Page:Popular Science Monthly Volume 53.djvu/93

Rh that the depth and solidity seen in some crystals when the photographs are mounted as stereoscopic views can not be in some adequate manner reproduced in engravings, for this adds not a little to an understanding of the manner in which the crystal has been formed. Yet something of this can be seen in the figures as here given. A careful study of this internal structure not only reveals new and far greater elegance of form than the simple outlines exhibit, but by means of these wonderfully delicate and exquisite figures much may be learned of the history of each crystal, and the changes through which it has passed in its journey through cloud-land. Was ever life history written in more dainty hieroglyphics! It is well known that crystals which form in a low temperature are smaller and more compact than those formed in a warmer atmosphere. As the higher cloud strata are colder than those nearer the earth, the snow crystals which originate there are smaller and less branched than those from lower clouds. Such are shown in Figs. 2, 20, and 22, while crystals from the warmer clouds are more often like Figs. 24 and 25. The small, compact crystals of the upper clouds do not always remain of their original form or size, for, as they fall through layer after layer of clouds, each layer subjecting them to its own special conditions, they may be greatly modified, and by the time they reach the earth they may closely resemble the crystals from lower clouds, though they can usually be distinguished from them by an examination of the internal structure, as well as by, in some cases, their general form. All crystals falling from high cloud strata, the cirrus or cirro-stratus, are not changed; especially is this true in a great storm, or when the temperature of the lower clouds is low, and in any case some are much more completely transformed than others. One crystal may pass through cloud layers not very unlike that from which it came, and of course will not be greatly changed. Another may encounter here a quiet cloud layer and there a tumultuous layer, here a lower, there a higher temperature, here a dense and there a thin cloud mass, and by all of these conditions may be affected.

Examples of crystals which have been little changed are shown in Figs. 3, 7, and 8, while Figs. 12, 13, 14, and 16 show more completely modified crystals. Total transformation, such as the change of one type into another, does not often occur. The nucleus retains its original form, to which various additions are made during the downward passage. Composite crystals may, however, be formed during the passage through diverse cloud layers, though they are not common, as shown in Figs. 11 and 19. Usually, however, the tabular, compact, small crystals of the high clouds continue their development at lower levels upon the original plan, though