5 ON EVAPORATION. 



The evaporation from water of ISO® was from 18 to ^ 

 grains per minute, according to circumftancesj or about halt 

 of that3t 212°. 

 <iuant5ues At 16 1-^ it was about one third of the quantity at the boiU 



which were, J^g temperature; or from 10 to 16 grains per minute. 



At 152° it was only one fourth of that at boiling; or from 

 8 to 12 grains, according to circumftances. 



The temperature of 144« aflbrd { of the effea at boiling; 

 13S*>gave|, 8ic. 



Having previoufly to thefe experiments determined the force 



of aqueous vapour at all the temperatures under 21 2*^, I was 



naturally led to examine whether tlie quantity of water eva-r 



porated in a given time bore any proportion to the^force of va- 



— in every part pour of the fame temperature, and was agreeably farprifed to 



of the fcaiepro- ^„^ ^l^^^. ^|j^ exaaiv correfponded in every part of the therr 



portional to the -^ -^ , 



force of vapour mometric fcale ; thus the forces of vapour at 21 2*^, 180^, 164°, 



at that temper- 1.52°, M4^, and 138° are equal to 30, 15, 10, 7|, 6 and 5 

 inches of mercury refpec^ively, and the grains of water eva- 

 porated per minute in Ihofe temperatures were 30, 15, 10, 7f , 

 6 and 5 alio; or numbers proportional to thefe. Indeed it 

 fliould be fo from the elj-abliflied law of mechanics, that all 

 effects are proportional to the caufes producing them. The 

 atmofphere, it fliould feem, obftruds the diffufion of vapour, 

 which would otherwife be almoft inftantaneous, as in vacuo; 

 but this obllrudion is overcome in proportion to the force of 

 the vapour. The obftrudlion however cannot arife from the 

 zveight of the atmofphere, as has till now been fuppofed; for 

 then it would efFcdually prevent any vapour from arifing un- 

 der 212^ : but it is caufed by the vis inerlitz of the particles of 

 air; and is fimilar to that which a ftrcam of water meets with 

 in defcendingamongft pebbles. 

 In low temper- The theory of evaporation being thus manifefted from ex- 

 atures the eva- periments in high temperatures, I found that if it was to be 

 force of vapour verified by experiments in low temperatures, regard raufl be 

 at the temper- had to the force of vapour actually exifting in the atmofphere 

 water diminiflied ^^ *^® i\v^Q. For inftance, if water of 39<* were the fubjed, 

 by thatexifting the force of vapour of that temperature is ^ of the force at 

 'h-rcV^'"^^" 212^, and one might exped the quantity of evaporation -5'^ 

 alfo; but if it fliould happen, as it fometimes does in fummfer, 

 that an aqueous atmofphere to that amount does already exift, 

 the evaporation, inllead of being -^-^ of that from boiling water, 



would 



