PHILOSOPHY OF "DEW." 271 



within certain limits, by the laws of diffusion. 

 There is, however, this great distinction, that 

 watery vapour is liable to become condensed 

 and liquefied at common temperatures. Carbonic 

 acid gas* is likewise a liquefiable gas, but it is 

 only under the influence of cold and extreme 

 pressure. Watery vapour, on the contrary, is 

 readily made to assume the liquid condition. 

 When this takes place, that is, when the vapour 

 passes from the vaporous into the liquid state, 

 the phenomenon called " dew " is produced. 

 The philosophical explanation of the formation 

 of dew is simple. There exists for watery va- 

 pour a state of density which it cannot pass with- 

 out losing its gaseous condition, and becoming 

 liquid. This state is conveniently expressed 

 by the term, " the maximum density " of the 

 vapour. The point on the thermometer at which 

 watery vapour attains its maximum density is 

 dependent upon the temperature of the air, 

 increasing as the temperature increases, and 

 sinking as the temperature falls. Consequently, 

 if the temperature of the evening were 50, 

 the point of the maximum density of watery 

 vapour would be lower, and the facility with 



* Whether oxygen and hydrogen may become liquefied in 

 future experiments is uncertain ; at present it seems little 

 probable. Professor Faraday's laborious researches have 

 failed to show the least tendency of tliis kind. 



