DE\V. 



until the mixture becomes perfectly uniform, every part of it 

 containing the two gases in the proportion of their entire 

 quantities. 



The same law prevails in the case of vapours mixed with gases ; 

 and thus may be explained the fact, that although the aqueous 

 vapour suspended in the air, and having the same temperature, is 

 always lighter bulk for bulk than the air, it does not ascend to 

 the upper strata of the atmosphere, but is uniformly diffused 

 through it. 



63. It may be stated generally, that the effect of a column of 

 air superposed upon the surface of water is only to retard, but not 

 either to prevent or diminish, the evaporation. The same quantity 

 of vapour will be developed as would be produced at the same 

 temperature if no air were superposed on the water ; but while in 

 the latter case the entire quantity of vapour would be developed 

 instantaneously, it is produced gradually, and completed only 

 after a certain interval of time when the air is present. The 

 quantity of vapour developed, and its density and pressure, are 

 however exactly the same, whether the space through which it ia 

 diffused be a vacuum, or be filled by air, no matter what the 

 density of the air may be. The properties of the air, therefore, 

 neither modify nor are modified by those of the vapour which is 

 diffused through it. 



Since, at the same temperature and pressure, the density of the 

 vapour of water is less than that of air in the ratio of 5 to 8, it 

 follows that when air becomes charged with vapour of its own 

 temperature, the volume will be augmented, but the density 

 diminished. If a certain volume of air weigh 8 grains, an 

 equal volume of vapour will weigh 5 grains, the two volumes- 

 mixed together will weigh 13 grains, and, consequently, an 

 equal volume of the mixture will weigh 6^ grains. In this 

 case, therefore, the density of the air charged with vapour is 

 less than the density of dry air of the same temperature in the 

 ratio of 6| to 8. 



64. The evaporation produced during the day by the action ot 

 solar heat on the surface of water, and on all bodies charged with 

 moisture, causes the atmosphere at the time of sunset to be more 

 or less charged with vapour, especially in the warm season. On 

 hot days, and in the absence of winds, the atmosphere at sunset is 

 generally at or near the point of saturation. 



Immediately after sunset the temperature of the air falls. If it 

 were previously in a state of saturation condensation must ensue, 

 which will be considerable if the heat of the day and the con- 

 sequent change of temperature after sunset be great. In such 

 case, the vapour condensed often assumes the appearance of a fine 



91 



