BRIGHT CLOUDS ON A DARK NIGHT SKY. 19 



the air for any length of time and dropping no water ; whereas the latter is the 

 very preparation for rain-fall. This latter too is the condition of least reflection 

 and most absorption of light, while the former is that of least absorption and 

 most reflection. 



These propositions may be practically established most easily by earth- sur- 

 face examples, where the nature of the molecules may be most easily examined. 

 Falling rain drops, even when directly shone on by the Sun and forming a rain- 

 bow, are anything but bright, unless indeed they are mixed with hail. And if 

 we take water in the largest shape, what is there so dark as the deep blue sea, 

 whence comes to our eyes only a faint, "first surface " reflection, from a sphere 

 of stupendous size ; and yet what is so brightly white as the foam of the same 

 material when arranged in numerous, small coalescing convex vesicles, eveiy 

 one of them reflecting from both the first, and more especially the second, 

 surface of its film, whatever point or gleam of light there may be in the whole 

 hemisphere illuminating them. 



Or again how dark brown is the water of a peaty hill in the Highlands when 

 in a placid state, reflecting by means of only its first surface ; and yet how 

 brightly white is the course of a stream of it viewed from a distance on the 

 mountain side and even under a cloudy sky, when it has to chafe and 

 tumble down rocky channels, wherein it covers itself with foam, or in- 

 numerable little hemispherical bubbles on bubbles, each of which gives us, 

 both the weak first, and the strong second, reflection of a film of water, or glass 

 plate in air. 



That is, such a frothy surface is bright when we look upon it from above, 

 or the side whence, by day, comes its chief, or only illuminating light ; as snow 

 also then appears most intensely white. But when we look at either snow, 

 or froth of water between us and the chief light, they appear dark rather 

 than bright ; for they reflect far too well, to allow much to be transmitted 

 through them. 



But still more particularly what can be brighter than those fields of apparent 

 snow, the upper surfaces of the clouds composing the great cloud stratum of 

 the low N. East, or commencing Trade, wind, as seen day after day in the 

 summer season from the summit of the Peak of Teneriffe, at a depth of several 

 thousand feet below : and they are known to be in the watery, as contra-dis- 

 tinguished to the frozen, condition. And yet what is darker and more threaten- 

 ing than the appearance of the same clouds, when seen from below, or between 

 the observer and the Sun-illumined hemisphere of sky. 



There too, at or near the sea level, in a moist air, rain does sometimes fall 

 from those clouds' lower dark surfaces, where their component molecules may 

 be increased in size with acquired outside moisture until they become practi- 

 cally drops of water. But above, on their upper surfaces, where to an observer 



VOL. XXXII. PART I. D 



