28 Dr. Masons Description 



and that where they happen to be mixed together, they 

 exist as independent atmospheres." 



The second, that the final tension of vapour given off 

 in the process of evaporation is determined not by the 

 temperature of the evaporating surface, but by the elasti- 

 city of the aqueous atmosphere already existing ; which 

 law I will endeavour to establish in my next paper. 



If we take the trouble to inquire how the error has arisen 

 with regard to this objection, I am endeavouring to remove, 

 we shall find that evaporation has been considered to be 

 essentially promoted by the application or presence of heat, 

 and the agitation of the aerial medium. 



Mr. Dalton when speaking of this subject observes, 

 *' that air when calm and still, merely obstructs the pro- 

 gress of evaporation ; but when in motion it increases its 

 effect in direct proportion to its velocity by removing the va- 

 pour as it forms." He has fixed the extremes that are likely 

 to occur in ordinary circumstances at 120 and 189 grains 

 per minute, from a vessel of six inches diameter, at a tem- 

 perature of 212°, giving 79 grains increase per minute, for 

 the effects of a strong wind. This law would be perfectly 

 applicable in a dry air, but will by no means hold in one 

 containing almost its full charge of humidity. 



The true law, I conceive to be this, that evaporation takes 

 place from a humid surface in direct proportion to the tem- 

 perature and velocity of the air, diminished by the force 

 of the vapour already existing in the atmosphere; this I 

 shall prove by direct experiment. The agitation of the 

 air has hitherto been conceived to perform the principle 

 part in reducing the temperature of an evaporating surface, 

 but I hope to prove, that it is the dryness of the air on 

 which we shall find the effect alone to depend. 



For example, if air be perfectly saturated with humidity, 

 neither the increased temperature of the humid surface 

 equally with that of the surrounding air, nor the strongest 

 agitation of the medium can produce further evaporation 

 or depression of temperature ; under those circumstances, 

 both thermometers would indicate the same degree, and 

 show that the air was absolutely saturated with humidity ; 

 these conditions would continue until one of the two follow- 



