Fig. 22. Fig. 23. Fig. 24.
somewhat different, but scarcely less beautiful, of which [Fig. 27] (page 142) may be considered a type. This is analogous to the fanciful forms of frost seen on the interior of a pane of glass, and is frequently to be found where the water is very shallow, and where its mixture with some gritty substance, or blade of grass, or other obstruction, has in all probability interfered with a more geometric arrangement. By degrees the whole surface of the water becomes interlaced with needles and pinnæ, whether singly or in groups, and thin laminated surfaces of ice which cover all interstices. Then, according to external influences, the ice either thickens, obliterating all this beautiful tracery, or it melts away before the rising temperature of the day. It often happens, however, that these processes occur after dark, or that the water freezes so rapidly as to disappoint the wishes of the observer. At moderate temperatures these changes are best observed; but, in our opinion, they are somewhat dependent on other atmospheric conditions. The formation of the needles is common to the freezing of water under all circumstances, and they vary from a few inches to a few feet in length.
Fig. 25.
To return to the crystals of snow. [Fig. 3] ([page 136]) is another elementary figure, common to temperatures about the freezing-point; it is not often less than half an inch in diameter, and is a miniature copy of the water crystal.