June 24, 1909] 



NA TURE 



493 



sphere obtaining within a dwelling provide more 

 extensive ranges of temperature and humidity, greater 

 diversity in the type of crystals is to be expected; it is 

 not surprising, therefore, to find that three-fourths of 

 the illustrations record forms that appeared on win- 

 dows. This group is, of course, distinguished from 

 the frost and ice groups, not by any essential differ- 

 ence in the characters of the crystals, but merely by 

 their site. Crystallisation which has resulted from 

 sublimation shows greater variety, and by far the 

 larger number of examples are devoted to window- 

 frost ; but the window-ice forms, which occur in com- 



the windows of both warm and cold rooms, but are 

 most common in unheated rooms of which the tem- 

 perature ranges from 32° to 5° F. (0° to —15° C.) and 

 the percentage of humidity from 55 to 70. Fig. 3 

 illustrates a nearly perfect example of the stelliform 

 of window-frost, a slow-growing type that occurs only 

 in cold weather when the temperature indoors is as 

 low as 20° F. ( — 6'6° C). The two crystals which we 

 illustrate are fairly typical of the delicate crystallisation 

 that embellishes the windows in frosty weather. Even 

 in photographs their beauty is evident, but, to quote 

 Mr. Bentley, " Only those who have seen frosted 



Fic. 4.- 



Fl ;. 6.— Ptar-shaped hailsl 



paratively mild weather when a film of dew has first 

 condensed on the surface of the glass, are not without 

 interest. Mr. Bentley has greatly enhanced the value 

 of the series by noting for each example the tempera- 

 ture out of doors and the temperature and degree of 

 humidity within doors. Fig. 2 represents a beautiful 

 example of " branching " window-frost. Its sym- 

 metry has been slightly affected by the disposition 

 of the surrounding crystals, one arm exceeding the 

 others in size ; indeed, the limitations set by the chance 

 position of adjacent crystals rarely permit of the almost 

 perfect symmetry characteristic of snow-crystals. 

 These fern- or tree-like forms are frequently seen on 



NO. 2069, VOL. 80] 



window-panes lit up by a bright winter moon, or seen 

 them flash and sparkle under the rays of a winter 

 sun, have seen the full beauty of the frost." Ice, 

 though in appearance a uniform, solid mass, is really 

 composed by the accretion of innumerable discrete 

 crystals. The separate individuals are generally in- 

 distinguishable in the mass, but certain of them may 

 be brought to light by slight heating — such as the 

 warmth due to the sun's rays. 



Mr. Bentley includes in his series reproductions of 

 three admirable photographs — one is shown in Fig. 4 — - 

 of " ice-flowers," Tyndall's appropriate term for them, 

 embedded in solid ice ; these particular photographs 



