640 Sroney—On the Cause of Iridescence in Clouds. 
real existence in Nature. We may, in fact, recapitulate our knowledge on the 
subject as follows :—Captain Scoresby’s observations show that the crystals of ice 
formed in the atmosphere do not agglutinate into snow-flakes, except at tempera- 
tures bordering on the freezing-point. At temperatures even a few degrees lower, 
the crystals remain distinct from one another, and at low temperatures are for the 
most part unmutilated and perfect geometrical figures. He has also shown that 
all the forms which crystals of ice can assume do not present themselves together, 
but that some one or two forms generally preponderate over the others, the pre- 
ponderating form varying according to atmospheric conditions, which he does not 
seem to have fully traced out. Thin tabular crystals are frequently the preponde- 
rating form, and become more delicate and thin and diminish in size as the cold 
increases. Hence it is to be presumed that the state of the atmosphere in the region 
of the cirrus clouds will sometimes be such as to produce crystalline plates of a 
tolerably uniform thickness. When the atmosphere has been in this state at the for- 
mation of the cloud, we shall accordingly have either the phenomenon of iridescence, 
or the twin phenomenon of a vertical column of reflected light passing through 
the sun. ‘This latter meteor will present itself but seldom, as it requires unusual 
quietude in the region of the cloud to allow the crystals to settle down sufficiently 
into the horizontal position. And, accordingly, on the rare occasions when the 
vertical column is seen, the cloud has been observed to present a gauze-like 
uniformity of appearance, which is an independent evidence of the calmness which 
is essential. But it much more frequently happens that the cloud is flocculent 
in its structure, and exposed to little breezes blowing in various directions, excited 
by one or more of the causes that have been indicated above. Whenever 
draughts of this kind intervene, the little tabular crystals are every now and then 
tossed about, and will then flutter; since being flat plates, subsiding through a 
resisting fluid, they will oscillate after each such disturbance in their progress 
towards the horizontal position. Whenever these events happen, we have condi- 
tions which must result in that iridescent phenomenon which is the subject of 
our inquiry. 
ALPE EN DiLxXe 
Extracts from Captain Scoresby’s record of his Observations on Snow Crystals 
formed at low temperatures, in his Account of the Arctic Regions, Vol. I., from p. 425 
to p. 433, and Plates vii., x., x., and x1, 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 irregular flakes, such as are common in Britain.’”’ ‘ But in severe 
