138 PROCEEDINGS OF SOCIETIES. 
salts combine in two proportions, and that it is by the loss of 
some of the atoms of their water of crystallization that these 
varied effects are produced. Water it is which gives the crystals 
their naturally free and beautifully leafy structure; and I find 
that by allowing them to form at higher temperatures in a drier 
atmosphere, where crystallization can take place with fewer atoms 
of water, they assume a more rigid appearance. At about 80° 
the cross becomes more angular or pointed (see diagram 2), while 
the crystals become smaller and more numerous. 
At 85°, 90°, and 100°, the waves become smaller, and resemble 
fine wires, flowing into each other, and forming apparently a 
system of delicate rings round the centre (see diagrams 3 and 4). 
At these temperatures the foliated structure is nearly lost in a, 
and B has almost entirely vanished from the slide. It is some- 
times possible to allow B to form a level granular structure on 
the slide, and then, by suddenly reducing the temperature, to 
make a form upon it, by which a most beautiful effect is produced 
(see diagram 5). By raising the temperature to about 117°, the 
highest point at which these crystals will form, the system of 
rings becomes obliterated, and the crystal arranges itself in planes 
radiating from the centre, with a black cross similar to that in a 
crystal with one axis (see diagram 6). The higher the tempera- 
ture, the longer is the time required for crystallization. Thus, at 
117° about three hours are necessary for the perfecting of the 
