236 TRANSACTIONS OF THE ROYAL CANADIAN INSTITUTE  [VOL. XI 
currents are avoided by keeping the solution in constant motion, the 
effect was tried of directing a small jet of the corrosive against the 
crystal and then quickly stopping the action by plunging it below the 
surface of water. Under these conditions no ascending current of 
sufficient force to overcome the jet which was being directed against 
the crystal could have been formed, and yet clearly defined pits, though 
small ones, were produced. 
It becomes quite evident that we must look for some other explana- 
tion for the origin of pits. When we examine an etched crystal we 
cannot help noticing that the pits are not scattered evenly over the 
surface, but that some parts are comparatively free and some densely 
covered. This fact had already been observed by Baumhauer,*? who 
remarks: ‘‘Die Vertheilung der Aetzfiguren tiber die geatzte Flache 
ist nie eine gleichmdssige, wie man nach der Theorie erwarten sollte’. 
Tentative explanations for this irregularity have from time to time 
been brought forward, and as this problem includes the fundamental 
one of accounting for the origin of the individual pit, it was necessary 
to devise experiments to put the various theories to practical tests. 
Several fragments which had been marked with fine scratches were 
etched, but instead of the etch-figures being more numerous over the 
scratches, the latter were at once smoothed away by the corrosive. 
It had been suggested that possibly small particles of dust might pro- 
vide the corrosive with points of attack, and that the fortuitous scatter- 
ing of the dust particles might reveal itself in the apparently unaccount- 
able distribution of the pits. In order to test this several pieces which 
haid been covered with dust were exposed, but in no case was there any 
increase in the number of pits, or any concentration on that part of the 
surface where a greater amount of dust had collected. 
Another possible explanation for this bunching of the pits was 
that the crystal might have been under strain, but a careful microscopic 
examination of several very thin slices from crystals which had shown 
this concentration of pits, failed to give the irregular extinction under 
crossed nicols, which is to be expected in strained crystals. This is of 
course what one might have expected, since pitting is a function of the 
cohesion of a crystal and strain is the result of abnormal elasticity. 
Among the crystals used in these experiments some were transparent 
while others were translucent. The transparent crystals, in some cases 
at least, produced large and clear pits, which were not set very closely 
together, while the surface of the milky crystals was closely covered 
with small, indistinct pits, making the crystal still less transparent. 
2 BE. Cits, Pil, 
