92 Dr. GL J. Stoney on the Cause 



tacle can only reveal itself on those rare occasions on which 

 the crystals are tossed about in an unusual degree.] 



Appendix. 



Extracts from Captain Scoresby's record of his Observa- 

 tions on Snow Crystals formed at Low Temperatures, in his 

 ' Account of the Arctic Regions/ vol. i., from pages 425 to 

 433, and plates 8 ; 9, 10, and 11 in vol. ii. : — 



" When the temperature of the air is within a degree or two of the 

 freezing-point, and much snow falls, it frequently consists of large irre- 

 gular Hakes, such as are common in Britain." "But in severe frosts, 

 though the sky appears perfectly clear, lamellar flakes of snow, of the 

 most regular and beautiful forms, are always seen floating in the air and 

 sparkling in the sunbeams, and the snow which falls, in general, is of the 

 most elegant texture and appearance." " The various modifications of 

 crystals may be classed under five general kinds or genera : — 1. Lamellar. 

 2. A lamellar, or spherical nucleus, with spinous ramifications in different 

 planes. 3. Fine spiculae, or six-sided prisms. 4. Hexagonal pyramids. 

 5. Spiculae, having one or both extremities affixed to the centre of a 

 lamellar crystal." 



We are more particularly concerned with the first and the 

 last of these genera. About the first, Captain Scoresby says : — 



"1. Lamellar Crystals. — The varieties of this kind are almost infinite. 

 They occur at all temperatures, and in the greatest abundance, and most 

 of the specimens are extremely thin, transparent, and of an exquisitely 

 delicate structure. They may be subdivided into several distinct species: — 

 (a) Stelliform ; having six points radiating from a centre, with parallel 

 collateral ramifications in the same plane. This species is the 

 most general form met with." " It occurs in greatest profusion 

 when the temperature approaches the freezing-point. 

 (6) Regular hexagon. This occurs in moderate, as well as in the lowest 

 temperature ; but it becomes more delicate and thin, and dimi- 

 nishes in size as the cold increases. Some specimens consist of 

 simple transparent plates, others are beautifully variegated within 

 the perimeter by white lines, forming smaller hexagons or other 

 regular figures in immense variety." 



(c) " Aggregations of hexagons. This beautiful species admits of im- 



mense variety. It occurs chiefly at low temperatures." 



(d) li Combinations of hexagons, with radii or spines and projecting 



angles. This constitutes the most extensive species in the 

 arrangement." 



About the last, or fifth, genus, Captain Scoresby says : — 



u 5. Spicules or prisms having one or both extremities inserted in the centre 

 of a Lamellar Crystal. — This is the most singular genus I have ever seen, 

 and has been observed but twice. It resembles a pair of wheels, united 

 by an axle-tree ; the wheels consisting of hexagonal or other lamellar 

 crystals, and the axle of a slender crystal." " Some of this extraordinary 



