EVAPORATION. 269 



which it contains. This water, in passing through a very 

 cold stratum of air, will freeze. Again, a very cold current 

 of air may suddenly meet with a body of air much warmer 

 than itself, and freeze it. The hail-stones at first will 

 be small. As they descend, they will freeze the parti- 

 cles of vapor, with which they come in contact, and thus 

 grow larger. Observations made in elevated situations 

 show that they do thus grow larger. In the winter the 

 vapor, as it condenses, is probably frozen before it forms 

 into large drops, and thus becomes snow. Hence, when 

 particles come together, they do not unite into one dense 

 mass, as in rain and hail, but each particle exhibits its 

 own formation and structure, and even when hail is form- 

 ed in winter, the hail-stones are very seldom large. Hail- 

 stones are generally hardest in the centre, and of a looser 

 texture towards the outside. In passing through succes- 

 sive portions of vapor, the intense cold of the hail-stones 

 is gradually diminished, and consequently those particles, 

 which unite with them last, are not frozen so hard, as 

 those which coalesced at an earlier period. It is fre- 

 quently the case, that it snows on mountains and rains 

 in valleys at the same time. Here we have evidence that 

 the condensed vapor, during a part of the period of its 

 descent, had the form of snow, and retained it, till it had 

 descended as low as the tops of the hills, but melted in 

 descending into the valleys. Hence the tops of the White 

 Mountains and many others are robed in white, long be- 

 fore snow begins to fall in places near the level of the 

 sea. Probably many of the rain-storms, which we expe- 

 rience, would be snow-storms to one who should ascend a 

 mile or two in the air. 



As the earth cools more rapidly than the air, when it 

 ceases to receive heat from the sun, the moisture, which is 

 contained in the portion of air in contact with the earth 

 imparts heat to it, and becomes condensed in the form of 

 dew. When the cold is great this condensed moisture 

 freezes, forming what is termed white frost. Dew col- 

 lects most abundantly on those substances which give off 

 heat most rapidly, as the difference between the warmtk. 



VOL. i. NO. xi. 24 



