DEW AND HOARFROST. 51 



minute drops freeze together so as to form feathery crystals or 

 snowflakes. 



This condensation of mist or liquid water in small particles from 

 moist air on cooling is due to the fact that air (like all other gases) 

 is capable of taking up in an invisible form a certain quantity, but 

 no more, of water- vapour ; the higher the temperature the more 

 water can be thus dissolved, as it were, in the air. Suppose then 

 that air containing as much water-vapour as it can take up at the 

 particular temperature existing at the moment be somewhat cooled, 

 the surplus amount of vapour present over and above the quantity 

 sufficient to " saturate " the air at the lower temperature, becomes 

 condensed to liquid water, making its appearance either as mist 

 suspended in the air or as dew on the surface of the vessel con- 

 taining the air ; and this is the explanation of the formation of 

 dew in nature. On a clear night the surface of the earth cools 

 down spontaneously by the process called radiation (Chapter 

 XXII. ), and consequently any moist air coming in contact with 

 objects thus cooled becomes also chilled and deposits its surplus 

 moisture as dew on the cold object. Accordingly those things 

 that radiate heat best become most chilled during the night and 

 most copiously bathed with dew by condensation of moisture from 

 the air ; whilst objects prevented from cooling so much by radia- 

 tion by a cover of any kind are correspondingly less bedewed. If 

 the chilling action is sufficiently great to freeze the moisture thus 

 deposited, hoarfrost results instead of liquid dew. 



Expt. 43. To cause Artificial Dew. If *we examine our 

 breath on a warm summer's day nothing is visible ; but in colder 

 weather it is obviously misty, for the reason that, as the moist air 

 exhaled from the lungs becomes mixed with the cold external air 

 and cooled down, the temperature is not sufficient to enable the 

 total air to retain permanently vaporous all the /moisture present, 

 so that the surplus condenses as liquid water mist-particles. If 

 we breathe on a cool window-pane the moisture condenses thereon 

 as visible dew ; similarly on putting a cool chimney over a burning 

 lamp or gas jet a deposition of dew becomes visible for a short 

 time until the chimney becomes hot ; in this case the moisture 

 comes in virtue of the chemical changes produced during the 

 burning of the lamp or gas (Chapter XIV.). 



Fill a flask or glass decanter with ice-cold water or with frag- 

 ments of ice, and then bring it into an ordinary sitting-room, 

 preferably one where many people are congregated ; immediately 

 the glass will be dimmed with dew, which after a few minutes will 

 roll down as good sized drops. 



Expt. 44. To cause Artifici 1 Hoarfrost. Kepeat the preced 



