RATIO GF EVAPORATION TO HUMIDITY. 



335 



elementary parallelogram E lei: but by the hypothesis the 

 fluxion of time is constant; Ee therefore is inversely pro- 

 portional to EI, which is inversely proportional toCE; 

 and consequently Ee is directly proportional to CE: that 

 is to say, the evaporation is at each instant proportional to 

 the difference between the quantity of water necessary to 

 saturate completely the body of air in which the evapo- 

 ration takes place, and the quantity of water actually eva- 

 porated and suspended in that air ; or, in other words, the 

 evaporation is proportional to the excess of the moisture 

 of the air at the point of saturation over the present hu- 

 midity. This is the general law, which evaporation follows 

 with respect to the humidity of the air. 



This law appears to confirm the opinion of Muschem- Tends to con. 

 brceck* and Leroy t, that the evaporation of water is no- fi ™ the opi " 



,. , * .' » i . rt •» . i . . nionthateva- 



thing but a solution of this fluid in the ambient air ; for the poration is a 

 law just announced must take place generally in all S0 Iu- true solullon - 

 tions. In fact, if the menstruum did not exert on the body 

 to be dissolved, in every portion of time, an action pro- 

 portionate to the quantity that remained to be dissolved to 

 produce complete saturation, but a greater or less action, 

 it would follow, that, in the first case, when the menstru- 

 um is completely saturated it would still retain a part of its 

 solvent action, which would remain without effect; and in 

 the second, that the effect would be greater than its cause, 

 which is equally absurd. 



The application of this law supposes a knowledge of the Attempt to 

 quantity of water necessary to saturate completely a given £" d f G 1"™~ 

 quantity of air at a given temperature. To endeavour to that would sa- 



accomplish this object, I repeated the elegant experiments turateair ata 

 r J 7 * or given temper- 



of Mr. de Saussure J : but not having a large globe, I could ature. 



only employ glass jars, the apertures of which I closed 

 with a plate of metal cemented all round. Notwithstand- 

 ing the imperfection of this apparatus, I had the satisfac- 

 tion to obtain the same result as that celebrated philoso- 

 pher; namely, that it required about 10 grains of water to 

 saturate completely a cubic foot of air at the temperature 



* Diss. Phys. Leyden, 1751, vol.11, p. 721, 



f Mem. Acad. 1751, p. 484 and foil. 



1 Essais sur l'Hygrometrie, N- 97 and foK 



of 



