232 TRANSACTIONS OF THE ROYAL CANADIAN INSTITUTE [VOL. XI 
which finally become practically indistinguishable from the surface of 
the crystal face. Then, newly developed eddies may form a fresh lot of 
pits in the enlarged bottoms of the old ones, and so the growth goes on. 
At first sight the explanation of the phenomena of etching offered 
by this theory seemed to be quite adequate, but some facts which came 
to light in the course of an investigation made by the writer, of the 
symmetry of diopside seemed to point so clearly to a cause which lies 
much closer to the molecular structure of crystalline matter, that the 
present research was undertaken in order to attain to a fuller knowledge 
of these interesting and somewhat obscure phenomena. To this end it 
was necessary to devise special methods of investigation. In the first 
place, in order to observe the origin and subsequent stages in the growth 
of the pit, and to compare the rapidity of the action on the various faces 
of the crystal, it was exposed to successive, short applications of a cor- 
rosive in somewhat dilute form. In the second place, in order to trace 
these stages with greater certainty, the corroded crystal was examined 
at each step by means of the goniometer, and in the third place in order 
to study the distribution of the pits a series of parallel cleavage plates 
was etched. By these methods, the various stages in the growth of an 
etch-figure could be traced from a shapeless, shallow depression, through 
the next stage of the greatest perfection, with almost flat figure-faces 
which approximate true crystal planes, and finally, to the gradually 
disintegrating pit, whose sides become shallower and rounder until the 
whole disappears. 
In order to observe these stages of growth various monoclinic minerals 
were etched. Colemanite, spodumene and diopside, on account of their 
transparency, their perfect cleavage and the ease with which satis- 
factory pits could be produced, were found to be the most suitable. 
The material was supplied by the Mineralogical Laboratory of the 
University of Toronto, where most of the experiments were carried out 
under the supervision of Professor T. L. Walker. 
PART I—EXPERIMENTAL. 
I. CORROSION OF COLEMANITE. 
In several ways colemanite was found to be particularly well adapted 
to these experiments. It is obtainable in clear and transparent crystals 
and it has a perfect cleavage parallel to the clinopinacoid. It is thus 
possible to prepare plates so thin that the effects of corrosion on opposite 
sides can readily be observed and compared, and whose broad faces are 
