^ ON EVAPORATION, 



and becomes a diftinfl elaftic fluid, called jteam: whether 

 water firft chemically combined with air, and then heated above 

 212®, is detached from the air or remains with it, the advo- 

 — is obfcurc and cates of the theory have not determined. — This theory has al- 

 dittcut. ways been confidered as complex and attended with difficulties; 



fo much that M. Pi6let and others have rejeded it, and adopted 

 Theory of dlf- that which admits of diftind elailic vapours in the atmofphere 

 vapours^n"thc ^* ^^ temperatures, uncombined with either of the principal 

 atmofphere. conftituent gafes; as being much more iiraple and eafy of ex-» 

 plication than the other; though they do not remove the grand 

 objeflion to it, arifing from atmofpheric preflure. It has how- 

 ever been made to appear in thefe eflays, I prefume, that the 

 objedion to it from preflure, is itfelf founded upon an un- 

 grounded hypothefis. 



Leaving the theory of evaporation for the prefent, we fliall 

 proceed to the experiments. 



The following pofitions have been eftablifljed by others, and 

 need therefore only to be mentioned here. 

 Pofitions. Dif- 1 • Some fluids evaporate much more quickly than others, 

 ferent fluids 2. The quantity evaporated is in direct proportion to the fur- 



drfferent rapi- face expofed, all other circumflances alike, 

 dity. Evapo- 3. An increafe of temperature in the liquid is attended with 

 fur'face*- k in- ^" increafe 6f evaporation, not diredly proportionable, 

 crcafes with the 4. Evaporation is greater where there is a flream of air than 

 temperature} ^^iQie the air is ftagnant. 



and when the ° . i i r » i • i- 



air is in motion, 5. Evaporation from water is greater the lefs the humidity 



and is drier. previoufly exifting in the atmofphere, all other circumflances 

 the fame. 



The objects in view in this eflay, are, 

 Objefts of the 1. To determine the precife effe<5l that a variation of tem- 

 prcfent eflay. perature has upon the quantity evaporated. 



2. To determine the ratio of evaporability of difierent fluids, 



3. To find a rule by which the quantity and efled of previous 

 humidity in the air may be afcerlained. 



4. From thefe ^nd other fads to obtain a true theory of eva- 

 poration. 



On the Evaporation of Water at 2\2^ 

 Water boiled in I took a fmall cylindrical veflfel of tin, its diameter 3i and 

 finTteti e \ ft^" ^^P^^ ^z ^"^^^^ ; and having fixed three pieces of wire to equi- 

 by evaporation diftant points of the circumference, they were faftened toge- 

 a quantity, ttjey 



