260 Professor Kaemtz on the more important 



ways approaches more and more a state of saturation, and 

 at a sufficiently low temperature a portion of the vapour 

 is converted into water, and precipitated. It is thus that 

 there is produced a moist coating on a glass of cold water when 

 it is brought into a warm moist room : thence may be explained 

 the moisture on windows during winter, inasmuch as the vapour 

 is precipitated on the cold glass ; and from the same cause a 

 mist is formed over a vessel of warm water. What we thus 

 perceive on a small scale, Nature is constantly performing on a 

 great. When, for example, the sky is clear and no wind blows, 

 the ground is cooled rapidly during the night by the radiation 

 of the heat, and the stratum of air next the ground is some de- 

 grees colder than the air a few feet above. At last the ground 

 is so much reduced in temperature, that the strata of air lying 

 next it are saturated with vapour, and, by a continuance of the 

 cooling, vapour is precipitated on grass and other objects, in the 

 form of drops, or in winter in a crystalline condition. The 

 dew or hoar-frost is so much the more considerable the greater 

 the cooling, and hence the older natural philosophers ascribed 

 to dew a cooling power, until at length Wells proved that the 

 cold is not the effect but the cause of the dew, just as in winter 

 the windows must be cold before they begin to shew their co- 

 vering of moisture. Exactly the same phenomenon, which we 

 perceive, when warm water evaporates in cold air, is presented 

 to us by Nature in the colder periods of the year, when, for ex- 

 ample, in autumn, the heat of the air diminishes very rapidly. 

 From rivers and from smooth sheets of water, which still possess 

 a high temperature caused by the summer, a quantity of vapours 

 arise ; the air which is more especially cold in the morning is 

 saturated in a short time, and the vapours ascending further, 

 become condensed, and float as water in the form of hollow vesi- 

 cles (Bldschen) in the air, giving rise to a fog, from whose posi- 

 tion we can often at a distance trace all the windings of a 

 river. If this fog becomes denser, several such vesicles unite to- 

 gether in drops and fall to the ground as fog-rain. 



In general, we must suppose, that all clouds arise from the 

 circumstance, of the air in which they float containing more 

 vapour than is enough for saturation ; so that we must regard 

 the clouds as fogs which are continued upwards, and from which 



