ON AQUEOUS AND IGNEOUS METEORS. 555 



rated with moisture, they must contain 8 grains of water when separate ; 

 but when mixed they will he too cold by 2 to contain the same quantity ; 

 since air at 48 can only contain four grains for each foot ; and it has been 

 supposed that such mixtures frequently occasion a precipitation in nature. 

 Thus, it often happens that the breath of an animal, which is in itself 

 transparent, becomes visible when mixed with a cold atmosphere ; 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 sus- 

 pended, in the form of a mist or of a cloud : sometimes, however, it appears 

 to be at once deposited 011 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 as distinct aggregates.t 



The dew, which is commonly deposited on vegetables, is partly derived, 

 in 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 morn- 

 ing, 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 within, but sometimes with- 

 out, they call givre. [The cause of the deposition of dew has been satis- 

 factorily assigned by Dr. Wells. ^ It is traceable to two circumstances, the 

 radiation of heat, and the condensation of vapour by cold. Owing to the 

 former circumstance, different substances on the earth's surface become 

 cool with different degrees of rapidity, according to their mechanical tex- 

 ture, or their position, or whatever it may be. When they have cooled 

 down to such a point that the existing vapour in the atmosphere near them 

 can no longer be retained in its elastic state, it becomes water, and is depo- 

 sited on their surface. The cause of deposition is the previous cooling 

 of the substance on which it takes place. Dr. Wells found, as Mr. Six 

 had done before him, that a thermometer laid on a grass plot in a clear 

 night, indicates a cold many degrees lower than a thermometer hung at 

 some height from the ground. This is owing to the fact that grass radiates 

 heat well; and accordingly it receives a copious deposition of dew, which 

 a worse radiator would not do. Moreover, if the sky becomes overcast, or 

 if any substance be interposed between it and the grass, radiation is checked, 

 or it may be that the grass receives more heat from the surrounding objects 

 or clouds than it radiates, and thus its temperature becomes raised. Under 

 these circumstances the deposition of dew ceases.] 



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

 these are called dry mists, and they have been supposed to blight vege- 



* Hatton, Dissertation on various Subjects of Natural Philosophy, 4to, Edin. 

 1792. 



t See Howard's Essay on the Modification of Clouds, 1832. 

 : Wolls on Dew, 1814. 



