408 Prof. J. P. Cooke on the Variation of Constitution in a 
stals formed were found to have the theoretical composition of 
Sb Zn?, indicating of course a tendency towards this point ; but 
on increasing or diminishing the amount of zinc in the alloy 
beyond these limits, the composition of the crystals immediately 
began to vary in the same direction as that of the alloy. The 
erystals of Sb Zn? containing an excess of zinc are smaller and 
more frequently isolated than those having the exact theoretical 
composition. A similar fact, it will be remembered, is true of 
the crystals of Sb Zn’. 
At the alloy of 33 per cent. of zinc, the definite crystals of 
Sb Zn? begin to disappear, and are succeeded by thin metallic 
scales, which are obviously imperfect crystals of the same form. 
This was established, not only by the obvious law of continuity 
noticed in the different specimens (the perfect crystals gradually 
passing into the scales), but also by the peculiar mode of twin- 
ing, which was the same with the scales as with the large ery- 
stals, forming the peculiar cellular structure already referred to. 
Moreover, the angle between two scales thus united was found 
to be equal to the basal angle of the perfect crystals, at least as 
nearly as could be measured. These scales continue up to the 
alloy of 41°8 per cent. of zinc, becoming, however, less abundant 
and less distinct. Several specimens of them were analysed ; but 
no regularity could be detected in their composition, except that 
they all contained a much larger amount of zinc than the alloys 
in which they were formed. 
Crystals of Sb Zn? containing an excess of antimony were readily 
obtained from alloys containing less than 27 per cent. of zine. 
They became more and more imperfect as the excess of antimony 
increased, and finally faded out altogether in the alloys below 
20 per cent. of zinc. It is evident, therefore, that definite and 
perfect crystals of Sb Zn® can be obtained with a large excess 
either of zinc or antimony above the theoretical composition. It 
is also evident that, of the two compounds, Sb Zn? is the most 
stable,—first, because it is formed to the exclusion of Sb Zn° in 
all alloys containing less zinc than the amount corresponding to 
the typical composition of the last compound; and secondly, 
because the erystals retain the typical composition under quite a 
wide variation (viz. between 31°5 and 27 per cent.) in the com- 
position of the alloy. 
The facts above stated are fully illustrated by the following 
Table, which gives the results of a large number of analyses of 
erystals of both compounds formed in alloys containing different 
proportions of the two metals :— 
