78 On the Production of Vapor, 



Is thrown off in vapor, more is drawn from the neigh- 

 boring parts, and propelled to the surface by the endeav- 

 or of the particles of water to come into union. And I 

 suspect that like other fluids, having formed a current, 

 it continues the same for a while after an equilibrium is 

 restored. There are some phenomena of nature, that 

 seem to indicate this ; and cannot well be explained with- 

 out the supposition. Unless it be admitted that there 

 exists a stratum of rarefied water at the surface, it can- 

 not well be conceived how any evaporation can take place. 

 We cannot say how those small portions of water, that 

 constitute particles of vapor, can be torn off and detach- 

 ed from the body of water ; unless by the agency of heat 

 the integrant parts are first separated to a greater distance 

 from each other, and the affinity of aggregation thereby 

 weakened. A small force, such as the agitation occa- 

 sioned by the motion of the air, the action of the solar 

 rays, or ebullition, will then be sufficient to throw off the 

 water in minute particles ; but if it should remain in its 

 common state of density, the particles would be so strong- 

 ly held down by attraction, as not to be dislodged by 

 these causes. But the whole body of water is not thus 

 rarefied ; there must, therefore, exist a rarefied stratum 

 at the surface, as a foundation for the formation of parti- 

 cles of vapor. The depth and rarity of this vaporific 

 stratum, varies with the state of the atmosphere. When 

 the air has a greater specific gravit}^, it so compresses as 

 to render it less rare, and of less depth, than when the 

 air possesses less gravity. This superficial stratum is of 

 a variable density, increasing from the exterior toward 

 the interior parts ; this is evident, and results from the 

 elasticity of the fluid. A particle of vapor is an assem- 

 blage of the exceedingly minute particles of water, rare- 

 fied by heat to such a degree as to be lighter than air, and 

 increasing in density from the surface toward the central 

 parts. When such a particle is detached from the body 

 of water, it parts with a portion of its caloric, which be- 

 fore was ret^iined by the attraction of the water. It hence 

 arises that the gravity of vapor is,coeteris paribus,propor- 

 tional to the magnitude of its particles. Since this rare- 

 fied stratum at the surface of water is necessary to the 



