AQUEOUS ATMOSPHERIC PHENOMENA. 



483 



Hoar-frost the ice of dew and mist is formed by the temperature of the atmosphere 

 falling below 32, the freezing point ; or by the powerful radiation of heat from the sub- 

 stances receiving moisture depressing their temperature to the point of refrigeration, in 

 which case the particles freeze, even while a thermometer may be several degrees above 

 32. Nature never appears more beautiful than on the mornings of a strong hoar-frost, 

 when the fogs have vanished, and the bright cerulean of the sky is expanded over the 

 productions of the earth, which, from a state of nakedness, have put on vestal garments of 

 the most exquisite purity and delicate finish, the suddenness of the change giving it the 

 aspect of a work of enchantment. Every part of the vegetable kingdom, from the humble 

 blade of grass to the gnarled and majestic oak of the forest, has acquired a character of 

 mysterious loveliness, which surpasses any idea of the scenes of fairy-land, and affects the 

 mind by the silence, rapidity, and extent of the creation, as well as by the consciousness 

 of its speedy departure. In the woods, the dark boles of the trees render more impressive 

 their silvery crests, from which the rime descends in snowy showers as the birds twitter 

 among their branches. The forms assumed by the ice of dew are classed by Mr. Howard 

 into the spicular and granular. The former are very minute icicles which appear upon 

 fibrous surfaces ; the latter are globules frozen as they hang pendant from the substances 

 upon which moisture has been deposited. But in whatever solid form the vapours of the 

 atmosphere are returned to their native terrestrial home, their reduction to a liquid state 

 speedily follows, except towards the poles, and on high mountain elevations. The snow 

 of the fields, the hoar-frost, and the ice of the rivers, dissolve under a change of temper- 

 ature, yet in obedience to a very peculiar law which ensures their gradual retirement. 

 The moment the change arrives at a particular and invariable degree of heat, thaw com- 

 mences ; but any further addition of heat is absorbed or rendered latent, and the temper- 

 ature of the thawing mass remains stationary till the dissolving operation is complete. 

 But for this circumstance, which has been called a violation of law, the operation would 

 be instantaneous, and productive of the most disastrous effects. Under the first touch of 

 the warmth necessary to ensure a thaw, the magical spectacle of hoar-frost would be gone 

 in a moment. The snow would rush from the fields in a resistless inundation to .the 

 rivers, and the frost-bound streams would be relieved of their ice before the skater could 

 reach their banks. 



