BIKT oh ANDY APFINITIES OF “CRYSTALS 129 
that the point is continually in contact with the strong solution, 
and therefore that the crystal continues to grow rapidly in 
this direction and slowly in others. That the liquid in con- 
tact with a slowly growing crystal is supersaturated but only 
metastable, I have proved by actually measuring its refractive 
index. Again, when a rapidly growing needle approaches a 
crystal at a part of its surface which is growing slowly, it ceases 
to grow rapidly just before actual contact takes place—z.e. so 
soon as it enters the zone of metastable solution surrounding 
the latter. 
If a thin drop of solution of potassium bichromate be 
watched as it crystallises on a warm slide, the ring of crystals 
which form on the edge of the drop will be seen to advance 
rapidly in a beautiful growth of needles; the process is then 
suddenly arrested, and the ring continues to grow quite slowly 
with a thickening of the crystals. After a short interval the 
sudden growth is repeated, and is then as suddenly checked ; 
and so on until the drop is dry. The explanation is probably 
the following: the labile solution in which the crystals first 
appear is reduced in strength by their rapid growth, and 
falls to the metastable condition, in which the growth is slow— 
evaporation then raises the strength to the labile point; rapid 
growth again sets in, and the process is repeated. 
Other illustrations of the same recurrent process are to be 
found (1) in “ Liesegang’s Rings” of silver chromate produced 
by a solution of silver nitrate creeping through gelatine which 
has been impregnated with potassium bichromate; (2) in the 
stripes of silver chloride deposited in nerve fibres by the opera- 
tion of acid nitrate of silver acting upon the chlorides in the 
fibre, as described by Macallum, and in capillary tubes as 
described by Boehm. 
Another feature of crystals growing rapidly in a labile 
solution is that they are not only apt to grow in long prisms 
and needles, but to group themselves into the branching and 
tree-like forms which lend such a beautiful appearance to many 
of these growths. For example, the frost pattern on a window 
pane, and the rapid growths of potassium bichromate just 
described. 
This branching of needles is generally due to “twinning,” the 
process by which two crystals grow out of each other in different 
directions at a fixed angle which depends upon the angles of 
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