468 
PROFESSOR HENRY A. MTERS ; AN ENQUIRY INTO 
■R' 42° 49' 
7-' 327° 474' 
ll'r' = 75° 1^' after etching. 
RR' = 0° lU' 
V) 
2 ’ 
■' = 0 ° 64 '. 
In another series of experiments good readings were oljtained from etched facets 
upon a cleavage rhomb immersed in very dilute acid; these were inclined to the 
cleavage faces at 0° 13f' after 2 hours, 
„ 0° I5|' „ 4 „ 
„ 0° 17f' „ 8 „ 
Tliese images Ijelong to facets lying in the zone R?' and are accompanied hy other 
imao-es not lying in this zone and l)elonging to facets on the other side of the 
triangular etched figures. 
These and other experiments indicate that well-defined facefs may he developed hy 
the action of very dilute acid, hut that they are inclined at different angles to the 
original face, and that the inclination of the etched facets changes as the etching- 
proceeds, the value in general increasing. 
(b.) Parallel Growth of NaN 03 with Oalcite. 
A cleavage rhomlj of calcite was adjusted and immersed in a concentrated solution 
of NaNOg. The cry.stals of sodium nitrate which grew upon the surface of the calcite 
could not, of course, have all their faces parallel to those of the calcite, for^ the 
rhombohedron angle of calcite is 74° 55', whereas that of sodium nitrate is 73° 30'. 
It was found that the sodium nitrate always lies on the calcite m such a way that 
the two rhomliohedra have one edge parallel, and generally also one face containing 
that edge in the sodium nitrate is parallel, or nearly parallel, to the corresponding 
face of "the calcite. This result is in accordance with that arrived at in another way 
by O. Mugge, ‘Neues Jahrhiich,’ 1896 (2), p. 74. 
It is also analogous to the union of twin crystals, in which the two individuals are 
not always exactly in the position demanded hy the law of twinning hut may deviate 
therefrom liy several minutes or even by a degree or more. In these cases the 
deviation takes place in such a way that the crystals retain the prrallelism of at least 
one face or one edge, as I have shown for Proustite and Bouinoiiite." 
* ‘ Mineralogical Magazine,’ vol. 6, 1884, p. 77; vol. 8, 1888, p. 75. 
