BIRTH AND AFFINITIES OF ‘CRYSTALS 131 
calcite is the only one which possesses this power of bringing 
down sodium nitrate in parallel position upon itself. Yet calcite 
is by no means the one which most nearly resembles sodium 
nitrate in form ; in fact, it differs from it in angle more than any 
of the other carbonates. It is clear then that some other 
criterion than equality of angle must be applied. On comparing 
the molecular volumes, however, it is found that calcite is the 
only one mineral in this group which possesses a molecular 
volume nearly equal to that of sodium nitrate. A reason why this 
equality of molecular volume should be essential for a parallel 
growth to take place is made evident from the following 
considerations. 
In order that two structures of particles may fit together it 
Fig. 4. 
is necessary not only that the arrangement of the particles should 
be the same in both, but also that the spacing of the particles 
should be identical. Consider, for example, a cubic structure 
consisting of particles arranged as in fig. 3. The particles 
are situated at the corners of a series of equal cubic cells. Let 
fig. 4 be a similar structure, but smaller in dimensions. Both 3 
and 4 are cubic structures, and will yield crystals having the 
same angles; but they will not fit together, because the cubic 
cells of 3 are larger than those of 4. If the structures are to 
be identical they must be made up of cells whose volume is 
the same. . 
Now the specific gravities, or relative weights of equal 
volumes, of the two crystals will depend both upon the intrinsic 
