130 SCIENCE PROGRESS 
their own facets. Twinning can generally be explained as a 
union of the two crystals along a plane face which is the same 
in the two crystals, but is reversed in one of them, so that the 
two crystals grow, as it were, back to back. (Sometimes it is 
an edge which is reversed.) It is highly probable that twinning 
is due to the fact that the structures which build up the two 
crystals can fit together in two positions, either parallel, or the 
one reversed with regard to the other. 
In the rapid and turbulent growth in a labile solution it 
may well happen that this reversal is more apt to take place 
than in the more placid growth in a metastable solution. Hence 
the beautiful stellate groups and tree-like forms which are more 
characteristic of rapid than of slow growth. 
Now just as two crystals of any substance may grow on each 
other whether in parallel position or in twin (reversed) position, 
because they have the same structure and fit together in these 
positions, so it may happen that two crystals of different sub- 
stances may grow together in parallel position for the same 
reason. This is well known in such groups as that of the 
alums, where one alum, if introduced into the solution of another 
alum, may continue to grow as a single crystal, and become 
enveloped by the latter—as is clearly visible when the two 
are of different colours. In such a case the two substances 
are absolutely similar in chemical composition, only differing 
by the substitution of metals which exhibit the greatest 
resemblances in chemical behaviour; and this is no doubt the 
reason why they possess identical structures. Another still 
more remarkable case is that of calcite and sodium nitrate. 
A drop of strong solution of sodium nitrate placed on a clean 
cleavage surface of calcite at once deposits small rhombohedra 
which are parallel to one another, and to the rhombohedron 
on which they are growing. The angles of sodium nitrate 
are very near to those of calcite, but not identical with them. 
Further, there is clearly a certain similarity of composition in 
CaCO, and NaNO.,. 
In the course of some systematic experiments upon the 
parallel growths of different substances, carried out in the 
Mineralogical Laboratory at Oxford, Mr. T. V. Barker investi- 
gated the question whether sodium nitrate also grows in parallel 
position upon the other minerals of the calcite group, the carbon- 
ates of iron, zinc, magnesium, and manganese. He found that 
