DEW. 361 



theorists ought to be, dew forms very much more freely on 

 some substances than on others. 



Here was a difficulty which long perplexed physicists. 

 It appeared that dew neither fell from the sky nor arose 

 from the earth. The object itself on which the dew was 

 formed seemed to play an important part in determining the 

 amount of deposition. 



At length it was suggested that Aristotle's long-neglected 

 explanation might, with a slight change, account for the 

 observed phenomena. The formation of dew was now 

 looked upon as a discharge of vapour from the air, this dis- 

 charge not taking place necessarily upwards or downwards, 

 but always from the air next to the object. But it was easy 

 to test this view. It was understood that the coldness of the 

 object, as compared with the air, was a necessary element 

 in the phenomenon. It followed, that if a cold object is 

 suddenly brought into warm air, there ought to be a deposi- 

 tion of moisture upon the object. This was found to be the 

 case. Any one can readily repeat the experiment. If a 

 decanter of ice-cold water is brought into a warm room, in 

 which the air is not dry a crowded room, for example 

 the deposition of moisture is immediately detected by the 

 clouding of the glass. But there is, in fact, a much simpler 

 experiment When we breathe, the moisture in the breath 

 generally continues in the form of vapour. But if we breathe 

 upon a window-pane, the vapour is immediately condensed, 

 because the glass is considerably colder than the exhaled air. 



But although this is the correct view, and though physi- 

 cists had made a noteworthy advance in getting rid of 

 erroneous notions, yet a theory of dew still remained to be 

 formed ; for it was not yet shown how the cold, which causes 

 the deposition of dew, is itself occasioned. The remarkable 

 effects of a clear sky and serene weather in encouraging the 

 formatipn of dew, were also still unaccounted for. On the 

 explanation of these and similar points, the chief interest of 

 the subject depends. Science owes the elucidation of these 

 difficulties to Dr. Wells, a London physician, who studied 



