1916] THE GROWTH OF ETCH-FIGURES ' 233 
those which exhibit the maximum cohesion. Experience has shown that 
the most perfect examples of pits are produced upon such faces. This 
has already been observed by Baumhauer, who presents the facts in 
the following words: ‘‘Am besten entstehen die einzelnen Aetzein- 
drucke auf solchen Flachen, welche nicht allzu leicht vom Aetzmittel 
angegriffen werden, welche demselben also einen verhaltnissmassig 
grossen Wiederstand entgegensetzen. Dies ist haiifig der Fall bei 
Spaltungsflachen’”. This necessarily follows from the fact that the 
phenomena of etching as well as those of cleavage are dependent upon 
the cohesion of the crystal. 
It is a well-known fact that strikingly different results are obtained 
by the use of various corrosives, and so it is important to choose with 
care that one which is to be employed. In these experiments with 
colemanite three acids, each in a very dilute form, were successively 
tried, namely citric, acetic and hydrochloric. 
Citric acid produced etch hills. In cases where the solution was 
very dilute and the action had been early discontinued, the surface of 
the cleavage plate was covered with closely lying hills of polygonal 
form, fitted together like tiles in a pavement. As the process progressed, 
certain of the hills became undercut leaving an overhanging part some- 
what resembling the beaked extensions of pits to be subsequently des- 
cribed. Frequently these projections, all pointing in the same direction, 
had the appearance of being oriented. 
As a result of many experiments to determine the effect of the cor- 
rosive in various degrees of dilution, it was found that with extremely 
dilute acetic or hydrochloric acid, the pits were very small, and that 
with corrosives of greater strength the pits became larger. A 0.5% 
solution of glacial acetic acid acting on the clinopinacoid, produced 
great numbers of four-sided pits so small that it required the highest 
available power of the microscope to observe their details. Along with 
these well-defined pits there were also shallow depressions with indefinite 
outline. Some of these had a deeper part which approached somewhat 
tc the form of a pit. An interesting feature was the occurrence in 
almost every clear pit, of a beak-shaped extension, sometimes many 
times longer than the pit itself, growing from the deepest part of the 
figure. As solutions of greater strength were used, the percentage of 
beaked pits grew steadily less, and the beaks became shorter, until, 
when the optimum strength was reached, they were practically 
absent. 
2 “Die Resultate der Aetzmethode in der krystallographischen Forschung.” 
Leipzig, 1894, p. 3. 
