Sronry— On the Cause of Iridescence in Clouds. 641 
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. 8. Fine spicule, or six-sided prisms. 4. Hexagonal 
pyramids. 5. Spicule, 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 tem- 
peratures, 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. 
«(b) Regular hexagon. This occurs in moderate, as well as in the lowest temperature ; but it 
becomes more delicate and thin, and diminishes in size as the cold increases. Some speci- 
mens 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 immense variety. It occurs 
chiefly at low temperatures. 
«(d) 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 :— 
“5, Spicule 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 figure occurred along with the last-described 
genus [Hexagonal pyramids]. Of which kinds, principally, a quantity of snow, three or four inches in 
depth, once fell on the deck of the ship in which I sailed, in the course of a few hours. The tempera- 
ture when this kind of crystal fell was in one instance 22°, and in the other 20°” [ Fahrenheit ]. 
In four engraved plates Captain Scoresby delineates ninety-six different forms, 
magnified from three to seven times. And he attaches a letter to most of them, 
referring to an annexed Table, in which the state of the atmosphere and weather, 
when each form was observed, stand recorded. In what he further adds to his 
record, he says :— 
‘Many instances, it may be observed, occur of mutilated and irregular specimens, some wanting 
two or three radii, and others haying radii of different shapes. But in low temperatures the greatest 
proportion of crystals that fall are probably perfect geometrical figures.” 
The foregoing are the parts of Captain Scoresby’s record of his invaluable 
observations upon ice crystals, which more particularly throw light on the cause 
TRANS. ROY. DUB. SOC., N.S. VOL. III. 48 * 
