240 TRANSACTIONS OF THE ROYAL CANADIAN INSTITUTE  [VOL. XI 
the fact that the action of solution will take place with the maximum 
of rapidity on any edge, more particularly if that edge is at all acute, and 
in this way the pits become flatter as action continues. But here are 
examples of the persistence of a well defined and sharp edge. 
Along with the forms described above there were others which from 
their distinctness of outline and depth would appear to be somewhat 
more mature still. They were bounded by three unequal, triangular 
figure faces of which the largest was curved. An additional peculiarity 
was that which has been referred to in the previous paragraph: the 
deepest part was not in the centre of the pit but was very much to one 
side, being in fact under an overhanging edge. In all the pits of this 
type, the figure-faces appeared to be striated parallel with the surface 
of the etched face. The significance of the phenomena just described 
will be dealt with in a subsequent part of this paper. 
Pits of a still more mature form were obtained by exposing the 
specimen to the corrosive action of the commercial solution of hydro- 
fluoric acid. After ten minutes’ heating over a water bath the corrosive 
had produced, not the forms which we usually think of when we speak 
of pits, but fine tubes. The openings to these tubes were larger than the 
tubes themselves and to a certain degree resembled true pits. On con- 
tinued action they increased in a remarkable manner, not only in number, 
but also in length, becoming twisted and interwoven like a bunch of 
hairs. The growth of these tubes continued with further exposure to 
the corrosive long after the surface originally acted upon had been com- 
pletely dissolved away. After an exposure of fifty minutes when the 
action was discontinued, some of the tubes seemed to have penetrated 
nearly half through the fragment. This rather extraordinary result 
seems to have a bearing upon the nature of schillerization as will be 
seen from the discussion in the second part of this paper. 
In the experiment described above, in addition to the corrosion 
which produced the fine tubes in the interior of the crystal, another and 
different type was observable on the surface. Gradually the pits which 
were first formed enlarged their borders until by the coalescence of 
numerous flat bottomed pits the whole surface with the exception of a 
few residuary etch-hills had been removed. On these freshly formed 
surfaces new etch-figures began to appear. While they were still of a 
somewhat triangular form, being composed of three figure-faces, one 
long and shallow, a second slightly shorter and steeper and a third 
small, steep and triangular, yet they showed considerable variation 
from the first crop produced upon the unetched face. After an exposure 
of thirty minutes they were fairly large, of clear general outlines but 
the form of the figure-faces was not easily seen. After forty minutes 
