cleav- 
age plane. The deepest part of one pit will then be found opposite the 
shallowest part of the other. In fact the two pits are enantiomorphous 
4 Zeitsch. fiir Kryst., 30: 97. 1889. 
