Physics and Chemistry. 147 



fringe, in which the several tints are arranged in stripes following 

 the sinuosities of the outline of the cloud. 



I cannot find in any of the books an explanation of this beauti- 

 ful spectacle, all the more pleasing because it generally presents 

 itself in delightful summer weather. It is not mentioned in the 

 part of Moigno's great Repertoire cV Oyrtique which treats of 

 meteorological optics, nor in any other work which I have con- 

 sulted. It seems desirable, therefore, to make an attempt to 

 search out what appears to be its explanation. 



At the elevation in our atmosphere at which these delicate 

 clouds are formed the tempei'ature is too low, even in midsummer, 

 for water to exist in the liquid state ; and, accordingly, the atten- 

 uated vapor from which they were condensed passed at once 

 into a solid form. They consist, in fact, of tiny crystals of ice, 

 not of little drops of water. If the precipitation has been hasty, 

 the crystals will, though all small, be of many sizes jumbled to- 

 gether, and in that case the beautiful optical phenomenon with 

 which we are now dealing wiU not occur. But if the opposite 

 conditions prevail (which they do on rare occasions), if the vapor 

 had been evenly distributed, and if the precipitation took place 

 slowly, then will the crystals in any one neighborhood be little 

 ice-crystals of nearly the same form and size, and from one 

 neighborhood to another they will differ chiefly in number and 

 size, owing to the process having gone on longer or taken place 

 somewhat faster, or through a greater depth, in some neighbor- 

 hoods than others. This will give rise to the patched appearance 

 of the clouds which prevails when this phenomenon presents itself. 

 It also causes the tiny crystals, of which the cloud consists, to 

 grow larger in some places than others. 



Captain Scoresby, in his ' Account of the Arctic Regions,' 

 gives the best description of snow-crystals formed at low tem- 

 peratures with which I am acquainted. From his observations 

 it appears — (a) that when formed at temperatures several degrees 

 below the freezing-point, the crystals, whether simple or com- 

 pound, are nearly all of symmetrical forms ; (b) that thin tabular 

 crystals are extremely numerous, consisting either of simple 

 transverse slices of the fundamental hexagon, or, more frequently, 

 of aggregations of these attached edgewise and lying in one 

 plane ; and (c) that, according as atmospheric conditions vary, 

 one form of crystal or another largely preponderates. A fuller 

 account of these most significant observations is given in the 

 Appendix to this paper. 



Let us then consider the crystals in any one neighborhood in 

 the sky, where the conditions that prevail are such as to produce 

 lamellar crystals of nearly the same thickness. The tabular 

 plates are subsiding through the atmosphere — in fact falling 

 towards the earth. And although their descent is very slow, 

 owing to their minute size, the resistance of the air will act upon 

 them as it does upon a falling feather ; it will cause them, if dis- 

 turbed, to oscillate before they settle into that horizontal position 



