Another very interesting example of ice-growth and formation which we occasionally discover upon the window-panes, and which should not be confounded with the frost work etchings, as it is in reality a thin transparent ice film, which frequently assumes exquisite formations and patterns. In [No. 141 a] the coral-like branch is a beautiful example of this window-pane ice, while upon the same pane of glass in precisely the same temperature we find another type, as [141 b] resembling somewhat the transparent wing of an insect. There are but two distinct types of the ice film etchings, and they are classified as the feather form and the arborescent types. It is presumed that the forming of types is largely governed by the varying thickness of the glass upon which the ice film is deposited, and to the presence or absence of minute frozen particles of ice beneath the film. The feather form seems to develop upon cooler positions of the glass, and where the water film is thinnest.
122. One of the most elaborate crystals shown. Notable for its very dark centre and the curious detail of its border
123. A rare design because of its open petal-like formation. Usually such crystals are solid
124. Very frigid altitude crystal having remarkably etched centre
125. A snow crystal covered with granular deposit of frost
Nos. [142], [143], and [144] are all rare examples of window ice. No. 16 shows to perfection a greatly magnified section of the coral-like tracings in detail. This window-pane ice, unlike the frost etchings, is always transparent or opaque water ice.