ON THE CRYSTALLINE STRUCTURE OF LEAD. 
233 
can be contrasted with the curved irregidar outline surrounding them. Similar cases 
can also be seen in fig. 10, though surrounded by new orientatioiis and not, as in 
fig. 11 , by the original one. In other cases there would be two or more sets of parallel 
straight-line boundaries, the two orientations occurring alternately ; and in others a 
patch showing one orientation would be surrounded on more than one side by straight 
lines, these being always parallel to an edge of the pits on either side. Figs. 12 
and 13 illustrate these cases; all are taken at a magnification of 45 diameters. 
In fig. 12 we see two 23ortions at the bottom, one bright and one dark, joining in a 
straight line, but surrounded by an irregular boundary ; while in the centre we have 
a crystal with a band with jjarallel sides running across it, the jDarts on either side of 
the band being in similar orientation. In fig. 13 (Plate 4) we have a joatch including 
various jDortions of a different orientation, though similar to one another. Some of the 
boundaries in this case appear irregular, but a rather Ifigher magnification shows that 
these aj^parently irregular boundaries are made uj) of numerous short straight lines. 
This photograj^h is also interesting as the etching had not been carried so far as in. 
most of the other cases and the etched j)its are not contiguous. It hence gives some 
idea as to the manner in whicli they are formed. 
Such straight boundaries are a strong indication that the jDortioiis on either side 
are connected by a twin relation, and the fact is further Ijrought out when we 
examine the geometrical relation of the jDits on either side. It has been said above 
that these j^its, which are 2 :»ortions of a cubo-octahedron, are jfiaced in a correct 
direction to the crystalline axes, and are in fact representations in miniature of what 
the external form of the crystal would jDrobably l)e if allowed to assume its pro23er 
shajDe. Take the case shown in fig. 14 : we have two main orientations in the iDicture, 
one having pits with a large hexagonal face nearly jiarallel to, and the other having 
pits with a cubic face j^arallel to, the surface. In the latter case, the cubic faces 
of the pits have, for the most jiart, been etched away, and the jiits are jiractically 
octahedra. If we draw the comjjlete form of tlie latter, we shall obtain the figure 
shown in fig. 6a, the dotted line being at right angles to the boundary l)etween tbe 
two jjarts in fig. 14. Now, if we give this figure a turn of 180° about the octahedral 
axis marked xx, that is to say, bring it into a 2 )Osition in twin orientation to its 
original one, it will apj^ear as shown in fig. 6d. Then, on comi^aring the etched jiits 
on either side of the boundary in fig. 14 with 6a and 6(7, the twin relation which 
exists between tbem is seen. It was found in all cases that the pits on either side 
of a straight boundary agreed with figures drawn in such a way. The nietbod is 
of course merely a rough one, and no accurate residts could be obtained from it, l^iit 
it should be quite possible with a suitable goniometer to actually measure the 
crystallograjihic angles of the pits on either side. Such measurement of etched j^its 
in lead have already been made by Professor Miers, but the work is rather beyond 
the scope of the present jiaj^er. 
The tension side of a sj^ecimen such as that described alcove was of course sub- 
VOL. cc, — A. 2 H 
