ON AQUEOUS AND IGNEOUS METEOKS, 711 



Sphere; and in such cases the deposition may perhaps be facilitated by the 

 cooling of the warmer air to a certain degree, even before a perfect mixture 

 has taken place 



When visible vapour has been thus deposited from transparent ^air, by means 

 either of cold or of mixture, it generally remains for some time suspended, in 

 the form of a mist or of a cloud : sometimes, however, it appears to be at once 

 deposited on the surface of a solid, in the form of dew or of hoar frost; for 

 it is not probable that the crystallized. form, in which hoar frost is arranged, 

 can be derived from the union of the particles already existing in the air g 

 distinct aggregates. 



The dew, which is commonly deposited on vegetables, is paitly derived, TiL.^/f^nr;.^i^ -^ ■ i}' 

 the evening, from the vapours ascending from the heated earth, since it is then 

 found on the internal surface of a bell glass; and towards the morning, from 

 the moisture descending from the air above, as it begins to cool. Sometimes, 

 however, in warmer weather, the dew begins to descend in the evening; this 

 the French call serein: the humidity deposited by mists on trees, and by 

 moist air on windows, generally Avithin, but sometimes without, they call givre. 



Mists are said to consist sometimes of other particles than pure water: 

 these are called dry mists, and they have been swpposed to blight vegetables. 

 Such mists are sometimes attended by a smell, resembling that which is occa- 

 sioned by an electric spark. Rain falling after a dry season deposits, when it 

 has been suffered to stand, some particles of foreign matter which it has 

 brought down from 4:he atmosphere. There must indeed frequently be a 

 multiplicity of substances of various kinds floating in the air; the wind has 

 been found to carry the farina of plants as far as 30 or 40 miles, and the 

 ashes of a volcano more than 200. It only requires that the magnitude of the 

 particles of any substance be sufficiently reduced in size, in order to render 

 them incapable of falling with any given velocity; and when this velocity 

 is very small, it may easily be overpowered by any accidental motions of the 

 air. The diameter of a sphere of water, falling at the rate of one inch only in 

 a second, ought to be one six hundred thousandth of an inch, which is about 

 the thickness of the upper part of a soap bubble at the instant when it bursts ; 

 but the particles of mists are incomparably larger than this, since they would 

 otherwise be perfectly iii visible as separate drops: the least particle, that could 



