ON THE CliYSTALLINE STRUCTURE OF LEAD. 
229 
cubic faces is parallel to the plane ol the paper, and in (cZ) an octahedral face is 
nearly parallel to the same plane. It joossesses faces which are alternately squares 
and hexagons, and it will be seen that in the following photographs the faces of the 
pits always consist of all or part of one of these figures. Fig. 6 (h), (e), are drawings 
of the pits which (a) and (cZ) respectively would make if made to penetrate a certain 
distance into the j^lane of the paper, and may be compared to the etched pits seen in 
the photographs. In some cases the octahedral faces were very strongly developed. 
and the cubical faces were practically non-existent. The figure then developed into 
the regular octahedron. Fig. 6 (c) shows the type of pit we should expect in such a 
case when a cubic face is parallel to the surface, and may be compared with the pits 
seen in figs. 9 and 14. The relation of these pits to the directions of tlie skeletal 
crystal formation was found by scratching lines to show these directions upon the 
surface before etching, taking care to make the scratches deep enough not to be 
eaten away. It was then seen that where a S(|iiare face of the pits was parallel to 
the surface of the specimen, the sides of the square made angles of 45° with the 
direction of the ridges. That is to say, the sides of the cube from which the cubo- 
octahedron was formed were parallel to these directions. Hence the pits are correctly 
placed as regards the crystalline axes, if we assume that the ridges are parallel to the 
cubic axes of the crystal. From the manner in which these ridges are formed by 
the crystal growing by successive branches at right angles to one another, as is 
illustrated in fig. .3, such an assumption seems not improbalfie. (See Plates 3 and 4.) 
