1916] THE GROWTH OF ETCH-FIGURES 231 
THE GROWTH OF ETCH-FIGURES. 
By WILLIAM Harvey MCNAIRN. 
INTRODUCTORY. 
The extensive and ever growing literature which contains the results 
of investigators who have made use of the method of etching in order 
to obtain a more exact knowledge of various phases of crystal symmetry, 
is an evidence of the importance of this series of phenomena as a means 
of crystallographic research. Among these publications there have 
appeared from time to time papers which have been devoted to the 
study of etch-figures themselves rather than to their secondary applica- 
tion. It is evident from a study of these articles that many of the pro- 
cesses involved in the origin and growth of etchfigures have not yet 
been fully explained, and as these processes are intimately connected 
with the molecular structure of crystalline matter, the value of fuller 
information will readily be conceded. 
A leader in the group of investigators who have devoted themselves 
to this phase of the subject is Professor V. Goldschmidt of Heidelberg, 
who published some years ago, two papers, among others, embodying 
a new theory for the formation of etch-figures.! According to this, 
both pits and etch-hills are the result of movements developed in the 
solvent. The chemical action between the corrosive and the substance 
upon which it is acting, gives rise to currents, some of which are directed 
towards and some away from the surface which is being etched. The 
interference of ascending with descending currents tends to form eddies, 
each of which is the starting point of a pit. The tendency of any solvent 
would be to produce regular, hemispherical excavations, but this is 
offset by the force of crystallization, which constantly endeavours to 
keep the corroded surfaces bounded by crystal planes. The resultant 
of these opposing tendencies is the typical etch-pit, whose sides are 
neither wholly irregular, nor yet normal crystal faces, but, as a com- 
promise, planes of similar nature to vicinal faces. When a beginning 
has been made, the growth of the pit proceeds at a diminishing rate, and 
since the edges of the pit dissolve with greater rapidity than the walls, 
the natural result will be an increasingly indistinct outline and walls, 
1 Zeitsch. fiir Kryst., 38:273, 656. 1904. 
