[BARNES] THE PHYSICAL CONSTANTS OF ICE 21 



Those formed in moderate weather and light winds or in low clouds are 

 apt to have frail branches and to be of a feathery type. Mixed forms 

 grow partly in low and partly in high clouds. High winds give broken 

 and irregular forms, and much moisture the very granular crystals. 



" Tliese heavy granular-covered crystals are peculiarly a product of 

 the lower or intermediate cloud strata, and especially of moist snow- 

 storms. In intense cold they are rare, while the columnar and solid 

 tabular then become common. 



" About four-fifths of the perfect forms occur within the west and 

 north quadrants of great storms. 



" The most common forms outlined within the nuclear or central 

 portions of the crystals are a simple star of six rays, a solid hexagon and 

 a circle. The subsequent additions assume a bcAvildering variety of 

 shapes, each of which usually differs widely from the one that preceded 

 it and from the primitive nuclear form at its centre. 



" By bearing in mind the fact that crystals evolved within the upper 

 clouds tend towards solidity, and the crystals formed in lower clouds 

 tend toward open branches and feathery forms, it is possible to trace the 

 history and travels of a great many of the crystals. 



" Columnar forms or solid tabular are naturally heavier than the 

 open forms. They are not, therefore, likely to be wafted about in so 

 many directions, and hence to be modified and become so intricate as the 

 light, feathery crystals. 



" Perfect crystals are frequently covered over and lines of beaut}' 

 obliterated by granular coatings. Such heavy granular-covered crystals 

 possess great interest for many reasons. They show when the character 

 of the snow is due to the aggregation of relatively coarse cloud-particles 

 of minute raindrops, and not to the aggregation of the much smaller 

 molecules of water presumably floating freely about between them. They 

 also offer a complete explanation of the formation and growth of the 

 very large raindrops that often fall from thunder-clouds and other rain- 

 storms, if we accept the conclusion that such large drops result from the 

 melting or merging together of one or more of the large granular 

 crystals. 



" While most granular forms possess true crystallic nuclei, there is 

 reason to suppose that they sometimes form directly from the particles 

 of cloud or mist. 



" Minute inclusions of air prevent a complete joining of the water- 

 molecules; the walls of the resultant air-tubes cause the absorption and 

 refraction of a part of the rays of light entering the crystal, hence those 

 portions appear darker by transmitted light than do the other portions. 

 The softer and broader interior sliadings may perhaps also be due, in 



