OUTLINES OF NATURAL PHILOSOPHY. 



When the aqueous particles first form a drop, and 

 are afterwards frozen in their descent, they be- 

 come Hail, which is sometimes found crystallised 

 with some degree of regularity. The whiteness 

 and opacity of the hail, is probably owing to the 

 congelation being performed where the air is very 

 rare. Professor LESLIE has remarked, in the cu- 

 rious experiments he has made on the production 

 of ice by evaporation, in a receiver where the air 

 was considerably rarefied, that the ice is more 

 porous, and less transparent, than that which is 

 formed under the ordinary pressure of the atmo- 

 sphere. 



436. Dew is a precipitation of humidity from 

 the lower strata of the atmosphere, which does not 

 disturb the transparency of the air, and to which 

 the mixture of different streams of air does not 

 seem necessary. 



When air containing humidity cools below a certain 

 point, it must begin to deposite its humidity. It 

 is in this way that dew is formed in warm wea- 

 ther, when, on the sun's going down, the heat of 

 the air at the surface is greatly diminished. The 

 temperature at which the deposition of dew takes 

 place, is therefore an indication of the quantity of 

 humidity contained in the air. BRUCE has remark- 

 ed, that during the dry season no dew falls in Egypt, 

 but that it is usually observed in the Delta five or 

 six days before the inundation. 



437. Besides 



