ME. T. HOPKINS ON THE FORMATION OP DEW- 



47 



vapour" of the atmosphere, advanced by the French savant, 

 is an old one — the diflfierent opinions which then existed 

 having reference merely to the manner in which the liquid 

 dew was formed from the vapour. 



In this as in many other cases disputes appear to have 

 arisen from the same word having been used to express 

 diflFerent ideas. The word dew is sometimes used to ex- 

 press the drops of water on the leaves of grass, as, in speak- 

 ing of the "dew on the grass," meaning the drops of water 

 that under certain circumstances are found on grass. At 

 other times it is meant to convey the idea we have of the small 

 globules of water that float in the air near to the ground, and 

 then the word is synonymous with "low mist;" whilst it 

 is occasionally spoken of as the aeriform material from 

 which both the drops and globules are formed, and is then 

 used to designate the aqueous vapour itself. 



Dew and mist are formed from the aqueous vapour that 

 exists in the atmosphere, by a degree of cold that is suffi- 

 cient to produce condensation of a part of the vapour. The 

 two names designate, not different substances, but the same 

 substance produced in different ways. Dew has, therefore, 

 to be distinguished from mist only by the mode and place 

 of its formation, and the shape in which it exists. 



Heat is found to leave all substances by radiation. In 

 the middle of the day, under ordinary circumstances, the ra- 

 diant heat received from the sun more than counterbalances 

 the loss of heat radiated from the earth j but as the radiated 

 solar heat diminishes on the approach of night, terrestrial 

 radiation continues, and reduces the earth's temperature. 

 And when the temperature of the earth and the air that is 

 near to it are thus reduced below the point of condensation, 

 or the dew-point, a part of the aqueous vapour of the atmo- 

 sphere close to the siu-face of the earth is condensed, and 

 forms particles of water so minute as to be sustained by the 

 atmosphere, and may therefore be called mist or ^'fioating 



