1821.] Causes of Calorific Capacity, Latent Heat, fyc. 37? 



Cor. 1. — Hence if the incremental evaporation of the fluid be 

 denoted by F, and the contemporaneous condensation of the 

 superincumbent air by A, the apparent evaporation of the fluid, 

 will beF-A = F-ET, Prop. XI. Therefore, though the 

 temperature being the same, the fluid would continue to evapo- 

 rate equally fast under every circumstance of humidity and 

 pressure in the superincumbent air, the apparent evaporation 

 would only be as the excess of the evaporation of the fluid above 

 the condensation of the vapour in the air. Let t be the tension 

 of the vapour in vacuo at the temperature T of the fluid, then, by 

 Prop. XI. the apparent evaporation of the water is as (r — E) T, 

 E being the elasticity the vapour in the atmosphere would have, 

 were it left to occupy the same space with all the air withdrawn. 



From this last formula it appears, that if the vapour in the 

 atmosphere be very small compared to the evaporation from the 

 fluid, the evaporation will be nearly as the tension of the vapour 

 corresponding to the true temperature of the fluid, and the true 

 temperature of the fluid conjointly. Mr. Dalton, to whose ability 

 we owe almost the whole of our knowledge of the laws of evapo- 

 ration, has found that water at 212° Fahr. evaporates at the rate 

 of 30 grains per minute. At this temperature the tension of 

 vapour is 30 inches. Let t be the true temperature correspond- 

 ing to 212° Fahr. then the evaporation of water in a dry atmo- 

 sphere at any other temperature, in grains per minute, is equal to 



T T . 



grains, supposing the atmosphere in which Mr. Dalton 



operated, was perfectly dry. 



The following table, whose first, third, and fourth columns I 

 have copied from M. Biot's Traite de Physique, will show how 

 this theorem agrees with the actual experiments of Mr. Dalton^ 



Mean error, —'015 



* In this fourth experiment, I think the printer of M. Biot's or of Mr. Dalton's work 

 must have made an error, which I have endeavoured to rectify. It stands 8-5 in the 

 Traite de Physique; but from the well-known care and skill of Mr. Dalton, and the 

 relation of the preceding and subsequent experiments with the tension in the third 

 column, I would venture to say 8*5 cannot be correct. There is no question, I think, 

 but 7-5 is what Mr. Dalton observed ; and that the 8 crept in by the inattention of the 

 compositor of one of the works. 



