MISTS. 251 



taming, within their thin bulbs, air fully charged 

 with moisture. Such is the so-called visible steam 

 or vapour, which rises from hot water, or which is 

 formed in the air we breathe out from our lungs, 

 when the weather is cold and moist. When the 

 weather is warm and the air dry, the breath 

 forms no clouds before the mouth, because the 

 mixture of the moist outbreathed air with that 

 of the dry atmosphere, is not cooled down below 

 its dew-point. And if am visible mist should be 

 formed under such conditions, it immediately 

 vanishes again ; that is, it returns to the proper 

 state of true vapour as soon as it is mixed with 

 the quantity of air that has enough capacity for 

 vapour to take it back to the state of gas. 



The vapours that float over rivers in cool even- 

 ings are due to the same cause. It has always 

 been found, that when these occur the temperature 

 of the water is higher than that of the air that lies 

 over it. By the same cause we account for the 

 morning mists which float over moist ground, over 

 meadows and wooded mountain slopes. When 

 seen from a distance, such masses of cloud seem to 

 be at rest. However, the process that is actually 

 going on is exactly the same as that which attends 

 the evaporation of water from any vessel. The 

 vapours as they rise are first pa] tly condensed into 

 mist, and are then dissolved again on mixture with 

 a farther quantity of air. In dry warm air the 



